= CTerm Manual
:toc:

== CTerm terminal characteristics

=== End of line behaviour (wrapping):
The cursor is moved to the first character of the next line
as soon as a character is written to the last column of the
current line, not on the next character.  A tab will wrap
to the next line only if the current cursor position is the
last character on the line.  This behavior is often surprising
to people who are used to VT emulators which implement the LCF
as documented in <<STD-070>>, who expect the cursor to "stick" in
the last column until the next character is received.

There are two settable flags that will impact the default
behaviour.

`CSI ? 7 l` will disable wrapping at the end of line
completely, and any characters written to the last column will
not move the cursor at all, overwriting the existing charater.
Default behaviour can be restored with `CSI ? 7 h`.

If the `CSI = 4` h sequence is received, CTerm will enable LCF
mode as documented in <<STD-070>>, and `CSI = 4 l` will restore
default behaviour.  `CSI = 5 h` will set LCF mode and disable
`CSI = 4 l`, as well as cause LCF to remain enabled across
an `ESC c` (RIS).

Specifically, the LCF will be
set when displaying a printable character advances the cursor
to the right margin, and cleared by any of the following being
received:
`CSI ? 6 h`, `CSI ? 6 l`, `CSI ? 7 l`, `CSI @`, `CSI A`, `CSI B`, `CSI a`
`CSI j`, `CSI H`, `CSI f`, `CSI I`, `CSI Y`, `CSI J`, `CSI K`, `CSI P`
`CSI X`, `CSI r`, `ESC E`, `ESC M`, `CR`, `LF`, `BS`, `TAB`
Any normal printable character when the cursor is at the right
margin (of the screen or scrollable area).

== C0 Control characters

=== `0x00` NUL (`NUL`)
In doorway mode, indicates that the next character is
a literal character.  The IBM CP437 character will
be displayed.  This allows ESC and other control
characters to be placed on the screen.

SOURCE: <<BANSI>>

=== `0x07` Bell (`BEL`)
Beep

=== `0x08` Backspace (`BS`)
Non-destructive backspace.  Moves cursor position to
the previous column unless the current column is the
first, in which case no operation is performed.

SOURCE: <<ECMA-48>>

=== `0x09` Character Tabulation (`HT`)
Moves to the next character tabulation stop.  Does not overwrite
any characters in between.  If there are no character tabulation stops
left in the line, moves to the first position of the next line.
If the starting position is on the last line, will perform
a scroll, filling the new line at bottom with the current
attribute.

NOTE: `0x0B` (VT) is NOT treated as a line tabulation control character.
It is displayed as the CP437 glyph (♂).  Use CVT (`CSI Pn Y`) for
line tabulation.

SOURCE: <<ECMA-48>>

=== `0x0A` Line Feed (`LF`)
Move cursor position to same column of the next row.
If current row is the last row, scrolls the screen up
and fills the new row with the current attribute.

SOURCE: <<ECMA-48>>

=== `0x0D` Carriage Return (`CR`)
Move cursor position to column 1 of the current line

SOURCE: <<ECMA-48>>

=== `0x1B` Escape (`ESC`)
Introduces a control code.  The `ESC` and the next byte
together form the control code.  If the control code is
not valid, the `ESC` is ignored.

SOURCE: <<ECMA-48>>

== nF Escape Sequences

nF Escape Sequences are in the following format: +
`ESC {SPACE to '/}{'0'` to `'~'}` +
There may be multiple characters from the {SPACE to '/'} before the
terminating {'@' to '~'} character.

At present, CTerm does not support any nF escape sequences.

SOURCE: <<ECMA-35>>

== Fp Escape Sequences (Private control functions)

Private control functions are in the following format: +
`ESC {'0'` to `'?'}` +

SOURCE: <<ECMA-35>>

Legal combinations not handled are silently dropped.

=== `ESC :` Escaped String Terminator (`ESCST`)
Reserved for use within STS transmitted content (FETM=INSERT mode).
OSC 8 hyperlink sequences inside an SOS-framed STS response cannot use
bare ST (`ESC \`) as their terminator because it would prematurely close
the SOS frame.  Instead, `ESC : \` (0x1B 0x3A 0x5C) is used as a
substitute terminator.

`ESC :` is an unassigned Fp private-use escape sequence (per <<ECMA-35>>).
A conformant parser that encounters `ESC :` outside of STS content
silently drops it (per the rule above).  Within STS FETM=INSERT content,
the host parser recognizes the three-byte sequence `ESC : \` as "escaped
ST" and converts it to `ESC \` before replay.

=== `ESC 7` Save Cursor (`DECSC`)
Saves the current cursor position same as `CSI s`

SOURCE: <<VT102>>

=== `ESC 8` Restore Cursor (`DECRC`)
Restores the current cursor position same as `CSI u`

SOURCE: <<VT102>>

== Fe Escape Sequences (Control functions in the C1 set)

Control codes are in the following format: +
`ESC {'@'` to `'_'}`
Legal combinations which are not handled are silently dropped.

=== `ESC E` Next Line (`NEL`)
Moves to the line home position of the next line.
(Same as `CR` `LF`)

SOURCE: <<ECMA-48>>

=== `ESC F` Start of Selected Area (`SSA`)
Marks the active presentation position as the first character position
of a selected area.  The content from SSA to ESA (or to the cursor
position, depending on TTM) is eligible for transmission when STS is
received.

SSA and ESA are cleared by changes to scroll margins (DECSTBM, DECSLRM),
origin mode (DECOM), or RIS.

Scrolling does not invalidate SSA — it marks a position in the
presentation component, not the content at that position.  When STS
triggers, the terminal reads whatever content is currently at the
marked positions.

SOURCE: <<ECMA-48>> (Section 8.3.138)

=== `ESC G` End of Selected Area (`ESA`)
Marks the active presentation position as the last character position
(inclusive) of a selected area begun by SSA.

ESA is required when TTM is set to ALL (`CSI 16 h`).  When TTM is
CURSOR (the default), the cursor position defines the end instead.

If no ESA has been issued, the selected area extends to the last cell
of the viewport.

SOURCE: <<ECMA-48>> (Section 8.3.47)

=== `ESC H` Character Tabulation Set (`HTS`)
Sets a character tabulation stop at the current column.

SOURCE: <<ECMA-48>>

=== `ESC J` Line Tabulation Set (`VTS`)
Sets a line tabulation stop at the current line.  Line tabulation stops
are used by CVT (`CSI Pn Y`) and can be cleared by TBC with parameter
values 1, 4, or 5.

Line tabulation stops are at fixed row numbers and are not affected by
scrolling.  No default line tabulation stops are set.

SOURCE: <<ECMA-48>>

=== `ESC M` Reverse Line Feed (`RI`)
Move up one line

SOURCE: <<ECMA-48>>

=== `ESC P`	Device Control String (`DCS`)
Begins a string consisting of the characters 0x08 - 0x0d and
0x20-0x7e, terminated by a String Terminator (`ST`)

If a byte outside the valid range (other than `ESC`) is received, the
string is discarded and the byte is re-processed as normal input.
This applies to all command strings: DCS, OSC, PM, and APC.

Introduces <<Device Control Strings>>

SOURCE: <<ECMA-48>>

=== `ESC S` Set Transmit State (`STS`)
Triggers transmission of the selected area content established by SSA
(and optionally ESA) back to the host.

The response is framed as `SOS CTerm:STS:<N>: <content> ST` where
`<N>` is 0 for FETM=INSERT (attributed) or 1 for FETM=EXCLUDE
(text only).

**With TTM=CURSOR (default, `CSI 16 l`):** Content from SSA up to but
excluding the cursor position is transmitted.

**With TTM=ALL (`CSI 16 h`):** Content from SSA through ESA (inclusive)
is transmitted regardless of cursor position.

**With FETM=INSERT (default, `CSI 14 l`):** Characters are transmitted
with SGR sequences for attribute changes, doorway mode encoding for C0
control characters in cells, and OSC 8 for hyperlink changes.  The
stream is valid ECMA-48 that can be replayed to reproduce the region
(after escaped-ST expansion; see below).

**Escaped ST in attributed content:** OSC 8 sequences require ST (`ESC \`)
as their terminator, but bare ST would prematurely close the SOS frame.
Within STS FETM=INSERT content, OSC 8 sequences are terminated with
`ESC : \` (0x1B 0x3A 0x5C) instead of `ESC \` (0x1B 0x5C).
`ESC :` is an unassigned Fp private-use escape sequence (per ECMA-35)
that any conformant parser silently drops.  The host parser recognizes
`ESC : \` within STS content as "escaped ST" and converts it to `ESC \`
before replay.

**With FETM=EXCLUDE (`CSI 14 h`):** Only graphic characters are
transmitted.  C0 and DEL bytes in cells are replaced with SPACE.

The transmitted stream is a linear sequence of character positions in
presentation order (left to right, top to bottom).  No line boundary
markers are emitted — the host slices by the known screen width.

If no SSA has been issued, or the eligible area is empty, an empty
response is returned: `SOS CTerm:STS:<N>: ST`

STS automatically clears the transmit state after transmission
completes.

SOURCE: <<ECMA-48>> (Section 8.3.145)

=== `ESC X` Start Of String (`SOS`)
As the above strings, but may contain any characters except
a Start Of String sequence or a String Terminator sequence.

SOURCE: <<ECMA-48>>

=== `ESC [` Control Sequence Introducer (`CSI`)
Introduces <<Control Sequences>>

=== `ESC \` String Terminator (`ST`)
Ends a string.

SOURCE: <<ECMA-48>>

=== `ESC ]` Operating System Command (`OSC`)
Begins a string consisting of the characters 0x08 - 0x0d and
0x20-0x7e, terminated by a String Terminator (ST)

Introduces <<Operating System Commands>>

SOURCE: <<ECMA-48>>

=== `ESC ^` Privacy Message (`PM`)
Begins a string consisting of the characters 0x08 - 0x0d and
0x20-0x7e, terminated by a String Terminator (`ST`)
The string is currently ignored.

SOURCE: <<ECMA-48>>

=== `ESC _` Application Program Command (`APC`)
Begins a string consisting of the characters 0x08 - 0x0d and
0x20-0x7e, terminated by a String Terminator (ST)

Introduces <<Application Program Commands>>

SOURCE: <<ECMA-48>>

== Fs Escape Sequences (Standardized single control functions)

Standardized single control functions are in the following format: +
`ESC {'\`'` to `'~'}` +

SOURCE: <<ECMA-35>>

Legal combinations not handled are silently dropped.

=== `ESC c` Reset to Initial State (`RIS`)
Resets all the terminal settings, clears the screen, and homes
the cursor.

SOURCE: <<ECMA-48>>

== Control Sequences

Control sequences start with the Control Sequence Introducer which is
`ESC [`.  `CSI` will be used to express this from now on.

Control sequences are in the following format: +
`CSI {'0' (ZERO) to '?'}{SPACE to '/'}{'@' to '~'}` +
There may be multiple characters from the {'0' (ZERO) to '?'}
and {SPACE to '/'} before the terminating {'@' to '~'} character.

Legal combinations not handled are silently dropped.
Illegal combinations are displayed.

=== Sequence Parameters
Parameters are expressed by the {'0' (ZERO) to '?'} character set.

Sequences which use parameters use decimal parameters separated by
a ';'.  The use of a ':' from the set is reserved.

If the parameter string begins with '<', '=', '>', or '?' then
this is a non-standard extension to the ANSI spec.

.Sequence Paramters
[%autowidth,cols="1,2"]
|===
|`Pn`
|Indicates a single numeric parameter

|`Pn1 ; Pn2`
|Two numeric parameters

|`Pn...`
|Any number of numeric parameters

|`Ps`
|Single selective parameter

|`Ps1 ; Ps1`
|Two selective parameters

|`Ps...`
|Any numer of selective parameters
|===

If a default is defined, the parameter is optional

=== `CSI Pn @` Insert Character(s) (`ICH`)
Defaults: `Pn` = 1 +
Moves text from the current position to the right edge Pn characters
to the right, with rightmost characters going off-screen and the
resulting hole being filled with the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI Pn SP @` Scroll Left (`SL`)
Defaults: `Pn` = 1 +
Shifts the contents of the screen left `Pn` columns(s) with
leftmost columns going off-screen and the resulting hole being
filled with the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI Pn A` Cursor Up (`CUU`)
Defaults: `Pn` = 1 +
Moves the cursor position up `Pn` lines from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn SP A` Scroll Right (`SR`)
Defaults: `Pn` = 1 +
Shifts the contents of the screen right `Pn` columns(s) with
rightmost columns going off-screen and the resulting hole being
filled with the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI Pn B` Cursor Down (`CUD`)
Defaults: `Pn` = 1 +
Moves the cursor position down `Pn` lines from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn C` Cursor Right (`CUF`)
Defaults: `Pn` = 1
Moves the cursor position right `Pn` columns from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn D` Cursor Left (`CUB`)
Defaults: `Pn` = 1
Moves the cursor position left `Pn` columns from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Ps1 ; Ps2 sp D` Font Selection (`FNT`)
Defaults: `Ps1` = 0  Ps2 = 0
"sp" indicates a single space character.
Sets font `Ps1` to be the one indicated by `Ps2`.  Currently four fonts are
supported.  `Ps2` must be between 0 and 255.  Not all output types support
font selection.  Only X11 and SDL currently do.

.Supported `Ps1` values
[%autowidth,cols="1,2"]
|===
|0
|Default font

|1
|Font selected by the high intensity bit when `CSI ? 31 h` is enabled

|2
|Font selected by the blink intensity bit when `CSI ? 34 h` is enabled

|3
|Font selected by both the high intensity and blink bits when both `CSI ? 31 h` and `CSI ? 34 h` are enabled
|===

.Currently included fonts
[%autowidth,cols="1,2"]
|===
|0
|Codepage 437 English

|1
|Codepage 1251 Cyrillic, (swiss)

|2
|Russian koi8-r

|3
|ISO-8859-2 Central European

|4
|ISO-8859-4 Baltic wide (VGA 9bit mapped)

|5
|Codepage 866 (c) Russian

|6
|ISO-8859-9 Turkish

|7
|haik8 codepage (use only with armscii8 screenmap)

|8
|ISO-8859-8 Hebrew

|9
|Ukrainian font koi8-u

|10
|ISO-8859-15 West European, (thin)

|11
|ISO-8859-4 Baltic (VGA 9bit mapped)

|12
|Russian koi8-r (b)

|13
|ISO-8859-4 Baltic wide

|14
|ISO-8859-5 Cyrillic

|15
|ARMSCII-8 Character set

|16
|ISO-8859-15 West European

|17
|Codepage 850 Multilingual Latin I, (thin)

|18
|Codepage 850 Multilingual Latin I

|19
|Codepage 885 Norwegian, (thin)

|20
|Codepage 1251 Cyrillic

|21
|ISO-8859-7 Greek

|22
|Russian koi8-r (c)

|23
|ISO-8859-4 Baltic

|24
|ISO-8859-1 West European

|25
|Codepage 866 Russian

|26
|Codepage 437 English, (thin)

|27
|Codepage 866 (b) Russian

|28
|Codepage 885 Norwegian

|29
|Ukrainian font cp866u

|30
|ISO-8859-1 West European, (thin)

|31
|Codepage 1131 Belarusian, (swiss)

|32
|Commodore 64 (UPPER)

|33
|Commodore 64 (Lower)

|34
|Commodore 128 (UPPER)

|35
|Commodore 128 (Lower)

|36
|Atari

|37
|P0T NOoDLE (Amiga)

|38
|mO'sOul (Amiga)

|39
|MicroKnight Plus (Amiga)

|40
|Topaz Plus (Amiga)

|41
|MicroKnight (Amiga)

|42
|Topaz (Amiga)

|43
|Prestel

|44
|Atari ST

|45
|RIPterm
|===

Not all fonts are supported in all modes.  If a font is not supported
in the current mode, no action is taken, but there should be a
non-zero 'Font Selection result' value in the Font State Report.

SOURCE: <<ECMA-48>>

=== `CSI Pn E` Cursor Next Line (`CNL`)
Defaults: `Pn` = 1 +
Moves the cursor to the first column of the line `Pn` down from the
current position. Attempting to move past the screen boundaries
stops the cursor at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn F` Cursor Preceding Line (`CPL`)
Defaults: `Pn` = 1 +
Moves the cursor to the first column of the row `Pn` up from the
current position.  Attempting to move past the screen boundaries
stops the cursor at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn G` Cursor Character Absolute (`CHA`)
Defaults: `Pn` = 1 +
Movies the cursor to column Pn of the current row.

SOURCE: <<ECMA-48>>

=== `CSI Pn1 ; Pn2 H` Cursor Position (`CUP`)
Defaults: `Pn1` = 1  `Pn2` = 1 +
Moves the cursor to the `Pn2`th column of the `Pn1`th line.

SOURCE: <<ECMA-48>>

=== `CSI Pn I` Cursor Forward Tabulation (`CHT`)
Defaults: `Pn` = 1 +
Move the cursor to the Pn-th next tab stop.
Basically the same as sending TAB Pn times.

SOURCE: <<ECMA-48>>

=== `CSI Ps J` Erase in Page (`ED`)
Defaults: `Ps` = 0 +
Erases from the current screen according to the value of `Ps`

[%autowidth,cols="1,2"]
|===
|0
|Erase from the current position to the end of the screen.

|1
|Erase from the current position to the start of the screen.

|2
|Erase entire screen.  As a violation of ECMA-048, also moves
the cursor to position 1/1 as a number of BBS programs assume
this behaviour.
|===

Erased characters are set to the current attribute.

SOURCE: <<ECMA-48>>, <<BANSI>>

=== `CSI Ps K` Erase in Line (`EL`)
Defaults: `Ps` = 0 +
Erases from the current line according to the value pf `Ps`

[%autowidth,cols="1,2"]
|===
|0
|Erase from the current position to the end of the line.

|1
|Erase from the current position to the start of the line.

|2
|Erase entire line.
|===

Erased characters are set to the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI Pn L` Insert Line(s) (`IL`)
Defaults: `Pn` = 1 +
Inserts `Pn` lines at the current line position.  The current line and
those after it are scrolled down and the new empty lines are filled
with the current attribute.  If the cursor is not currently inside
the scrolling margins, has no effect.

SOURCE: <<ECMA-48>>

=== `CSI Pn M` Delete Line(s) / "ANSI" Music (`DL`)
Defaults: `Pn` = 1
Deletes the current line and the `Pn` - 1 lines after it scrolling the
first non-deleted line up to the current line and filling the newly
empty lines at the end of the screen with the current attribute.
If the cursor is not currently inside the scrolling margins, has no
effect.
If "ANSI" Music is fully enabled (CSI = 2 M), and no parameter is
specified, performs "ANSI" music instead.
See <<_ansi_music,"ANSI" MUSIC>> section for more details.

SOURCE: <<ECMA-48>>, <<BANSI>>

=== `CSI = Ps M` CTerm Set ANSI Music (`CTSAM`)
NON-STANDARD EXTENSION. +
Defaults:  `Ps` = 0 +
Sets the current state of ANSI music parsing.
0 - Only `CSI |` will introduce an ANSI music string.
1 - Both `CSI |` and `CSI N` will introduce an ANSI music string.
2 - `CSI |`, `CSI N`, and `CSI M` will all introduce an ANSI music string.
In this mode, Delete Line will not be available.

=== `CSI N` BananaCom ANSI Music (`BCAM`)
"ANSI" Music / Not implemented.
If "ANSI" Music is set to BananaCom (`CSI = 1 M`) or fully enabled
(`CSI = 2 M`) performs "ANSI" music.  See <<_ansi_music,"ANSI" MUSIC>> section for more
details.

SOURCE: <<BANSI>>

=== `CSI Pn P` Delete Character (`DCH`)
Defaults: `Pn` = 1 +
Deletes the character at the current position by shifting all
characters from the current column + `Pn` left to the current column.
Opened blanks at the end of the line are filled with the current
attribute.  If the cursor is not currently inside the scrolling
margins, has no effect.

SOURCE: <<ECMA-48>>

=== `CSI Pn S` Scroll Up (`SU`)
Defaults: `Pn` = 1 +
Scrolls the screen up `Pn` lines.  New lines emptied at the
bottom are filled with the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI ? Ps1 ; Ps2 S` XTerm Set or Request Graphics Attribute (`XTSRGA`)
If `Ps1` is 2, and `Ps2` is 1, replies with the graphics screen information
in the following format: `CSI ? 2 ; 0 ; Px ; Py S`
Where `Px` is the width of the screen in pixels and `Py` is the height.

SOURCE: <<XTerm>>

=== `CSI Pn T` Scroll Down (`SD`)
Defaults: `Pn` = 1 +
Scrolls all text on the screen down `Pn` lines.  New lines emptied at the
top are filled with the current attribute.

SOURCE: <<ECMA-48>>

=== `CSI Pn X` Erase Character (`ECH`)
Defaults: `Pn` = 1 +
Erase `p1` characters starting at the current character.  Will not erase
past the end of line.
Erased characters are set to the current attribute.
This can erase across scroll margins.

SOURCE: <<ECMA-48>>

=== `CSI Pn Y` Cursor Line Tabulation (`CVT`)
Defaults: `Pn` = 1 +
Moves the cursor to the corresponding character position (same column)
of the line corresponding to the `Pn`-th following line tabulation stop.
Line tabulation stops are set by VTS (`ESC J`).

If there is no next line tabulation stop, the screen scrolls up one line
and the cursor moves to the last row, preserving the current column.

SOURCE: <<ECMA-48>>

=== `CSI Pn Z` Cursor Backward Tabulation (`CBT`)
Defaults: `Pn` = 1 +
Move the cursor to the ``Pn``th preceding tab stop.  Will not go past the
start of the line.

SOURCE: <<ECMA-48>>

=== `CSI Pn `` Character Position Absolute (`HPA`)
Defaults: `Pn` = 1 +
Move the cursor to the specified position on the current row.
Will not go past the end of the line.

SOURCE: <<ECMA-48>>

=== `CSI Pn a` Cursor Position Forward (`HPR`)
Defaults: `Pn` = 1 +
Moves the cursor position forward `Pn` columns from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn b` Repeat (`REP`)
Defaults: `Pn` = 1 +
Repeats the previous graphic character `Pn` times.  Will not repeat
escape sequences.

SOURCE: <<ECMA-48>>

=== `CSI Ps c` Device Attributes (`DA`)
Defaults: `Ps` = 0 +
If `Ps` is 0, CTerm will reply with the sequence:
`CSI = 67;84;101;114;109;pN c`
`67;84;101;114;109` is the ASCII values of the "CTerm" string.  `pN` is the
revision ID of CTerm with dots converted to semi-colons
(e.g. "1;156").  Use the revision to detect if a specific feature
is available.  If you are adding features to a forked version of cterm,
please do so by	adding an extra parameter to the end, not by
incrementing any existing one!

SOURCE: <<ECMA-48>>

=== `CSI < Ps c` CTerm Device Attributes (`CTDA`)
Defaults: `Ps` = 0 +
If `Pn` is 0, CTerm will reply with the sequence:
`CSI < 0 ; Ps... c`

.Possible values for `Ps`
[%autowidth,cols="1,2"]
|===
|1
|Loadable fonts are availabe via Device Control Strings

|2
|Bright Background (ie: DECSET 32) is supported

|3
|Palette entries may be modified via an Operating System Command string
|4
|Pixel operations are supported (currently, sixel and PPM graphics)

|5
|The current font may be selected via `CSI Ps1 ; Ps2 sp D`

|6
|Extended palette is available

|7
|Mouse is available
|===

=== `CSI PN d` Line Position Absolute (`VPA`)
Defaults: `Pn` = 1 +
Moves to row specified by `Pn`.

SOURCE: <<ECMA-48>>

=== `CSI Pn SP d` Tab Stop Remove (`TSR`)
Defaults: None +
Removes a tab stop at postion `Pn`.

SOURCE: <<ECMA-48>>

=== `CSI Pn e` Line Position Forward (`VPR`)
Defaults: `Pn` = 1 +
Moves forward Pn rows.

SOURCE: <<ECMA-48>>

=== `CSI Pn1 ; Pn2 f` Character and Line Position (`HVP`)
Defaults: `Pn1` = 1  `Pn2` = 1 +
Moves the cursor to the ``Pn``2th column of the ``Pn``1th line.

SOURCE: <<ECMA-48>>

=== `CSI Ps g` Tabulation Clear (`TBC`)
Defaults: `Ps` = 0 +
Clears tabulation stops according to the value of `Ps`:

[%autowidth,cols="1,2"]
|===
|0
|Clears the character tabulation stop at the current position.

|1
|Clears the line tabulation stop at the current line.

|3
|Clears all character tabulation stops.

|4
|Clears all line tabulation stops.

|5
|Clears all tabulation stops (both character and line).
|===

SOURCE: <<ECMA-48>>

=== `CSI Ps... h` Set Mode (`SM`)
Sets one or more ANSI modes.  The following modes are supported:

[%autowidth,cols="1,2"]
|===
|14
|FORMAT EFFECTOR TRANSFER MODE (FETM) = EXCLUDE.
When set, transmitted data (via STS) contains only graphic characters.
C0/DEL bytes in cells are replaced with SPACE.  SGR, positioning, and
other formator functions are excluded.

|16
|TRANSFER TERMINATION MODE (TTM) = ALL.
When set, the complete selected area from SSA to ESA is transmitted
regardless of cursor position.  When reset (default), only content
preceding the cursor is transmitted (cursor position excluded).
|===

SOURCE: <<ECMA-48>> (Sections 7.2.6, 7.2.18)

=== `CSI = 255 h` (`BCSET`)
NON-STANDARD EXTENSION +
Enable DoorWay Mode

SOURCE: <<BANSI>>

=== `CSI = 4 h` Enable Last Column Flag (`CTELCF`)
NON-STANDARD EXTENSION +
Enable Last Column Flag mode

=== `CSI = 5 h` Force Last Column Flag (`CTFLCF`)
NON-STANDARD EXTENSION +
Force Last Column Flag mode

=== `CSI ? Ps... h` Set Mode (DECSET)
NON-STANDARD EXTENSION +
Sets one or more mode.  The following modes are supported:

[%autowidth,cols="1,2"]
|===
|6
|Enable origin mode.

In this mode, position parameters are relative to the top left of
the scrolling region, not the screen.  Defaults to reset.

SOURCE: <<VT102>>
|7
|Enable auto wrap

This is the normal mode in which a write to the last column of a
row will move the cursor to the start of the next line triggering
a scroll if required to create a new line.  Defaults to set.

SOURCE: <<VT102>>
|9
|X10 compatible mouse reporting

Mouse button presses will send a CSI M <button> <x> <y>
Where <button> is ' ' + button number (0-based)
<x> and <y> are '!' + position (0-based)

SOURCE: <<XTerm>>
|25
|Display the cursor.  Defaults to set.

SOURCE: <<VT320>>
|31
|Enable bright alt character set

With this mode set, the bright (1) graphic rendition selects
characters from an alternate character set.  Defaults to reset.
|32
|Bright Intensity Disable

This makes the bright intensity bit not control the intensity.
Mostly for use with `CSI ? 31 h` to permit fonts in the same
colours.  Defaults to reset.
|33
|Blink to Bright Intensity Background

With this mode set, the blink (5,6) graphic renditions cause the
background colour to be high intensity rather than causing blink.
Defaults to reset.
|34
|Enable blink alt character set

With this mode set, the blink (5, 6) graphic renditions selects
characters from an alternate character set.  Defaults to reset
|35
|Blink Disabled

This makes the blink (5, 6) graphic renditions not cause the
character to blink.  Mostly for use with `CSI ? 34 h` to permit
fonts to be used without blinking.  Defaults to reset.
|67
|When set, the backspace key sends a backspace character.

Defaults to set.
|69
|DEC Left Right Margin Mode enabled

Enables `CSI s` to set the left/right margins, and disables `CSI s`
from saving the current cursor position.
|80
|Sixel Scrolling Enabled

When this is set, the sixel active position begins in the
upper-left corner of the currently active text position.
When the sixel active position reaches the bottom of the
page, the page is scrolled up.  At the end of the sixel
string, a sixel newline is appended, and the current cursor
position is the one in which the bottom sixel is in.
Defaults to set.

SOURCE: <<VT330340,[VT330/340]>>
|1000
|Normal tracking mode mouse reporting

Mouse button presses will send a CSI M <button> <x> <y>
Where <button> is ' ' + button number (0-based)
Mouse button releases will use a button number of 4
<x> and <y> are '!' + position (0-based)

SOURCE: <<XTerm>>
|1001
|Highlight tracking mode mouse reporting

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1002
|Button-event tracking mode mouse reporting

Mouse button presses and movement when a button is pressed
will send a CSI M <button> <x> <y>
Where <button> is ' ' + button number (0-based)
32 is added to the button number for movement events.
Mouse button releases will use a button number of 4
<x> and <y> are '!' + position (0-based)

SOURCE: <<XTerm>>
|1003
|Any-event tracking mode mouse reporting

Mouse button presses and movement
will send a CSI M <button> <x> <y>
Where <button> is ' ' + button number (0-based)
32 is added to the button number for movement events.
Mouse button releases will use a button number of 4
<x> and <y> are '!' + position (0-based)
If no button is pressed, it acts as though button 0 is.

SOURCE: <<XTerm>>
|1004
|Focus-event tracking mode mouse reporting

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1005
|UTF-8 encoded extended coordinates

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1006
|SGR encoded extended coordinates

Instead of the CSI M method, the format of mouse reporting
is changed to CSI < Pb ; Px ; Py M for presses and
CSI < Pb ; Px ; Py m for releases.
Instead of CSI M
Px and Py are one-based.
Pb remains the same (32 added for movement)
Button 3 is not used for release (separate code)

SOURCE: <<XTerm>>
|1007
|Alternate scroll mode

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1015
|URXVT encoded extended coordinates

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|2004
|Set bracketed paste mode

SOURCE: <<XTerm>>
|===

=== `CSI Pn j` Character Position Backward (`HPB`)
Defaults: `Pn` = 1 +
Moves the cursor position left `Pn` columns from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Pn k` Line Position Backward (`VPB`)
Defaults: `Pn` = 1
Moves the cursor position up `Pn` lines from the current position.
Attempting to move past the screen boundaries stops the cursor
at the screen boundary.

SOURCE: <<ECMA-48>>

=== `CSI Ps... l` Reset Mode (`RM`)
Resets ANSI modes.  Same parameter values as SM above.  Resetting
mode 14 restores FETM=INSERT (attributed transmission).  Resetting
mode 16 restores TTM=CURSOR (cursor-exclusive termination).

SOURCE: <<ECMA-48>>

=== `CSI = 255 l` Disable DoorWay Mode (`BCRST`)
NON-STANDARD EXTENSION +

SOURCE: <<BANSI>>

=== `CSI = 4 l` (`CTDLCF`)
NON-STANDARD EXTENSION +
Disable Last Column Flag mode

=== `CSI ? Ps... l` Reset Mode (`DECRST`)
NON-STANDARD EXTENSION +
Resets one or more mode.  The following modes are supported:

[%autowidth,cols="1,2"]
|===
|6
|Origin Mode

With this mode reset, position parameters are relative to the
top left of the screen, not the scrolling region.  Defaults
to reset.

SOURCE: <<VT102>>
|7
|Disable auto wrap

Resetting this mode causes a write to the last column of a to
leave the cursor where it was before the write occurred,
overwriting anything which was previously written to the same
position.

SOURCE: <<VT102>>
|9
|Disable X10 compatible mouse reporting

|25
|Hide the cursor.  Defaults to set.

SOURCE: <<VT320>>
|31
|Disable bright alt character set

With this mode reset, the bright (1) graphic rendition does not
select an alternative font.  Defaults to reset.
|32
|Bright Intensity Enable

When reset, bright intensity graphics rendition behaves normally.
Defaults to reset.
|33
|Disable Blink to Bright Intensity Background

With this mode set, the blink (5,6) graphic renditions do not
affect the background colour.  Defaults to reset.
|34
|Disable blink alt character set

With this mode reset, the blink (5, 6) graphic renditions do not
select characters from an alternate character set.  Defaults to
reset.
|35
|Blink Enable

With this mode reset, the blink (5,6) graphic renditions behave
normally (cause the characters to blink).  Defaults to reset.
|67
|When reset, the backspace key sends a delete character.

Defaults to set.
|69
|DEC Left Right Margin Mode disabled

Disables CSI s from setting the left/right margins, and changes
it back to saving the current cursor position.  The current
left/right margins are maintained.
|80
|Sixel Scrolling Disabled

When this is reset, the sixel active position begins in the
upper-left corner of the page.  Any commands that attempt to
advance the sixel position past the bottom of the page are
ignored.  At the end of the sixel string, the current cursor
position is unchanged from where it was when the sixel string
started.  Defaults to set.

SOURCE: <<VT330340,[VT330/340]>>
|1000
|Disable Normal tracking mode mouse reporting

SOURCE: <<XTerm>>
|1001
|Disable Highlight tracking mode mouse reporting

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1002
|Disable Button-event tracking mode mouse reporting

SOURCE: <<XTerm>>
|1003
|Disable Any-event tracking mode mouse reporting

SOURCE: <<XTerm>>
|1004
|Disable Focus-event tracking mode mouse reporting

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1005
|Disable UTF-8 encoded extended coordinates

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1006
|Disable SGR encoded extended coordinates

SOURCE: <<XTerm>>
|1007
|Disable Alternate scroll mode
(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|1015
|Disable URXVT encoded extended coordinates

(Not supported by SyncTERM)

SOURCE: <<XTerm>>
|2004
|Disable bracketed paste mode

SOURCE: <<XTerm>> <<Paste64>>
|===

=== `CSI Ps... m` Select Graphic Rendition (`SGR`)
Defaults: `Ps1` = 0 +
Sets or clears one or more text attributes.  Unlimited parameters are
supported and are applied in received order.  The following are
supported:
[%autowidth,cols="1,2,^3,^4,^5,^6,^7,^8"]
|===
|`Ps` |Description |Blink |Bold |FG |BG |TF |TB
|0
|Default attribute, white on black
|√
|√
|√
|√
|√
|√

|1
|Bright Intensity
| 
|√
| 
| 
|√
| 

|2
|Dim intensity (clears the bright attribute; equivalent to SGR 22
in PC text mode since there is no distinct dim state)
| 
|√
| 
| 
|√
| 

|5
|Blink (By definition, slow blink)
|√
| 
| 
| 
| 
|√

|6
|Blink (By definition, fast blink)

NOTE: Both blinks are the same speed.
|√
| 
| 
| 
| 
|√

|7
|Negative Image - Reverses FG and BG
| 
| 
|√
|√
|√
|√

|8
|Concealed characters, sets the

      foreground colour to the background
      colour.
| 
| 
|√
| 
|√
|√

|22
|Normal intensity
| 
|√
| 
| 
|√
| 

|25
|Steady (Not blinking)
|√
| 
| 
| 
| 
|√

|27
|Positive Image - Restores FG and BG

NOTE: This should be a separate
attribute than 7 but this
implementation makes them equal
| 
| 
|√
|√
|√
|√

|30
|Black foreground
| 
| 
|√
| 
|√
| 

|31
|Red foreground
| 
| 
|√
| 
|√
| 

|32
|Green foreground
| 
| 
|√
| 
|√
| 

|33
|Yellow foreground
| 
| 
|√
| 
|√
| 

|34
|Blue foreground
| 
| 
|√
| 
|√
| 

|35
|Magenta foreground
| 
| 
|√
| 
|√
| 

|36
|Cyan foreground
| 
| 
|√
| 
|√
| 

|37
|White foreground
| 
| 
|√
| 
|√
| 

|38
|Extended Foreground (see notes)
| 
| 
| 
| 
|√
| 

|39
|Default foreground (same as white)
| 
| 
|√
| 
|√
| 

|40
|Black background
| 
| 
| 
|√
| 
|√

|41
|Red background
| 
| 
| 
|√
| 
|√

|42
|Green background
| 
| 
| 
|√
| 
|√

|43
|Yellow background
| 
| 
| 
|√
| 
|√

|44
|Blue background
| 
| 
| 
|√
| 
|√

|45
|Magenta background
| 
| 
| 
|√
| 
|√

|46
|Cyan background
| 
| 
| 
|√
| 
|√

|47
|White background
| 
| 
| 
|√
| 
|√

|48
|Extended Background (see notes)
| 
| 
| 
| 
| 
|√

|49
|Default background (same as black)
| 
| 
| 
|√
| 
|√

|91
|Bright Red foreground
| 
|√
|√
| 
|√
| 

|92
|Bright Green foreground
| 
|√
|√
| 
|√
| 

|93
|Bright Yellow foreground
| 
|√
|√
| 
|√
| 

|94
|Bright Blue foreground
| 
|√
|√
| 
|√
| 

|95
|Bright Magenta foreground
| 
|√
|√
| 
|√
| 

|96
|Bright Cyan foreground
| 
|√
|√
| 
|√
| 

|97
|Bright White foreground
| 
|√
|√
| 
|√
| 

|100
|Bright Black background
|√
| 
| 
|√
| 
|√

|101
|Bright Red background
|√
| 
| 
|√
| 
|√

|102
|Bright Green background
|√
| 
| 
|√
| 
|√

|103
|Bright Yellow background
|√
| 
| 
|√
| 
|√

|104
|Bright Blue background
|√
| 
| 
|√
| 
|√

|105
|Bright Magenta background
|√
| 
| 
|√
| 
|√

|106
|Bright Cyan background
|√
| 
| 
|√
| 
|√

|107
|Bright White background
|√
| 
| 
|√
| 
|√

|===

All others are ignored.

Blink indicates the blink bit.
Bold indicates the bold bit.
FG indicates the foreground colour.
BG indicates the background colour.
TF indicates that the Tru Colour foreground is changed.
TB indicates that the Tru Colour background is changed.

NOTE: For 90-97, there is no effect unless bright foreground colours
are enabled (i.e., DECSET mode 32 is not set, which is the default).

NOTE: For 100-107, there is no effect unless bright background colours
are enabled via DECSET mode 33.

NOTE: For 38 and 48, two additional formats are supported, a palette
selection and a direct colour selection.

For palette selection, an additional two parameters are required
after that value.  They are considered part of the 38/48, not separate
values.  The first additional parameter must be a 5.  The second
additional parameter specified the palette index to use.  To set the
foreground to orange, and the background to a fairly dark grey, you
would send:
`CSI 38 ; 5 ; 214 ; 48 ; 5 ; 238 m`

The default palette is the XTerm 256-colour palette.  <<colors256,[256colors]>>

For direct colour selection, an additional four parameters are required
after that value.  They are considered part of the 38/48, not separate
values.  The first additional parameter must be a 2.  The second,
third, and fourth specify the R/G/B values respectively. CTerm handles
this with an internal temporary palette, so scrollback may not have the
correct colours.  The internal palette is large enough for all cells in
a 132x60 screen to have unique foreground and background colours
though, so the current screen should always be as expected.

SOURCE: <<ECMA-48>>, <<XTerm>>

=== `CSI Ps n` Device Status Report (`DSR`)
Defaults: `Ps` = 0 +
A request for a status report.  CTerm handles the following three
requests:
[%autowidth,cols="1,2"]
|===
|5
|Request a DSR

CTerm will always reply with CSI 0 n indicating 
"ready, no malfunction detected"

|6
|Request active cursor position

CTerm will reply with CSI y ; x R where y is the current line
and x is
the current row.  When origin mode (`CSI ? 6 h`) is enabled,
the reported position is relative to the scroll region, not
the screen.  When the Last Column Flag is set, the reported
column is the last column (not last column + 1).

|255
|NON-STANDARD EXTENSION (BCDSR)

Replies as though a CSI 6 n was received with the cursor in
the bottom right corner.  i.e.: Returns the terminal size as
a position report.
|===

SOURCE: <<ECMA-48>> (parameters 5 and 6 only) <<BANSI>> (parameter 255)

=== `CSI = Ps n` State/Mode Request/Report (`CTSMRR`)
NON-STANDARD EXTENSION +
Defaults: `Ps` = 1 +
When `Ps` is 1, CTerm will respond with a Font State Report of the form
`CSI = 1 ;pF ;pR ;pS0 ;pS1 ;pS2 ;pS3 n`
`pF` is the first available loadable-font slot number
`pR` is the result of the previous "Font Selection" request:
[%autowidth,cols="1,2"]
|===
|0
|successful font selection

|1
|failed font selection

|99
|no font selection request has been received
|===

`pS0` - `pS3` contain the font slots numbers of previously successful
"Font Selection" requests into the 4 available alternate-font
style/attribute values:
[%autowidth,cols="1,2"]
|===
|`pS0`
|normal attribute font slot

|`pS1`
|high intensity foreground attribute font slot

|`pS2`
|blink attribute font slot

|`pS3`
|high intensity blink attribute font slot
|===

When `Ps` is 2, CTerm will respond with a Mode Report of the form
`CSI = 2[;pN [;pN] [...]] n`
Where pN represent zero or more mode values set previously
(e.g. via `CSI ? pN h`). Mode values cleared (disabled via `CSI ? pN l`)
will not be included in the set of values returned in the Mode
Report.  If no modes are currently set, an empty parameter will be
included as the first and only pN.

When `Ps` is 3, CTerm will respond with a Mode Report of the form
`CSI = 3 ; pH ; pW n`
Where `pH` is the height of a character cell in pixels, and `pW` is
the width of a character cell in pixels.

When `Ps` is 4, CTerm will respond with a Mode Report of the form
`CSI = 4 ; pF n`
Where `pF` is 1 if LCF mode is enabled, and 0 if it is disabled.

When `Ps` is 5, CTerm will respond with a Mode Report of the form
`CSI = 5 ; pF n`
Where pF is 1 if LCF mode is forced, and 0 if it is not.

When `Ps` is 6, CTerm will respond with a Mode Report of the form
`CSI = 6 ; pA n`
Where `pA` is 1 if OSC 8 hyperlink support is available, and 0 if
it is not.

=== `CSI ? Ps [ ; Pn ] n` Device Status Report (`DECDSR`)
When `Ps` is 62 (`DECMSR`) and there is no `Pn`, CTerm will respond
with a Mode Report of the form
`CSI 32767 * {`
This indicates that 524,272 bytes are available for macro storage.
This is not actually true, SyncTERM will use all available memory
for macro storage, but some software checks this value, and some
parsers don't allow more than INT16_MAX parameter values.

When `Ps` is 63 (DECCKSR) `Pn` defaults to 1, and CTerm will respond
with a checksum of the defined macros in the form
`DCS Pn ! xxxx ST`
Where xxxx is the hex checksum.

SOURCE: <<VT420>>

=== `CSI Ps $ p` Request Mode — ANSI (`DECRQM`)
Requests the current state of ANSI mode `Ps`.  The terminal responds
with `CSI Ps ; Pm $ y` (DECRPM).

The following ANSI modes can be queried:

[%autowidth,cols="1,2"]
|===
|1  |GATM — permanently reset (Pm=4)
|2  |KAM — permanently reset (Pm=4)
|3  |CRM — permanently reset (Pm=4)
|4  |IRM — permanently reset (Pm=4)
|5  |SRTM — permanently reset (Pm=4)
|6  |ERM — permanently reset (Pm=4)
|7  |VEM — permanently reset (Pm=4)
|8  |BDSM — permanently reset (Pm=4)
|9  |DCSM — permanently reset (Pm=4)
|10 |HEM — permanently reset (Pm=4)
|11 |PUM — permanently reset (Pm=4)
|12 |SRM — permanently reset (Pm=4)
|13 |FEAM — permanently reset (Pm=4)
|14 |FETM — changeable (Pm=1 when set, Pm=2 when reset)
|15 |MATM — permanently reset (Pm=4)
|16 |TTM — changeable (Pm=1 when set, Pm=2 when reset)
|17 |SATM — permanently reset (Pm=4)
|18 |TSM — permanently reset (Pm=4)
|21 |GRCM — permanently set (Pm=3)
|22 |ZDM — permanently set (Pm=3)
|===

All ECMA-48 standard modes are reported.  Modes 14 and 16 are
changeable; modes 21 and 22 are permanently set; all others are
permanently reset.
Any unrecognized mode returns Pm=0.

SOURCE: <<VT320>>, <<ECMA-48>>

=== `CSI = Ps $ p` Request Mode — CTerm Extension
NON-STANDARD EXTENSION +
Requests the current state of CTerm private mode `Ps`.  The terminal
responds with `CSI = Ps ; Pm $ y` using the same `Pm` values as
DECRQM above.

The following CTerm modes can be queried:

[%autowidth,cols="1,2"]
|===
|4  |Last Column Flag mode (CTELCF)
|5  |Forced Last Column Flag (CTFLCF) — reports 3 (permanently set) when forced
|255|DoorWay mode
|===

=== `CSI ? Ps $ p` Request Mode — Private (`DECRQM`)
Requests the current state of private mode `Ps`.  The terminal responds
with a mode report of the form `CSI ? Ps ; Pm $ y` (DECRPM) where `Pm`
indicates the mode state:

[%autowidth,cols="1,2"]
|===
|0
|Mode not recognized

|1
|Set

|2
|Reset

|3
|Permanently set

|4
|Permanently reset
|===

The following private modes can be queried:

[%autowidth,cols="1,2"]
|===
|6  |Origin mode (DECOM)
|7  |Auto-wrap mode (DECAWM)
|9  |X10 mouse reporting
|25 |Cursor visible (DECTCEM)
|31 |Alternate character set
|32 |No bright foreground
|33 |Bright background
|34 |Blink = alternate character set
|35 |No blink
|67 |Backarrow key mode (DECBKM)
|69 |Left/right margin mode (DECLRMM)
|80 |Sixel scrolling
|1000–1007, 1015 |Mouse tracking modes
|2004 |Bracketed paste mode
|===

Any unrecognized mode returns `Pm` = 0.

SOURCE: <<VT320>>

=== `CSI Ps SP q` Set Cursor Style (`DECSCUSR`)
Defaults: `Ps` = 1
Sets the cursor style

[%autowidth]
|===
|Ps|Result

|0
|Blinking block

|1
|Blinking block

|2
|Steady block

|3
|Blinking underline

|4
|Steady underline

|===

SOURCE: <<VT520>>

=== `CSI Pn1 ; Pn2 r` Set Top and Bottom Margins (`DECSTBM`)
Defaults: `Pn1` = 1  `Pn2` = last line on screen +
Selects top and bottom margins, defining the scrolling region. `Pn1` is
the line number of the first line in the scrolling region. `Pn2` is the
line number of the bottom line.

SOURCE: <<VT100>>

=== `CSI Pt ; Pl ; Pb ; Pr ; Ps ... Ps $ r` Change Attributes in Rectangular Area (`DECCARA`)
Defaults: `Pt` = 1  `Pl` = 1  `Pb` = last line  `Pr` = last column +
Sets SGR attributes in a rectangular screen area without changing
characters.  `Pt`, `Pl`, `Pb`, `Pr` define the rectangle.  The
remaining `Ps` parameters are SGR attribute values (same as `CSI m`),
including extended colour sequences (38;5;N, 38;2;R;G;B, 48;5;N,
48;2;R;G;B).

If DECSACE is set to 1 (stream mode), the operation applies as a
character stream from (`Pt`,`Pl`) to (`Pb`,`Pr`) wrapping at line
boundaries.  In stream mode, `Pl` > `Pr` is permitted when `Pt` <
`Pb`.

Coordinates are affected by DECOM (origin mode).  Does not change
cursor position.

SOURCE: <<VT420>>

=== `CSI Ps1 ; Ps2 * r` Select Communication Speed (`DECSCS`)
Set the output emulation speed.
If `Ps1` or `Ps2` are omitted, causes output speed emulation to stop
`Ps1` may be empty.
Sequence is ignored if `Ps1` is not empty, 0, or 1.
The value of `Ps2` sets the output speed emulation as follows:

[%autowidth]
|===
|Value |Speed

|empty, 0
|Unlimited

|1
|300

|2
|600

|3
|1200

|4
|2400

|5
|4800

|6
|9600

|7
|19200

|8
|38400

|9
|57600

|10
|76800

|11
|115200
|===

SOURCE: <<VT420>>

=== `CSI Pn1 ; Pn2 s` Set Left and Right Margins (`DECSLRM`)
(Only when DEC Left Right Margin Mode  - 69 - is enabled)

Defaults: `Pn1` = 1  `Pn2` = last column on screen +
If either `Pn1` or `Pn2` is zero, the current setting is retained.
Selects left and right margins, defining the scrolling region. `Pn1` is
the column number of the first column in the scrolling region. `Pn2` is
the column number of the right column.

SOURCE: <<XTerm>>

=== `CSI s` Save Current Position (`SCOSC`)
(Only when DEC Left Right Margin Mode  - 69 - is disabled)
NON-STANDARD EXTENSION
Saves the current cursor position for later restoring with `CSI u`
although this is non-standard, it's so widely used in the BBS world
that any terminal program MUST implement it.

SOURCE: <<ANSISYS>>

=== `CSI ? Ps... s` Save Mode Setting (`CTSMS`)
NON-STANDARD EXTENSION +
Saves the current mode states as specified by `CSI ? l` and `CSI ? h`.  If
`Ps1` is omitted, saves all such states.  If one or more values of `Ps` is
included, saves only the specified states (arguments to `CSI ? l`/`h`).

=== `CSI Ps ; Pn1 ; Pn2 ; Pn3 t` Select a 24-bit colour (`CT24BC`)
NON-STANDARD EXTENSION

If `Ps` is 0, sets the background colour.
If `Ps` is 1, sets the foreground colour.
`Pn1`, `Pn2`, `Pn3` contains the RGB value to set.
CTerm handles this with an internal temporary palette, so scrollback
may not have the correct colours.  The internal palette is large
enough for all cells in a 132x60 screen to have unique foreground
and background colours though, so the current screen should always
be as expected.

=== `CSI Pt ; Pl ; Pb ; Pr ; Ps ... Ps $ t` Reverse Attributes in Rectangular Area (`DECRARA`)
Defaults: `Pt` = 1  `Pl` = 1  `Pb` = last line  `Pr` = last column +
Toggles (reverses) SGR attributes in a rectangular screen area without
changing characters.  `Pt`, `Pl`, `Pb`, `Pr` define the rectangle.
The remaining `Ps` parameters specify which attributes to toggle:

[cols="1,3"]
|===
| Ps | Action

| 0 | Invert all toggleable attributes (bold, blink, negative)
| 1 or 22 | Toggle bold (XOR legacy attribute bit 3)
| 5 or 25 | Toggle blink (XOR legacy attribute bit 7)
| 7 or 27 | Toggle negative (swap foreground and background)
|===

All other `Ps` values are silently ignored.

If DECSACE is set to 1 (stream mode), the operation applies as a
character stream wrapping at line boundaries.

Coordinates are affected by DECOM (origin mode).  Does not change
cursor position.

SOURCE: <<VT420>>

=== `CSI u` Restore Cursor Position (`SCORC`)
Move the cursor to the last position saved by `CSI s`.  If no position
has been saved, the cursor is not moved.

SOURCE: <<ANSISYS>>

=== `CSI ? Ps...  u` Restore Mode Setting (`CTRMS`)
NON-STANDARD EXTENSION

Restores the mode states as saved via `CSI ? s`.  If `Ps` is omitted,
restores all such states.  If one or more values of `Ps` is included,
restores all the specified states (arguments to `CSI ? l`/`h`)

=== `CSI Pts ; Pls ; Pbs ; Prs ; Pps ; Ptd ; Pld ; Ppd $ v` Copy Rectangular Area (`DECCRA`)
Copies a rectangular area of characters from one location to another.
The copied text retains its character values, attributes, and hyperlinks.

`Pts`, `Pls`, `Pbs`, `Prs` define the source rectangle (top, left,
bottom, right).  `Pps` is the source page (ignored — single page).
`Ptd`, `Pld` define the destination top-left corner.  `Ppd` is the
destination page (ignored).

Defaults: `Pts` = 1, `Pls` = 1, `Pbs` = last line, `Prs` = last column,
`Ptd` = 1, `Pld` = 1.

If `Pbs` < `Pts` or `Prs` < `Pls`, DECCRA is ignored.
Coordinates are affected by origin mode (DECOM) but not by margins.
Values exceeding page dimensions are clamped.  The cursor does not move.
If the destination area extends past the screen, the off-screen portion
is clipped.

SOURCE: <<VT420>>

=== `CSI 2 $ w` Request Tab Stop Report (`DECTABSR`)
Requests a list of tab stops.
The list is in the form:
`DCS 2 $ u Pt ST`

The string `Pt` is a list of tab stops separated by `/`s.

SOURCE: <<VT320>>

=== `CSI Pch ; Pt ; Pl ; Pb ; Pr $ x` Fill Rectangular Area (`DECFRA`)
Fills a rectangular area with the character specified by `Pch`.
The fill character uses the visual attributes set by the most recent
SGR command.  Hyperlinks are cleared on filled cells.

`Pch` is the decimal value of the fill character.  Any value from
0x20 to 0x7E or 0x80 to 0xFF is accepted.  If `Pch` is in the C0
range (0x00–0x1F) or is DEL (0x7F), the command is ignored.

`Pt`, `Pl`, `Pb`, `Pr` define the rectangle (top, left, bottom, right).
Defaults: `Pt` = 1, `Pl` = 1, `Pb` = last line, `Pr` = last column.

If `Pb` < `Pt` or `Pr` < `Pl`, DECFRA is ignored.
Coordinates are affected by origin mode (DECOM) but not by margins.
Values exceeding page dimensions are clamped.  The cursor does not move.

SOURCE: <<VT420>>

=== `CSI Ps * x` Select Attribute Change Extent (`DECSACE`)
Controls whether DECCARA and DECRARA operate on a rectangular area or
a character stream.

[cols="1,3"]
|===
| Ps | Mode

| 0 | Rectangular (default)
| 1 | Stream (linear character sequence wrapping at line boundaries)
| 2 | Rectangular
|===

Reset to 0 by RIS.

SOURCE: <<VT420>>

=== `CSI Pn1 ; Ps ; Pn2 ; Pn3 ; Pn4 ; Pn5 * y` Request Checksum of Rectangular Area (`DECRQCRA`)
Returns a checksum for the specified rectangular area.
`Pn1` is an ID that is returned in the response.
`Ps` MUST be 1
`Pn2` specifies the top row of the rectangle
`Pn3` specifies the left column of the rectangle
`Pn4` specifies the bottom row of the rectangle
`Pn5` specifies the right column of the rectangle
The return value is in the format of `DCS Pn1 ! ~ xxxx ST`
Where xxxx is the hex value of the checksum.

Source: <<VT420>>

=== `CSI Pt ; Pl ; Pb ; Pr $ z` Erase Rectangular Area (`DECERA`)
Erases all character positions in the specified rectangular area.
Erased positions are set to SPACE with the visual attributes from the
most recent SGR command.  Hyperlinks are cleared on erased cells.

`Pt`, `Pl`, `Pb`, `Pr` define the rectangle (top, left, bottom, right).
Defaults: `Pt` = 1, `Pl` = 1, `Pb` = last line, `Pr` = last column.

If `Pb` < `Pt` or `Pr` < `Pl`, DECERA is ignored.
Coordinates are affected by origin mode (DECOM) but not by margins.
Values exceeding page dimensions are clamped.  The cursor does not move.

SOURCE: <<VT420>>

=== `CSI Pn * z` Invoke Macro (`DECINVM`)
Invokes a macro.
`Pn` specifies the macro number.  If `Pn` is not 0..63, no action is
taken.

SOURCE: <<VT420>>

=== `CSI = Ps1 ; Ps2 {` (`CTOSF`)
NON-STANDARD EXTENSION (Deprecated) +
Defaults:  `Ps1` = 255  `Ps2` = 0 +
Indicates that a font block is following.
`Ps1` indicates the font slot to place the loaded font into.  This must
be higher than the last default defined font (See `CSI sp D` for list
of predefined fonts)  `Ps2` indicates font size according to the
following table:

[%autowidth,cols="1,2"]
|===
|0
|8x16 font, 4096 bytes.

|1
|8x14 font, 3584 bytes.

|2
|8x8 font, 2048 bytes.
|===

The DCS font string should be used instead as of CTerm 1.213

=== `CSI Ps $ }` Select Active Status Display (`DECSASD`)
Defaults: `Ps` = 0 +
Selects which display subsequent output is directed to.  `Ps` = 0
sends output to the main display (default); `Ps` = 1 sends it to
the one-row status line.  `Ps` = 1 is silently ignored unless the
status display type is currently host-writable (DECSSDT `Ps` = 2).

When the status line is the active display, writes land in a
single-row sub-terminal with its own cursor, SGR attributes, and
saved-cursor slot.  Line-feed, wrap-to-next-line, and scroll do
not leave the status row.  Selecting `Ps` = 0 restores the main
display's cursor and resumes normal output there.

Switching back to `Ps` = 0 is also forced implicitly whenever
DECSSDT changes to a different type while the status display was
active.

SOURCE: <<VT320>>

=== `CSI Ps $ ~` Select Status Display Type (`DECSSDT`)
Defaults: `Ps` = 1 +
Selects the status display type.  `Ps` = 0 hides the status row and
extends the main display by one line.  `Ps` = 1 shows SyncTERM's
native indicator (clock, connection state, help hints) — this is
the default and matches the pre-existing user preference (overridable
from the BBS list or the `-n` CLI flag).  `Ps` = 2 gives the row to
the host: SyncTERM's indicator is cleared and the row becomes
writable via DECSASD.

The main display is resized in place across toggles; cursor
position and existing screen content are preserved.  Values of
`Ps` outside 0..2 are silently ignored.

SOURCE: <<VT320>>

=== `CSI Pn ' }` Insert Column (`DECIC`)
Defaults: `Pn` = 1 +
Inserts `Pn` blank columns into the scrolling region, starting at
the column that has the cursor.  Columns between the cursor and the
right margin shift to the right.  Columns shifted past the right
margin are lost.  The inserted columns are filled with SPACE using
the current SGR attributes.  Hyperlinks are cleared on inserted cells.

DECIC has no effect if the cursor is outside the scrolling margins.
The cursor does not move.

SOURCE: <<VT420>>

=== `CSI Pn ' ~` Delete Column (`DECDC`)
Defaults: `Pn` = 1 +
Deletes `Pn` columns from the scrolling region, starting at the column
that has the cursor.  The remaining columns between the cursor and the
right margin shift to the left.  Blank columns are added at the right
margin, filled with SPACE using the current SGR attributes.  Hyperlinks
are cleared on the new blank cells.

DECDC has no effect if the cursor is outside the scrolling margins.
The cursor does not move.

SOURCE: <<VT420>>

== Device Control Strings

A Device Control String Begins with a `DCS` and ends with a `ST`
The following commands are supported:

=== `DCS CTerm:Font:p1:<b64> ST` CTerm Loadable Font (`CTLF`)
Indicates the string is a loadable font. (CTerm 1.213)

`p1` is a font slot number, which must be higher than the last
default defined font (See `CSI sp D` for list of predefined
fonts). `<b64>` is the base64 encoded font data.  Font size is
deduced from the size of the data.  This replaces the now
deprecated `CSI = Ps1 ; Ps2 {`

=== `DCS [ p1 [ ; p2 ] ] q ST` Sixel Sequence
Defaults:  `p1` = 0  `p2` = 0
Indicates the string is a sixel sequence.

`p1` selects the vertical height of a single pixel.  This
may be overridden by the raster attributes command, and
is deprecated.  Supported values

.Supported Values of `p1`
[%autowidth]
|===
|Value |Vertical Size

|0,1,5,6
|2 pixels

|2
|5 pixels

|3,4
|3 pixels

|7,8,9
|1 pixel
|===

`p2` indicates if unset sixels should be set to the current
background colour.  If p2 is 1, positions specified as 0
remain at their current colour.

Any additional parameters are ignored.

The rest of the string is made up of sixel data characters and
sixel control functions.  Sixel data characters are in the
range of `?` (0x3f) to `~` (0x7e).  Each sixel data character
represents six vertical pixels.  The data is extracted by
subtracting 0x3f from the ASCII value of the character.
The least significant bit is the topmost pixel.

.Sixel Control Functions
[%autowidth]
|===
|Function |Parameters |Name |Description

|`!`
|`Pn X`
|Graphics Repeat Introducer
|The character X is repeated Pn times.

|`"`
|`p1 ; p2 [ ; p3 [ ; p4 ] ]`
|Raster Attributes
|p1 indicates the vertical size in pixels of each sixel. +
	p2 indicates the horizontal size in pixels. +
	p3 and p4 define the height and width (in sixels)
	respectively of a block to fill with the background
	colour.  This block may not extend past the current
	bottom of the screen.  If any pixel data characters
	proceed this command, it is ignored.

|`#`
|`p1`
|Colour Select
|Selects the current foreground colour from the	sixel palette.

|`#`
|`p1 ; p2 ; p3 ; p4 ; p5`
|Palette map
|Defines sixel palette entry p1 and sets it as the
	current foreground colour. +
	p2 specifies the colour	space to define the colour in,
	the only supported value is 2. +
	p3, p4, and p5 specify the red, green, and blue content as a
	percentage (0-100).

|`$`
|
|Graphics Carriage Return
|Returns the active position to the left border of
	the same sixel row.  Generally, one pass per colour
	is used.  In passes after the first one, sixels
	with a value of zero are not overwritten with the
	background colour.

|`-`
|
|Graphics New Line
|Moves the active position to the left border of the
	next sixel row.
|===

SOURCE: <<VT330340,[VT330/340]>>

=== `DCS $ q pt ST` Request Status String (`DECRQSS`)
`pt` is the intermediate and/or final characters of a control
function to query the status of.  The terminal will send a
response in the format

`DCS p1 $ r pt ST` 

`p1` is 1 if the terminal supports querying the control
function and 0 if it does not.

`pt` is the characters in the control function except the `CSI`
characters. If `p1` is zero, `pt` is zero-length.

.Currently supported values of pt:
[%autowidth]
|===
|`pt` |Description

|m
|Request SGR parameters

|r
|Request top and bottom margins

|s
|Request left and right margins

|t
|Request height in lines

|$\|
|Request width in columns
|*\|
|Request height in lines
|` q` (space + q)
|Request cursor style (`DECSCUSR`).  The response contains the
current cursor style number (0–4) as set by `CSI Ps SP q`.
|`*r`
|Request communication speed (`DECSCS`).  The response contains the
current speed parameters as `;Ps2*r` where `Ps2` is the speed index
(0=default, 1=300, 2=600, ..., 11=115200).
|`*x`
|Request attribute change extent (`DECSACE`).  The response contains
the current DECSACE value (0 or 1).
|`$~`
|Request status display type (`DECSSDT`).  The response contains the
current type (0 = none, 1 = indicator, 2 = host-writable).
|`$}`
|Request active status display (`DECSASD`).  The response contains
the current selection (0 = main, 1 = status).
|===

SOURCE: <<VT420>>

=== `DCS p1 [ ; p2 [ ; p3 ] ! z ST` Define Macro (`DECDMAC`)
Defaults: `p2` = 0  `p3` = 0

Sets a macro to be replayed using `CSI Pn * z`

`p1` is the macro number to set, and must be between 0 and
63 inclusive.

If `p2` is zero, the macro numbered `p1` will be deleted before the new
macro is set. If `p2` is one, all macros are deleted before the new
macro is set.  If the macro is zero length, only the delete action is
stored, you can't store a zero-length macro.

If `p3` is zero, the macro is defined using ASCII characters
(0x20 - 0x7e and 0xa0 - 0xff only).  Note that since ESC (0x1b)
is not in this range, ASCII-mode macros cannot contain escape
sequences; use hex-mode (`p3` = 1) instead.
If `p3` is one, the macro is defined using hex pairs.

When the macro is defined using hex pairs, a repeat
sequence may be included in the format of `! Pn ; D..D ;`
`Pn` specifies the number of repeats (default of one instance)+
`D..D` is the sequence of pairs to send Pn times.  The 
terminating ; may be left out if the sequence to be
repeated ends at the end of the string.

SOURCE: <<VT420>>

== Operating System Commands

An Operating System Command Begins with an `OSC` and ends with a `ST`
The following commands are supported:

=== `OSC 4;(pX;pY)... ST` Palette Redefinition/Query (`OSC 4`)
Specifies one or more palette redefinitions or queries. +
`pX` is the palette index, and `pY` is the colour definition +
Color format: `rgb:R/G/B`::
	Where `R`, `G`, and `B` are a sequence of one to four
	hex digits representing the value of the
	red, green, and blue channels respectively.

If `pY` is `?`, CTerm responds with the current colour for palette
index `pX` in the form `OSC 4;pX;rgb:RR/GG/BB ST` where
`RR`, `GG`, and `BB` are two hex digit colour values reflecting
the 8-bit-per-channel storage precision.

SOURCE: <<XTerm>>

=== `OSC 104 [ ; Ps ... ] ST` Reset Palette Entry (`OSC 104`)
Resets palette entry to default.  If the entire string
is "104" (ie: no `Ps` present), resets all colours.  Otherwise, only each index
separated by a semicolon is reset.
SOURCE: <<XTerm>>

=== `OSC 10 ; ? ST` Query Default Foreground Color (`OSC 10`)
Queries the default foreground color. +
CTerm responds with `OSC 10;rgb:RR/GG/BB ST` where `RR`, `GG`, and `BB`
are two hex digit color values.

SOURCE: <<XTerm>>

=== `OSC 11 ; ? ST` Query Default Background Color (`OSC 11`)
Queries the default background color. +
CTerm responds with `OSC 11;rgb:RR/GG/BB ST` where `RR`, `GG`, and `BB`
are two hex digit color values.

SOURCE: <<XTerm>>

=== `OSC 8 ; params ; uri ST` Hyperlink (`OSC 8`)
Sets a hyperlink on subsequent text output. Text printed after this
sequence will be associated with the given URI. An empty URI ends the
hyperlink region.

The `params` field is a colon-separated list of `key=value` pairs.
The `id=` parameter allows non-contiguous text runs to share the same
hyperlink.

Only `http`, `https`, `ftp`, and `ftps` URIs are supported. Other
schemes are silently ignored.

Users can open hyperlinks by clicking when BBS mouse capture is off,
or by Ctrl+clicking when mouse capture is active.

.Examples
----
ESC ] 8 ; ; https://example.com ST Click here ESC ] 8 ; ; ST
ESC ] 8 ; id=link1 ; https://example.com ST part1 ESC ] 8 ; ; ST ... ESC ] 8 ; id=link1 ; https://example.com ST part2 ESC ] 8 ; ; ST
----

SOURCE: https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda

== Application Program Commands

An Application Program Command begins with an `APC` and ends with a `ST`
SyncTERM implements the following APC commands:

=== `APC SyncTERM:VER ST` Get SyncTERM Version (`CTSV`)
SyncTERM responds with
an `APC` string of the form `APC SyncTERM:VER;version ST` where
`version` is the full version string of SyncTERM, either
`SyncTERM 1.7rc1` for release builds or `SyncTERM 1.7b Debug (Sep 27 2025)`
for debug builds

=== `APC SyncTERM:C;S Ps1 Ps2 ST` Store file (`CTSFI`)
Where `Ps1` is a filename and `Ps2` is the base64 encoded
contents of the file.  The named file is stored in the
cache directory for the current connection.

=== `APC SyncTERM:C;L [ ; Ps] ST` List Files (`CTLFI`)
Defaults: `Ps` = * +
`Ps` is the glob(3) pattern to use matching files. +
SyncTERM responds with
an `APC` string with lines separated by newlines.  The
first line is always `SyncTERM:C;L\n` and for each
matching file, a line in the form
`<Filename> TAB <MD5 sum> LF` is sent
(ie: "coolfont.fnt\t595f44fec1e92a71d3e9e77456ba80d1\n")

=== `APC SyncTERM:C;SetFont; Pn ; Ps ST` Set Font (`CTSF`)
Where `Pn` is a font slot number (max 255) and `Ps` is a
filename in the cache.  This sets font slot `Pn` to use
the specified font file.

=== `APC SyncTERM:C;DrawPPM Ps... Ps1 ST` Draw a PPM from Cache (`CTDPFC`)
Draws a PPM from the cache directory on the screen. +
`Ps1` is the filename and is required.  Arguments for
`Ps` are optional.  The following options can be included
(separated by semi-colons):

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description

|`SX=#`
|Sets the left X position in the specified image to copy from.  Default = 0.

|`SY=#`
|Sets the top Y position in the specified image to copy from.  Default = 0.

|`SW=#`
|Sets the width of the portion of the image to copy.  Default = Image width - `SX`

|`SH=#`
|Sets the height of the portion of the image to copy.  Default = Image height - `SH`

|`DX=#`
|Sets the X position on the screen to draw the image at.  Default = 0.

|`DY=#`
|Sets the Y position on the screen to draw the image at.  Default = 0.

|`MX=#`
|Sets the X position in the mask to start applying from.  Default = 0.

|`MY=#`
|Sets the Y position in the mask to start applying from.  Default = 0.

|`MW=#`
|Sets the overall width of the mask (not the width to apply).  If `MFILE`
	is not specified, and a mask is (ie: using `MASK=`), this is
	required.  If `MFILE` is specified, the width is read from the
	file.

|`MH=#`
|Sets the overall height of the mask (not the height to apply).  If
	MFILE is not specified, and a mask is (ie: using MASK=), this is
	required.  If MFILE is specified, the width is read from the
	file.

|`MFILE=<filename>`
|Specifies a filename in the cache directory of a PBM file specifying a
	mask of which pixels to copy.  Any pixel set to black (ie: 1) in
	the PBM will be drawn from the source image.  Pixels set to
	white (ie: 0) will be left untouched.

|`MASK=<maskbits>`
|Specifies a base64-encoded bitmap, each set bit will be drawn from the
	source image, cleared bits will not be drawn.  Requires MW= and
	MH= to be specified.

|`MBUF`
|Uses the loaded mask buffer.
|===

The PPM file may be raw (preferred) or text.  SyncTERM
does not support more than 255 values per colour channel
and assumes it is correctly using the BT.709 gamma
transfer.

=== `APC SyncTERM:C;DrawJXL Ps... Ps1 ST` Draw a JPEG XL from Cache (`CTDJFC`)
Draws a JPEG XL from the cache directory on the screen. +
`Ps1` is the filename and is required.  Arguments for
`Ps` are optional.  The following options can be included
(separated by semi-colons):

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description

|`SX=#`
|Sets the left X position in the specified image to copy from.  Default = 0.

|`SY=#`
|Sets the top Y position in the specified image to copy from.  Default = 0.

|`SW=#`
|Sets the width of the portion of the image to copy.  Default = Image width - `SX`

|`SH=#`
|Sets the height of the portion of the image to copy.  Default = Image height - `SH`

|`DX=#`
|Sets the X position on the screen to draw the image at.  Default = 0.

|`DY=#`
|Sets the Y position on the screen to draw the image at.  Default = 0.

|`MX=#`
|Sets the X position in the mask to start applying from.  Default = 0.

|`MY=#`
|Sets the Y position in the mask to start applying from.  Default = 0.

|`MW=#`
|Sets the overall width of the mask (not the width to apply).  If `MFILE`
	is not specified, and a mask is (ie: using `MASK=`), this is
	required.  If `MFILE` is specified, the width is read from the
	file.

|`MH=#`
|Sets the overall height of the mask (not the height to apply).  If
	MFILE is not specified, and a mask is (ie: using MASK=), this is
	required.  If MFILE is specified, the width is read from the
	file.

|`MFILE=<filename>`
|Specifies a filename in the cache directory of a PBM file specifying a
	mask of which pixels to copy.  Any pixel set to black (ie: 1) in
	the PBM will be drawn from the source image.  Pixels set to
	white (ie: 0) will be left untouched.

|`MASK=<maskbits>`
|Specifies a base64-encoded bitmap, each set bit will be drawn from the
	source image, cleared bits will not be drawn.  Requires MW= and
	MH= to be specified.

|`MBUF`
|Uses the loaded mask buffer.
|===

=== `APC SyncTERM:C;LoadPPM Ps... Ps0 ST` Load a PPM to Buffer (`CTLPTB`)
Loads a PPM to a buffer.  Ps0 is the filename +

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description
|`B=#`
|Selects the buffer (0 or 1 only) to paste from.
|===

=== `APC SyncTERM:C;LoadJXL Ps... Ps0 ST` Load a JPEG XL to Buffer (`CTLJTB`)
Loads a JPEG XL to a buffer.  Ps0 is the filename

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description
|`B=#`
|Selects the buffer (0 or 1 only) to paste from.
|===

=== `APC SyncTERM:C;LoadPBM Ps... Ps0 ST` Load a PBM to Buffer (`CTLPBTB`)
Loads a PBM to a buffer.  Ps0 is the filename

=== `APC SyncTERM:P;Copy Ps... ST` Copy Screen into Buffer (`CTCSIB`)
Copies a portion of the screen into an internal pixel buffer for use with the
Paste function.  Defaults to copying the entire screen.  All coordinates
and dimensions are in pixels.

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description
|`B=#`
|Selects the buffer (0 or 1 only) to copy to.

|`X=#`
|Sets the left X position on the screen to start copying at.  Default = 0.

|`Y=#`
|Sets the top Y position on the screen to start copying at.  Default = 0.

|`W=#`
|Sets the width to copy. Default = Screen width - X.

|`H=#`
|Sets the height to copy. Default = Screen height - X.
|===

=== `APC SyncTERM:P;Paste Ps... ST` Paste Buffer to Screen (`CTPBTS`)
Pastes from the copied pixel buffer.  Supports the same options as the Cache
DrawPPM command except for the filename, and adds the B= option.
All coordinates and dimensions are in pixels.

.Currently Supported Ps Arguments:
[%autowidth]
|===
|Argument |Description
|`SX=#`
|Sets the left X position in the specified image to copy from.  Default = 0.

|`SY=#`
|Sets the top Y position in the specified image to copy from.  Default = 0.

|`SW=#`
|Sets the width of the portion of the image to copy.  Default = Image width - `SX`

|`SH=#`
|Sets the height of the portion of the image to copy.  Default = Image height - `SH`

|`DX=#`
|Sets the X position on the screen to draw the image at.  Default = 0.

|`DY=#`
|Sets the Y position on the screen to draw the image at.  Default = 0.

|`MX=#`
|Sets the X position in the mask to start applying from.  Default = 0.

|`MY=#`
|Sets the Y position in the mask to start applying from.  Default = 0.

|`MW=#`
|Sets the overall width of the mask (not the width to apply).  If `MFILE`
	is not specified, and a mask is (ie: using `MASK=`), this is
	required.  If `MFILE` is specified, the width is read from the
	file.

|`MH=#`
|Sets the overall height of the mask (not the height to apply).  If
	MFILE is not specified, and a mask is (ie: using MASK=), this is
	required.  If MFILE is specified, the width is read from the
	file.

|`MFILE=<filename>`
|Specifies a filename in the cache directory of a PBM file specifying a
	mask of which pixels to copy.  Any pixel set to black (ie: 1) in
	the PBM will be drawn from the source image.  Pixels set to
	white (ie: 0) will be left untouched.

|`MASK=<maskbits>`
|Specifies a base64-encoded bitmap, each set bit will be drawn from the
	source image, cleared bits will not be drawn.  Requires MW= and
	MH= to be specified.

|`MBUF`
|Uses the loaded mask buffer.

|`B=#`
|Selects the buffer (0 or 1 only) to paste from.
|===

=== `APC SyncTERM:Q;JXL ST` Query JXL Support (`CTQJS`)
Queries support for the JXL image format. +
SyncTERM will respond with a CTerm APC State Report of the form

`CSI = 1 ; pR - n`

pR is 0 if JXL support is not available, and 1 if it is.

=== `APC SyncTERM:Q; Ps ST` Feature presence query (`CTQF`)
Generic feature-availability probe.  `Ps` is the feature name; currently
defined:

|===
|Feature |Meaning

|`libsndfile`
|Runtime-loaded libsndfile for audio file decoding (`APC SyncTERM:A;Load`).
Under the dynamic (`WITH_SNDFILE`) build mode, the first query also triggers
a lazy `xp_dlopen` attempt — no harm if the library is absent, just
reports unavailable.  The `STATIC_SNDFILE` build mode always reports
available; `WITHOUT_SNDFILE` always reports unavailable.
|===

The response is an audio DSR (see `CSI = 7 n` under "Audio APCs"), with
the feature ID (100 for libsndfile) and availability (0 / 1) as the
pair.  Example responses:

- libsndfile available: `CSI = 7 ; 100 ; 1 n`
- libsndfile unavailable: `CSI = 7 ; 100 ; 0 n`

== Audio APCs

SyncTERM exposes a BBS-facing audio playback API on top of its internal
mixer.  The model has two pieces:

*Patch slots* — 256 per-session buffers of S16 stereo 44100 Hz PCM,
addressed by slot number 0..255.  Slots are filled by `Load` (decoding
a file via libsndfile), `Synth` (in-terminal tone generator), or `Copy`
(duplicating an existing slot).

*Channels* — 16 mixer streams, addressed 0..15.  Channels 0 and 1 alias
cterm's internal music stream (ANSI/BananANSI/SyncTERM music) and
foreground-SFX stream (RIP / OOII) respectively; the BBS can use
`Flush`, `Volume`, `Wait`, and `Update` on them but cannot `Queue`
audio onto them.  Channels 2..15 are APC-dedicated and lazy-opened at
-12 dB base level on first use.

A `Queue` moves a slot's buffer into a channel's FIFO, after which the
slot is empty.  The mixer plays channels concurrently with sample-accurate
volume, fade-in/out envelopes, loop, and crossfade.

=== Parameter letters

Keys are single characters (`C`, `S`, `V`, etc.) in a `;`-delimited
`Key=Value` list.  Two keys (`X` and `L`) are presence flags — their
mere inclusion in the token list sets the flag; they take no value.

|===
|Letter |Meaning |Used by

|`S`   |Slot number (0..255) — patch source or dest  |Load, Synth, Queue, Copy
|`D`   |Destination slot (Copy only)                 |Copy
|`W`   |Waveform / shape                             |Synth
|`F`   |Frequency in Hz                              |Synth
|`T`   |Time / duration (ms default, see below)      |Synth
|`C`   |Channel number (0..15)                       |Queue, Flush, Volume, Wait, Update
|`I`   |Fade-in duration                             |Queue
|`O`   |Fade-out duration                            |Queue, Flush
|`X`   |Crossfade (presence flag, no value)          |Queue
|`L`   |Loop (presence flag, no value)               |Queue
|`V`   |Volume, both channels                        |Queue, Volume
|`VL`  |Volume, left channel                         |Queue, Volume
|`VR`  |Volume, right channel                        |Queue, Volume
|===

*Duration grammar* for `T`, `I`, `O`:

- No suffix → milliseconds (`T=500` = 500 ms).
- `ms` suffix → explicit milliseconds.
- `f` suffix → frames, sample-exact (`T=22050f` = half a second @ 44.1 kHz).
- `p` suffix → full periods of `F`, zero-crossing aligned.  Valid only
  in Synth's `T`; rejected for Queue/Flush fade durations (which apply
  to pre-rendered buffers with no single frequency).

Any parse error (unknown suffix, empty, non-numeric) yields a duration
of 0 — Synth with `T=0` just produces an empty slot; Queue with `I=0`
skips that envelope.

*Volume grammar* for `V`, `VL`, `VR`:

- No suffix → linear percentage 0..100 (100 = 0 dB unity, 50 ≈ -6 dB,
  0 = silence clamped to -60 dB).
- `dB` suffix (case-insensitive) → explicit dB value, parsed as a
  float with any sign (`V=-6.5dB`).

Volume parameters on `Queue` apply to that buf only (summed with the
stream's base dB during playback).  `Volume` verb updates the channel's
base level for subsequent playback.

Errors are silent.  The BBS can confirm an action succeeded by querying
`CSI = 7 ; <ch> n` afterward and inspecting the state.

=== `APC SyncTERM:A;Load;S=N;<filename> ST` Load patch from Store (`CTAL`)
Decodes `<filename>` (a file previously uploaded via `SyncTERM:C;S`)
into slot `N` using libsndfile.  Silently ignored if libsndfile is not
available — query `SyncTERM:Q;libsndfile` first to confirm.

=== `APC SyncTERM:A;Synth;S=N;W=<shape>;F=<hz>;T=<dur> ST` Synthesize patch (`CTAS`)
Generates a waveform into slot `N`.  Shapes: `SIN`, `SAW`, `SQ`,
`SINE_HARM`, `SINE_SAW`, `SINE_SAW_CHORD`, `SINE_SAW_HARM`, `SILENCE`.
`F=0` or `W=SILENCE` produces silence regardless of other parameters.

=== `APC SyncTERM:A;Copy;S=N;D=M ST` Copy patch (`CTAC`)
Deep-copies slot `N` into slot `M`.  Source slot is NOT emptied.

=== `APC SyncTERM:A;Queue;C=#;S=N;... ST` Queue patch on channel (`CTAQ`)
Moves slot `N`'s buffer onto channel `C`'s FIFO.  Slot becomes empty.
Rejected when `C=0` or `C=1` (those channels are cterm-owned).
Optional parameters: `I=` (fade-in), `O=` (fade-out), `X` (crossfade),
`L` (loop), `V=` / `VL=` / `VR=` (per-buf volume).

Crossfade (`X`): if the channel has a head buf already playing, its
decay overlaps with the new buf's fade-in over `fade_in_frames` frames
— both play concurrently during the overlap.  No-op if the channel
is empty (the new buf just fades in from silence).

Loop (`L`): the buf's mixer cursor wraps at end.  A subsequent Queue
(plain or crossfade) ends the loop.

=== `APC SyncTERM:A;Flush;C=#[;O=<dur>] ST` Flush channel (`CTAFL`)
Drains channel `C`'s queue.  If `O=` is provided, applies an overlay
fade-out of that length on the current head buf first; otherwise stops
immediately.

=== `APC SyncTERM:A;Volume;C=#;... ST` Set channel volume (`CTAV`)
Updates the channel's base dB.  `V=` sets both channels; `VL=` / `VR=`
override a specific side.  If only one side is given, the other is left
at its current value.

An optional `T=<duration>` parameter turns the change into a dB-linear
ramp that interpolates from the current volume to the target over
`<duration>` (same grammar as `Synth`'s `T` — milliseconds by default,
`f` suffix for frames, `ms` for explicit ms; `p` periods are rejected).
`T=0` or absent → instant change.

=== `APC SyncTERM:A;Wait;C=# ST` Wait for channel to idle (`CTAW`)
Blocks the terminal (stalling further BBS byte processing) until
channel `C`'s queue is empty.  No-op if the head is a looping buf.

=== `APC SyncTERM:A;Update;C=# ST` Arm idle notification (`CTAU`)
Arms a one-shot `CSI = 7 ; <ch> ; 0 n` emission the next time channel
`C` transitions from running to stopped.  BBS can poll channel state
(`SyncTERM:A;Queue` followed by a state query) without busy-waiting.

=== `CSI = 7 n` DSR — audio channel + feature state
Unified response sequence for audio channel state queries AND feature
presence queries.  The grammar is:

`CSI = 7 [ ; <id> ; <state> ]... n`

— zero or more `(id, state)` pairs packed into one sequence.
`id` is a channel number (0..15) or a feature ID (100+).

States: `0` = stopped, `1` = running.  For feature IDs: `0` =
unavailable, `1` = available.  Feature IDs defined:

- `100` — libsndfile

Query forms:

- `CSI = 7 n` — all running channels.  Emits one pair per running
  channel; non-running channels omitted.  A fully idle terminal
  replies with just `CSI = 7 n` (no pairs).
- `CSI = 7 ; <ch> n` — one specific channel.  Always emits exactly
  one pair.

Async notification:

- `CSI = 7 ; <ch> ; <state> n` — emitted when a channel armed via
  `Update` transitions running→stopped.  Each notification is a full
  DSR in its own right, emitted asynchronously from the idle-
  transition poll.

== "ANSI" Music
This is the place where the BBS world completely fell on it's face in ANSI
usage.  A programmer with either TeleMate or QModem (the first two programs to
support "ANSI" music as far as I can tell) decided they needed a method of
playing music on a BBS connection.  They decided to add an "unused" ANSI code
and go their merry way.  Since their product didn't implement `CSI M` (Delete
line) they assumed it was unused and blissfully broke the spec.  They defined
"ANSI" music as:
`CSI M <music string> 0x0e`

They used a subset of IBM BASICs PLAY statement functionality for ANSI music
strings which often start with "MF" or "MB", so the M after the CSI was often
considered as part of the music string.  You would see things such as:
`CSI MFABCD 0x0e` and the F would not be played as a note.  This just added
further confusion to the mess.

Later on, BananaCom realized the conflict between delete line and music, so
they added *another* broken code `CSI N` (Properly, erase in field... not
implemented in many BBS clients) which was to provide an "unbroken" method of
playing music strings.  They also used `CSI Y` to disambiguate delete line, `CSI Y`
is supposed to be a vertical tab (also not implemented in very many clients).
BananaCom also introduced many more non-standard and standard-breaking control
sequences which are not supported by CTerm.

CTerm has further introduced a standard compliant ANSI music introducer `CSI |`

By default, CTerm allows both `CSI N` and `CSI |` to introduce a music string.
Allowed introducers are set by `CSI = p1 M` as defined above.

The details of ANSI music then are as follows:
The following characters are allowed in music strings:
"aAbBcCdDeEfFgGlLmMnNoOpPsStT0123456789.-+#<> "
If any character not in this list is present, the music string is ignored as
is the introducing code.

If the introducing code is `CSI M` the first char is examined, and if it is
a one of "BbFfLlSs" or if it is "N" or "n" and is not followed by a decimal
digit, then the music string is treated as though an M is located in front
of the first character.

The music string is then parsed with the following sequences supported:

`Mx`::
sets misc. music parameters where x is one of the following:
+
[%autowidth, cols="1,2"]
|===
|`F`
|Plays music in the foreground, waiting for music to complete
playing before more characters are processed.

|`B`
|Play music in the background, allowing normal processing to continue.

|`N`
|"Normal" not legato, not staccato

|`L`
|Play notes legato

|`S`
|Play notes staccato
|===

`T###`::
Sets the tempo of the music where `+###+` is one or more decimal digits.
If the decimal number is greater than 255, it is forced to 255.
If it is less than 32, it is forced to 32.  The number signifies
quarter notes per minute.
The default tempo is 120.

`O###`::
Sets the octave of the music where `+###+` is one or more decimal digits.
If the decimal number is greater than 6, it is forced to 6.
The default octave is 4.

`N###`::
Plays a single note by number.  Valid values are 0 - 71.  Invalid
values are played as silence.  Note zero is C in octave 0.
See following section for valid note modifiers.

`A, B, C, D, E, F, G, or P`::
Plays the named note or pause from the current
octave.  An "Octave" is the rising sequence of the following notes:
C, C#, D, D#, E, F, F#, G, G#, A, A#, B
The special note `P` is a pause.
Notes may be followed by one or more modifier characters which
are applied in order.  If one overrides a previous one, the last
is used.  The valid modifiers are:
	`+` - Sharp:::
	The next highest semitone is played.
	Each sharp character will move up one semitone, so "C++"
	is equivalent to "D".

	`#` - Sharp:::
	The next highest semitone is played.
	Each sharp character will move up one semitone, so "C##"
	is equivalent to "D".

	`-` - Flat:::
	The next lowest semitone is played.
	Each flat character will move down one semitone, so "D--"
	is equivalent to "C".

	`.` - Duration is 1.5 times what it would otherwise be:::
	Dots are not cumulative, so `C..` is equivalent to `C.`

	`+###+` - Notelength as a reciprocal of the fraction of a whole note to play the note for:::
	For example, 4 would indicate a 1/4 note.
	The default note length is 4.

`L###`::
Set the notelength parameter for all following notes which do not have
one specified (ie: override the quarter-note default)  Legal note
lengths are 1-64 indicating the reciprocal of the fraction (ie: 4
indicates a 1/4 note).

`<`::
Move the next lowest octave.
Octave cannot go above six or below zero.

`>`::
Move to the next highest octave.
Octave cannot go above six or below zero.

The lowest playable character is C in octave zero.  The frequencies for the
six C notes for the seven octaves in rising order are:
65.406, 130.810, 261.620, 523.250, 1046.500, 2093.000, 4186.000

Purists will note that the lower three octaves are not exactly one half of
the next higher octave in frequency.  This is due to lost resolution of
low frequencies.  The notes *sound* correct to me.  If anyone can give me
an excellent reason to change them (and more correct integer values for all
notes) I am willing to do that assuming the notes still sound "right".

NMOTE: If you are playing some ANSI Music then ask the user if they
heard it, ALWAYS follow it with an 0x0f 0x0e is the shift lock character which
*will* cause people with anything but an ANSI-BBS terminal (ie: *nix users
using the bundled telnet app) to have their screen messed up.  0x0f "undoes"
the 0x0e.

== Sequences sent by SyncTERM
The following keys in SyncTERM result in the specified sequence being
sent to the remote.  This is not part of CTerm, but are documented here
for people who want to maintain compatibility.

[%autowidth,cols="1,2"]
|===
|Left Arrow
|"\033[D"

|Right Arrow
|"\033[C"

|Up Arrow
|"\033[A"

|Down Arrow
|"\033[B"

|Home
|"\033[H"

|End
|"\033[K"

|Select
|"\033[K" (Same as End due to termcap weirdness)

|Backspace
|"\b" when DECBKM is set (default), "\x7f" when reset

|Delete
|"\x7f" when DECBKM is set (default), "\033[3~" when reset

|Page Down
|"\033[U"

|Page Up
|"\033[V"

|F1
|"\033[11~"

|F2
|"\033[12~"

|F3
|"\033[13~"

|F4
|"\033[14~"

|F5
|"\033[15~"

|F6
|"\033[17~" (Note the jump from 15 to 17 here)

|F7
|"\033[18~"

|F8
|"\033[19~"

|F9
|"\033[20~"

|F10
|"\033[21~"

|F11
|"\033[23~" (Note the jump from 21 to 23 here)

|F12
|"\033[24~"

|Shift + F1
|"\033[11;2~"

|Shift + F2
|"\033[12;2~"

|Shift + F3
|"\033[13;2~"

|Shift + F4
|"\033[14;2~"

|Shift + F5
|"\033[15;2~"

|Shift + F6
|"\033[17;2~"

|Shift + F7
|"\033[18;2~"

|Shift + F8
|"\033[19;2~"

|Shift + F9
|"\033[20;2~"

|Shift + F10
|"\033[21;2~"

|Shift + F11
|"\033[23;2~"

|Shift + F12
|"\033[24;2~"

|Alt + F1
|"\033[11;3~"

|Alt + F2
|"\033[12;3~"

|Alt + F3
|"\033[13;3~"

|Alt + F4
|"\033[14;3~"

|Alt + F5
|"\033[15;3~"

|Alt + F6
|"\033[17;3~"

|Alt + F7
|"\033[18;3~"

|Alt + F8
|"\033[19;3~"

|Alt + F9
|"\033[20;3~"

|Alt + F10
|"\033[21;3~"

|Alt + F11
|"\033[23;3~"

|Alt + F12
|"\033[24;3~"

|Control + F1
|"\033[11;5~"

|Control + F2
|"\033[12;5~"

|Control + F3
|"\033[13;5~"

|Control + F4
|"\033[14;5~"

|Control + F5
|"\033[15;5~"

|Control + F6
|"\033[17;5~"

|Control + F7
|"\033[18;5~"

|Control + F8
|"\033[19;5~"

|Control + F9
|"\033[20;5~"

|Control + F10
|"\033[21;5~"

|Control + F11
|"\033[23;5~"

|Control + F12
|"\033[24;5~"

|Insert
|"\033[@"

|Back Tab
|"\033[Z"
|===

== Prestel and BBC Micro (Mode 7) Emulation

CTerm implements two related Videotex/Teletext display modes: Prestel
(the UK viewdata terminal standard) and BBC Micro Mode 7 (the BBC
Microcomputer's teletext display mode).  Both use the same 40×25
character grid with the same mosaic graphics and serial attribute
system, but differ in their C0 control character handling and cursor
movement behavior.

Both modes use the `PRESTEL_40X25` video mode: 40 columns × 25 rows,
12×20 character cells, with the Prestel 8-color palette (black, red,
green, yellow, blue, magenta, cyan, white).  Bright backgrounds and
no-blink are NOT enabled in this mode (unlike the Commodore and Atari
modes).

=== Serial Attributes

Unlike conventional terminals where attributes are invisible state
changes, Prestel/Mode 7 uses _serial attributes_.  Control codes in
the range 0x40–0x5F (accessed via ESC + character in Prestel mode, or
raw bytes 0x80–0x9F as C1 controls, or via ESC in BEEB mode) occupy a
character cell position on screen.  The control code cell is displayed
as either a space or a held mosaic character.

Serial attributes apply to all subsequent cells on the same row until
changed by another control code.  At the start of each new row, all
attributes reset to their defaults: white alphanumeric text, black
background, no flash, no conceal, no hold, contiguous mosaics, normal
height.

Control code effects are split into _before_ and _after_ phases:

- **Before** effects apply to the control code's own cell position
  (steady, normal height, conceal, contiguous/separated mosaics, black
  background, new background, hold mosaics)
- **After** effects apply to cells following the control code
  (alphanumeric colors, mosaic colors, flash, double height, release
  mosaics)

=== Serial Attribute Codes

These are the decoded control values (0x40–0x5F range after ESC, or
0x80–0x9F as raw C1 bytes minus 0x40):

.After-effect controls
[cols="1,1,3"]
|===
| Code | Name | Action

| 0x41 | Alpha Red | Set foreground to red, switch to alphanumeric mode
| 0x42 | Alpha Green | Set foreground to green, switch to alphanumeric mode
| 0x43 | Alpha Yellow | Set foreground to yellow, switch to alphanumeric mode
| 0x44 | Alpha Blue | Set foreground to blue, switch to alphanumeric mode
| 0x45 | Alpha Magenta | Set foreground to magenta, switch to alphanumeric mode
| 0x46 | Alpha Cyan | Set foreground to cyan, switch to alphanumeric mode
| 0x47 | Alpha White | Set foreground to white, switch to alphanumeric mode
| 0x48 | Flash | Enable flashing (blink attribute)
| 0x4D | Double Height | Enable double-height for this row (see below)
| 0x51 | Mosaic Red | Set foreground to red, switch to mosaic mode
| 0x52 | Mosaic Green | Set foreground to green, switch to mosaic mode
| 0x53 | Mosaic Yellow | Set foreground to yellow, switch to mosaic mode
| 0x54 | Mosaic Blue | Set foreground to blue, switch to mosaic mode
| 0x55 | Mosaic Magenta | Set foreground to magenta, switch to mosaic mode
| 0x56 | Mosaic Cyan | Set foreground to cyan, switch to mosaic mode
| 0x57 | Mosaic White | Set foreground to white, switch to mosaic mode
| 0x5F | Release Mosaics | Clear held mosaic, disable hold mode
|===

.Before-effect controls
[cols="1,1,3"]
|===
| Code | Name | Action

| 0x49 | Steady | Disable flashing (clear blink attribute)
| 0x4C | Normal Height | Disable double-height, clear hold and held mosaic
| 0x58 | Conceal | Enable concealed display (hidden until revealed)
| 0x59 | Contiguous Mosaics | Switch to contiguous (solid) mosaic rendering
| 0x5A | Separated Mosaics | Switch to separated (gapped) mosaic rendering
| 0x5C | Black Background | Set background to black
| 0x5D | New Background | Set background to current foreground color
| 0x5E | Hold Mosaics | Enable hold mode (see below)
|===

=== Alphanumeric vs Mosaic Mode

The alphanumeric color codes (0x41–0x47) switch to the G0 character
set where bytes 0x20–0x7F display as normal text characters.

The mosaic color codes (0x51–0x57) switch to the G1 character set.
In mosaic mode, characters in the ranges 0x20–0x3F and 0x60–0x7F are
displayed as 2×3 block mosaic graphics (with bit 7 set in the stored
character value).  Characters 0x40–0x5F remain alphanumeric even in
mosaic mode.

=== Hold Mosaics

When a mosaic color change occurs, the control code cell would
normally display as a space.  With _hold mosaics_ enabled (code 0x5E),
the control code cell instead displays the last mosaic character that
was output, preserving visual continuity across color changes.

Hold mode is cleared by: alphanumeric color codes, normal height,
double height, and release mosaics.

=== Double Height

Double-height mode (code 0x4D) causes characters on the current row
to be rendered at twice their normal height, spanning two physical
screen rows.  The top half is rendered on the row containing the
double-height code, and the bottom half on the row below.

Double-height tracking is complex:

- A row containing any double-height code becomes a "top" row
- The row immediately below a "top" row becomes a "bottom" row
- Bottom rows in Prestel terminal mode copy attributes and characters
  from the corresponding top row cell
- Bottom rows in BEEB mode display the bottom half of the same cell
- The last screen row cannot be a top row
- Double-height codes also clear hold mode and the held mosaic

=== Concealed Display

Code 0x58 enables concealed mode.  Concealed characters are stored
normally but rendered with the foreground hidden (displayed as
background color).  The user can toggle reveal mode (in SyncTERM, via
Alt-V) to show concealed content.

Concealment is cleared by any alphanumeric or mosaic color code.

=== Character Translation (BEEB only)

In BBC Micro mode, three characters are translated to match the
Mode 7 SAA5050 character set:

- `#` (0x23) becomes `_` (0x5F) — pound sign position
- `_` (0x5F) becomes `` ` `` (0x60)
- `` ` `` (0x60) becomes `#` (0x23)

=== C0 Control Characters — Prestel

[cols="1,1,3"]
|===
| Byte | Name | Action

| 0x00 | NUL | Time filling — no action (flushes print buffer)
| 0x05 | ENQ | Send memory slot 0 contents (Prestel identity)
| 0x08 | APB | Active Position Backward — move cursor left one position (wraps to end of previous row; top row wraps to bottom)
| 0x09 | APF | Active Position Forward — move cursor right one position (wraps to start of next row; bottom row wraps to top)
| 0x0A | APD | Active Position Down — move cursor down one row (wraps to top row)
| 0x0B | APU | Active Position Up — move cursor up one row (wraps to bottom row)
| 0x0C | CS | Clear Screen — home cursor and clear page memory, reset reveal mode
| 0x0D | APR | Active Position Return — move cursor to first position of current row
| 0x11 | CON | Cursor On — make cursor visible
| 0x14 | COF | Cursor Off — make cursor invisible
| 0x1B | ESC | Escape — prefix for serial attribute codes and programming sequences
| 0x1E | APH | Active Position Home — move cursor to first position of top row
|===

Per the specification, "for all cursor movements the first character of
each row is regarded as contiguous with the last character of the
previous row, and the top row is regarded as the row following the
bottom row."  This means all cursor movement wraps around the screen
in Prestel mode.

Bytes 0x20–0x7F are printable (with mosaic translation as described
above).  Bytes 0x80–0x9F are treated as raw C1 serial attribute
controls (value minus 0x40).  Other undefined C0 bytes are ignored
(section 2.6: "shall take no action on their receipt").

=== C0 Control Characters — BEEB (BBC Micro Mode 7)

[cols="1,1,3"]
|===
| Byte | Name | Action

| 0x00 | NUL | No-op (flushes print buffer)
| 0x07 | BEL | Audible bell
| 0x08 | APB | Active Position Backward — move cursor left (wraps to previous row; scrolls down at top)
| 0x09 | APF | Active Position Forward — move cursor right (wraps to next row; scrolls up at bottom)
| 0x0A | APD | Active Position Down — move cursor down one row (scrolls up at bottom)
| 0x0B | APU | Active Position Up — move cursor up one row (scrolls down at top)
| 0x0C | CS | Clear Screen — home cursor and clear screen, reset reveal mode
| 0x0D | APR | Active Position Return — move cursor to first position of current row (via ctputs CR handling)
| 0x17 | VDU 23 | Start 9-byte VDU 23 sequence (cursor control only)
| 0x1C | APS | Active Position Set — followed by 2 bytes: column (minus 0x20), row (minus 0x20), both 0-based
| 0x1E | APH | Active Position Home — move cursor to first position of top row
| 0x7F | DEL | Destructive backspace — move left, write space, move left
|===

NOTE: LF (0x0A) in BEEB mode is handled by the explicit APD case
before reaching ctputs, so it functions as cursor-down with scroll,
not as a ctputs line feed.

Unlike Prestel, BEEB mode scrolls the screen when cursor movement
reaches the edges rather than wrapping around.

=== Cursor Movement Differences

The key behavioral difference between Prestel and BEEB modes is cursor
wrapping at screen edges:

- **Prestel:** Cursor wraps around the screen. Moving up from the top
  row wraps to the bottom row.  Moving down from the bottom row wraps
  to the top row.
- **BEEB:** Cursor scrolls the screen. Moving up from the top row
  scrolls content down.  Moving down from the bottom row scrolls
  content up.

Left/right wrapping to adjacent rows is the same in both modes.

=== ESC Sequences — Prestel

In Prestel mode, ESC is followed by a character that is interpreted
as a serial attribute code (the character value is used directly as
the control code, range 0x40–0x5F for valid codes).

Additionally, Prestel supports a _programming_ protocol for memory
slot management:

- `ESC 1 ESC 2` — Begin memory query/program sequence
- `ENQ` (0x05) within programming — send current memory slot
- `ESC 3` — advance to next memory slot
- `ESC 4` — begin programming current memory slot (followed by 16
  data bytes using digits 0–9, `:`, `;`, `?`)
- 7 memory slots of 16 bytes each are available

=== ESC/VDU Sequences — BEEB

In BEEB mode, ESC does NOT trigger serial attribute codes (unlike
Prestel).  Serial attributes in BEEB mode are delivered as raw C1
bytes (0x80–0x9F) which are translated to attribute codes by
subtracting 0x40.

ESC in BEEB mode introduces VDU control sequences.  Two multi-byte
sequences are supported:

- **VDU 23,1,N;0;0;0;** (byte 23 + 9 data bytes): Cursor control.
  N=0 hides cursor, N=1 shows cursor.  All other bytes in the sequence
  must be zero.
- **APS** (byte 28 + 2 bytes): Direct cursor addressing.  First byte
  is column (minus 0x20), second is row (minus 0x20), both 0-based.

=== Bitmap Rendering

Prestel/Mode 7 rendering is handled specially in the bitmap driver
(`bitmap_con.c`) because it requires features not available in the
standard text rendering path:

- **Separated mosaics:** Mosaic characters are rendered with gaps
  between the 2×3 blocks, using the `CIOLIB_BG_SEPARATED` flag
- **Double-height rendering:** Characters are stretched vertically
  across two physical rows, with top/bottom half tracking per row
- **Concealed display:** The `CIOLIB_BG_PRESTEL` flag enables special
  concealment rendering where foreground pixels are suppressed unless
  `CONIO_OPT_PRESTEL_REVEAL` is set
- **Row state caching:** Double-height row states (top/bottom) are
  pre-computed to avoid O(rows²) scanning during rendering

The `CIOLIB_BG_PRESTEL_TERMINAL` flag distinguishes Prestel terminal
mode from BEEB mode for double-height bottom-row handling: Prestel
copies attributes from the row above, while BEEB uses the cell's own
attributes.

== PETSCII Emulation

CTerm's PETSCII mode emulates the Commodore 64 and Commodore 128
screen editors.  PETSCII (PET Standard Code of Information
Interchange) is the character encoding used by Commodore 8-bit
computers, with control codes for color, cursor movement, and screen
editing.

Three screen modes are supported:

[cols="2,1,1,2"]
|===
| Mode | Size | Colors | Default Attr

| C64 40×25 | 40×25 | 16 (C64 palette) | 0x6E (light blue on blue)
| C128 40×25 | 40×25 | 16 (C64 palette) | 0xBD (light green on dark green)
| C128 80×25 | 80×25 | 16 (CGA palette) | 0x07 (light grey on black)
|===

All three modes have bright backgrounds enabled and blinking disabled
(BGBRIGHT and NOBLINK video flags).

Each mode has two font variants: upper-case/graphics (the default) and
lower-case/upper-case (selected by control codes 14 and 142).

=== Control Codes

[cols="1,1,3"]
|===
| Byte | Name | Action

| 5 | White | Set foreground color to white
| 7 | Bell | Audible bell
| 13 | Return | Carriage return + line feed. Also disables reverse mode. Scrolls at bottom.
| 14 | Lower Case | Switch to lower-case/upper-case font
| 17 | Cursor Down | Move cursor down one row (scrolls at bottom)
| 18 | Reverse On | Enable reverse video mode
| 19 | Home | Move cursor to top-left corner
| 20 | Delete | Destructive backspace — move cursor left (wrapping to end of previous row), shift remaining characters left, blank inserted at right margin. At top-left, does nothing.
| 28 | Red | Set foreground color to red
| 29 | Cursor Right | Move cursor right (wraps to first column of next row, scrolls at bottom)
| 30 | Green | Set foreground color to green
| 31 | Blue | Set foreground color to blue
| 129 | Orange | Set foreground color to orange (C64/C128-40) or magenta (C128-80)
| 141 | Shift+Return | Line feed (move to first column of next row, scrolls at bottom). Does NOT disable reverse mode.
| 142 | Upper Case | Switch to upper-case/graphics font
| 144 | Black | Set foreground color to black
| 145 | Cursor Up | Move cursor up one row (clamps at top, no scroll)
| 146 | Reverse Off | Disable reverse video mode
| 147 | Clear Screen | Clear screen and home cursor
| 148 | Insert | Insert a blank space at cursor; characters to the right shift right. Character at right margin is lost.
| 149 | Brown | Set foreground color
| 150 | Light Red | Set foreground color
| 151 | Dark Grey | Set foreground color
| 152 | Grey | Set foreground color
| 153 | Light Green | Set foreground color
| 154 | Light Blue | Set foreground color
| 155 | Light Grey | Set foreground color
| 156 | Purple | Set foreground color
| 157 | Cursor Left | Move cursor left (wraps to last column of previous row if at left margin; clamps at top-left)
| 158 | Yellow | Set foreground color
| 159 | Cyan | Set foreground color
|===

Bytes 32–127 and 160–255 are printable characters (unless listed
above).  Bytes in ranges 0–31 and 128–159 that are not listed above
are ignored.

=== Known Differences from Hardware

The Commodore 128 KERNAL handles several control codes differently
from the C64.  CTerm currently uses C64 behavior for most of these.
The following codes differ between C64 and C128 hardware but are not
yet differentiated in CTerm:

[cols="1,2,2"]
|===
| Byte | C64 Hardware | C128 Hardware

| 0x02 | Ignored | UL ON (underline, C128 only)
| 0x08 | LOCK CASE | TAB SET/CLEAR (HTS)
| 0x09 | UNLOCK CASE | TAB (HT)
| 0x0A | Ignored | LINE FEED
| 0x0B | Ignored | UNLOCK CASE
| 0x0C | Ignored | LOCK CASE
| 0x0F | Ignored | FSH ON (flashing, 80-column only)
| 0x1B | Ignored | ESC
| 0x82 | Ignored | UL OFF (underline off, C128 only)
| 0x8F | Ignored | FSH OFF (flashing off, 80-column only)
|===

NOTE: The actual behavior desired may differ from raw hardware because
BBS terminal programs (CCGMS, DesTerm, NovaTerm, etc.) running on
these systems may have performed their own translation or stripping of
control codes before displaying them.

=== Color Mapping

The 16 color control codes map to different palette indices depending
on the screen mode.

.C64 and C128 40-column color mapping
[cols="1,2,1,1,2,1"]
|===
| Byte | Color Name | Index | Byte | Color Name | Index

| 144 | Black | 0 | 129 | Orange | 8
| 5 | White | 1 | 149 | Brown | 9
| 28 | Red | 2 | 150 | Light Red | 10
| 159 | Cyan | 3 | 151 | Dark Grey | 11
| 156 | Purple | 4 | 152 | Grey | 12
| 30 | Green | 5 | 153 | Light Green | 13
| 31 | Blue | 6 | 154 | Light Blue | 14
| 158 | Yellow | 7 | 155 | Light Grey | 15
|===

.C128 80-column color mapping
[cols="1,2,1,1,2,1"]
|===
| Byte | Color Name | Index | Byte | Color Name | Index

| 144 | Black | 0 | 152 | Grey | 8
| 31 | Blue | 1 | 154 | Light Blue | 9
| 30 | Green | 2 | 153 | Light Green | 10
| 151 | Dark Grey | 3 | 159 | Cyan | 11
| 28 | Red | 4 | 150 | Light Red | 12
| 129 | Orange | 5 | 156 | Purple | 13
| 149 | Brown | 6 | 158 | Yellow | 14
| 155 | Light Grey | 7 | 5 | White | 15
|===

NOTE: The C64 and C128 40-column modes use the C64 VIC-II palette.
The C128 80-column mode uses a standard CGA-compatible palette.  The
same control code byte may map to a different palette index depending
on the mode.

=== Reverse Video

Reverse video mode (byte 18 on, byte 146 off) swaps the foreground
and background nibbles of the attribute byte for all subsequent
characters.  Byte 13 (Return) also disables reverse mode.

=== Cursor Movement Details

- **Return (13):** Moves cursor to first column and down one row.
  Scrolls if at bottom.  Also disables reverse mode.
- **Shift+Return (141):** Same as Return but does NOT disable reverse.
- **Cursor Down (17):** Move down one row, scrolls at bottom.
- **Cursor Up (145):** Move up one row, clamps at top (no scroll).
- **Cursor Right (29):** Move right one column.  At right margin,
  wraps to first column of next row.  At bottom-right, scrolls.
- **Cursor Left (157):** Move left one column.  At left margin, wraps
  to last column of previous row.  At top-left, does nothing.

=== Font Switching

Two character sets are available per machine type:

[cols="1,2,2"]
|===
| Mode | Upper Case (142) | Lower Case (14)

| C64 | Font 32 (C64 UPPER) | Font 33 (C64 Lower)
| C128 | Font 34 (C128 UPPER) | Font 35 (C128 Lower)
|===

The upper-case set contains upper-case letters and PETSCII graphics
characters.  The lower-case set contains both upper and lower-case
letters.

=== Unimplemented Features

- Flashing on/off (bytes 2/0x82 and 15/0x8F) — C128 80-column only,
  not implemented

== ATASCII Emulation

CTerm's ATASCII mode emulates the Atari 8-bit computer's screen editor.
ATASCII (Atari ASCII) is the character encoding used by Atari 400/800
and XL/XE series computers, with control codes and cursor behavior
specific to the Atari hardware.

Two screen modes are supported:

[cols="2,1,1,2"]
|===
| Mode | Size | Colors | Notes

| Standard (Atari 40×24) | 40×24 | 2 | Dark blue background, light blue foreground
| XEP80 (Atari 80×25) | 80×25 | 2 | Greyscale (white and black)
|===

No escape sequences are used; all operations are performed via
single-byte control codes.

=== Character Encoding

In **normal mode**, printable bytes are stored directly as raw ATASCII
values in the screen buffer.  The font rendering maps these to the
appropriate Atari glyphs.

In **inverse (ESC) mode**, bytes are translated to Atari screen codes
before storage:

- Bytes 0–31 are mapped to screen codes 64–95
- Bytes 32–95 are mapped to screen codes 0–63
- Bytes 96–127 are not translated
- Bytes 128–159 are mapped to screen codes 192–223
- Bytes 160–223 are mapped to screen codes 128–191
- Bytes 224–255 are not translated

=== ESC Mode

Byte 27 (ESC) enables _inverse video mode_.  The next byte received is
translated to a screen code (see above) and displayed using the inverse
attribute (attr=1 instead of the normal attr=7).  After displaying the
inverse character, normal mode is automatically restored.  The ESC byte
itself is not displayed.

In ESC mode, byte 155 (Return) still functions as a control code.
All other bytes are translated to screen codes and displayed in
inverse.

=== Control Codes

[cols="1,1,3"]
|===
| Byte | Name | Action

| 27 | ESC | Enable inverse video for the next character
| 28 | Cursor Up | Move cursor up one row; wraps to bottom of same column
| 29 | Cursor Down | Move cursor down one row; wraps to top of same column
| 30 | Cursor Left | Move cursor left one column; wraps to right side of same row
| 31 | Cursor Right | Move cursor right one column; wraps to left side of same row
| 125 | Clear Screen | Clear entire screen and home cursor
| 126 | Backspace | Move cursor left one column and erase the character at the new position. Does NOT wrap; sticks at left margin.
| 127 | Tab | Advance to next tab stop. If no tab stop is found before end of line, wraps to first column of next row (scrolling if at bottom).
| 155 | Return (EOL) | Move cursor to first column of next row (CR+LF). Scrolls if at bottom.
| 156 | Delete Line | Delete the current line; lines below shift up. Cursor moves to first column.
| 157 | Insert Line | Insert a blank line at cursor row; lines below shift down. Current line is cleared.
| 158 | Clear Tab | Clear the tab stop at the current cursor column
| 159 | Set Tab | Set a tab stop at the current cursor column
| 253 | Bell | Audible bell
| 254 | Delete Char | Delete the character at the cursor; characters to the right shift left. A blank is inserted at the right margin.
| 255 | Insert Char | Insert a blank space at the cursor; characters to the right shift right. The character at the right margin is lost.
|===

All other byte values (including 0–26 except ESC, 32–124, 128–154,
160–252) are treated as printable characters and displayed using the
ATASCII-to-screen-code translation described above.

=== Cursor Wrapping Behavior

The four cursor movement keys wrap differently than most terminals:

- **Up (28):** Wraps from the top row to the bottom row of the _same
  column_. Does NOT scroll.
- **Down (29):** Wraps from the bottom row to the top row of the _same
  column_. Does NOT scroll.
- **Left (30):** Wraps from the left margin to the right margin of the
  _same row_. Does NOT change rows.
- **Right (31):** Wraps from the right margin to the left margin of the
  _same row_. Does NOT change rows.

This matches verified Atari 8-bit hardware behavior.

=== Tab Stops

Tab stops in ATASCII are set and cleared at individual column positions
using bytes 159 (Set Tab) and 158 (Clear Tab).  Default tab stops are
at the standard 8-column intervals.  The tab character (byte 127)
advances to the next set tab stop; if no stop exists before the end of
the line, it wraps to column 1 of the next row (scrolling if
necessary).

== Atari ST VT52 Emulation

CTerm's Atari ST VT52 mode emulates the VT52-compatible terminal built
into the Atari ST's TOS/GEM desktop.  This includes the standard VT52
command set plus GEMDOS/TOS extensions for color, line editing, and
additional cursor control.

Three screen modes are supported, matching the Atari ST's display
hardware:

[cols="2,1,1,2"]
|===
| Mode | Size | Colors | Notes

| Low resolution (ST 40×25) | 40×25 | 16 | Full 16-color palette
| Medium resolution (ST 80×25) | 80×25 | 4 | White, Red, Green, Black
| High resolution (ST 80×25 Mono) | 80×25 | 2 | White and Black
|===

Autowrap is disabled by default (unlike ANSI-BBS mode).  All three
modes have bright backgrounds enabled and blinking disabled (BGBRIGHT
and NOBLINK video flags).

The available colors depend on the screen mode — color commands
(`ESC b`, `ESC c`) index into the mode's palette, and indices beyond
the mode's color count wrap around (only the low bits are meaningful).

=== C0 Control Characters

[cols="1,1,3"]
|===
| Byte | Name | Action

| 0x00 | NUL | Ignored
| 0x01–0x06 | — | Ignored
| 0x07 | BEL | Audible bell
| 0x08 | BS | Backspace — move cursor left one column (no wrap, clamps at left margin)
| 0x09 | HT | Horizontal tab — advance to next tab stop
| 0x0A | LF | Line feed — move cursor down one row, scroll if at bottom
| 0x0B | VT | Vertical tab — same as LF (move down one row, scroll)
| 0x0C | FF | Form feed — same as LF (move down one row, scroll)
| 0x0D | CR | Carriage return — move cursor to left margin
| 0x0E–0x1A | — | Ignored
| 0x1B | ESC | Start escape sequence
| 0x1C–0x1F | — | Ignored
| 0x20–0x7E | — | Printable character, displayed at cursor position
|===

BS, HT, LF, and CR are processed by the shared `ctputs` output path.
VT and FF are handled explicitly before output and behave identically
to LF (move down one row with scroll).

=== Escape Sequences — Standard VT52

All sequences are two bytes: ESC followed by a single character, except
ESC Y which takes two additional parameter bytes.

[cols="1,1,3"]
|===
| Sequence | Name | Action

| `ESC A` | Cursor Up | Move cursor up one row (clamp at top, no scroll)
| `ESC B` | Cursor Down | Move cursor down one row (clamp at bottom, no scroll)
| `ESC C` | Cursor Right | Move cursor right one column (clamp at right margin)
| `ESC D` | Cursor Left | Move cursor left one column (clamp at left margin)
| `ESC H` | Cursor Home | Move cursor to top-left corner (row 1, column 1)
| `ESC I` | Reverse Line Feed | Move cursor up one row; if at top, scroll down
| `ESC J` | Erase to End of Page | Clear from cursor to end of line, then clear all lines below
| `ESC K` | Erase to End of Line | Clear from cursor to end of current line
| `ESC Y` _row col_ | Direct Cursor Address | Move cursor to position (_row_ − 32, _col_ − 32). Row and column are 0-based offsets encoded as the value + 32 (space = 0). Row is clamped to 0–23, column to 0–79. Parameters below 32 abort the sequence.
| `ESC ESC` | — | Ignored (ESC followed by ESC)
| `ESC =` | Alternate Keypad | Enable alternate keypad mode
| `ESC >` | Normal Keypad | Disable alternate keypad mode (normal numeric keypad)
| `ESC F` | — | Ignored (VT52 graphics mode — not applicable)
| `ESC G` | — | Ignored (VT52 graphics mode — not applicable)
| `ESC [` | — | Ignored (hold-screen mode — not implemented)
| `ESC \` | — | Ignored (hold-screen mode — not implemented)
|===

=== Escape Sequences — GEMDOS/TOS Extensions

These are extensions specific to the Atari ST's TOS operating system.

[cols="1,1,3"]
|===
| Sequence | Name | Action

| `ESC E` | Clear Screen | Clear entire screen and move cursor to home position
| `ESC L` | Insert Line | Insert a blank line at cursor row; lines below shift down. Cursor column is preserved.
| `ESC M` | Delete Line | Delete the line at cursor row; lines below shift up. A blank line is added at the bottom.
| `ESC b` _c_ | Set Foreground Color | Set foreground color to _c_ & 0x0F (4-bit palette index, 0–15)
| `ESC c` _c_ | Set Background Color | Set background color to _c_ & 0x0F (4-bit palette index, 0–15)
| `ESC d` | Erase to Start of Page | Clear from cursor to start of line (inclusive), then clear all lines above
| `ESC e` | Show Cursor | Make cursor visible (normal cursor)
| `ESC f` | Hide Cursor | Make cursor invisible
| `ESC j` | Save Cursor | Save current cursor position
| `ESC k` | Restore Cursor | Restore cursor to last saved position. No effect if no position was saved or if saved position is outside the current window.
| `ESC l` | Clear Line | Clear the entire current line (cursor position is preserved)
| `ESC o` | Erase to Start of Line | Clear from start of line to cursor position (inclusive)
| `ESC p` | Reverse Video On | Enable reverse video (swap foreground and background)
| `ESC q` | Reverse Video Off | Disable reverse video (restore normal foreground/background)
| `ESC v` | Enable Autowrap | Characters at the right margin wrap to the next line
| `ESC w` | Disable Autowrap | Characters at the right margin are clamped (no wrap)
|===

=== Color Palette

The color parameter for `ESC b` and `ESC c` is the low 4 bits of the
byte following the command character (value & 0x0F), giving an index
from 0 to 15 into the current mode's palette.

In **low resolution** (40×25), the full 16-color palette is available:

[cols="1,2,1,2"]
|===
| Index | Color | Index | Color

| 0 | White | 8 | Light Red
| 1 | Red | 9 | Light Green
| 2 | Green | 10 | Light Yellow
| 3 | Yellow | 11 | Light Blue
| 4 | Blue | 12 | Light Magenta
| 5 | Magenta | 13 | Light Cyan
| 6 | Cyan | 14 | Dark Grey
| 7 | Light Grey | 15 | Black
|===

In **medium resolution** (80×25), the default palette has 4 colors
repeated across all 16 slots: White (0), Red (1), Green (2), Black (3),
matching the Atari ST hardware where only 4 simultaneous colors were
available.

In **high resolution** (80×25 mono), the default palette alternates
White and Black across all 16 slots, matching the monochrome hardware.

NOTE: SyncTERM's per-entry custom palette feature allows users to
override the default palette with up to 16 unique colors in any mode.
This does not match hardware behavior where medium and high resolution
modes are physically limited to 4 and 2 colors respectively, but
provides flexibility for BBS content that uses the additional palette
slots.

NOTE: Reverse video (`ESC p` / `ESC q`) interacts with color commands.
When reverse video is active, `ESC b` sets the background color and
`ESC c` sets the foreground color (the sense is swapped).

=== Negative Image (Reverse Video)

The Atari ST VT52 mode uses the full 4-bit foreground and background
nibbles when swapping for reverse video, unlike ANSI-BBS mode which
only swaps the low 3 bits and preserves the bright/blink bits
independently.  In VT52 mode, `ESC p` swaps all 4 bits of the
foreground nibble with all 4 bits of the background nibble.

=== Differences from Standard VT52

* Autowrap is off by default (standard VT52 has no autowrap control)
* 16-color support via `ESC b` and `ESC c` (Atari ST extension)
* Insert/delete line (`ESC L` / `ESC M`) are TOS extensions
* Erase-to-start commands (`ESC d`, `ESC o`) are TOS extensions
* Cursor save/restore (`ESC j` / `ESC k`) are TOS extensions
* Show/hide cursor (`ESC e` / `ESC f`) are TOS extensions
* Autowrap control (`ESC v` / `ESC w`) are TOS extensions
* Reverse video (`ESC p` / `ESC q`) are TOS extensions
* Clear line (`ESC l`) is a TOS extension
* VT52 Identify (`ESC Z`) is not implemented
* VT52 graphics mode (`ESC F` / `ESC G`) is not implemented
* Hold-screen mode (`ESC [` / `ESC \`) is not implemented

== References

* [[STD-070]]https://vt100.net/manx/details/1,35074[[STD-070\]] Digital Equipment Corporation. Video Systems Reference Manual. 1989-04-14.
* [[ECMA-48]]https://www.ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf[[ECMA-48\]] ECMA. Control Functions for Coded Character Sets. June 1991
* [[XTerm]]https://invisible-island.net/xterm/ctlseqs/ctlseqs.pdf[[XTerm\]] Edward May. XTerm Control Sequences. University of California, Berkeley. 2024/09/19
* [[Paste64]]https://invisible-island.net/xterm/xterm-paste64.html[[Paste64\]] Thomas E. Dickey. XTerm -- bracketed paste. 2022
* [[BANSI]]http://www.bbsdocumentary.com/library/PROGRAMS/GRAPHICS/ANSI/bansi.txt[[BANSI\]] Paul Wheaton. BANSI.TXT. 1999
* [[VT102]]https://vt100.net/docs/vt102-ug/[[VT102\]] Digital. VT102 Video Terminal User Guide. 1982.
* [[VT330340]]https://vt100.net/docs/vt3xx-gp/[[VT330/340\]] Digital. VT330/VT340 Programmer Reference Manual, Volume 2: Graphics Programming. May 1988.
* [[VT320]]https://vt100.net/docs/vt320-uu/[[VT320\]] Digital. Installing and Using the VT320 Video Terminal. June 1987.
* [[colors256]]https://jonasjacek.github.io/colors/[[256colors\]] Jonas Jarad Jacek. 256 colors cheat sheet. 2023-12-24.
* [[VT420]]https://vt100.net/docs/vt420-uu/[[VT420\]] Digital. Installing and Using the VT420 Video Terminal. June 1990.
* [[ANSISYS]]https://en.wikipedia.org/wiki/ANSI.SYS[[ANSISYS\]] Wikipedia. ANSI.SYS.
* [[ECMA-35]]https://ecma-international.org/wp-content/uploads/ECMA-35_6th_edition_december_1994.pdf[[ECMA-35\]] ECMA. Character Code Structure and Extension Techniques. December 1994
* [[VT520]]https://vt100.net/dec/ek-vt520-rm.pdf[[VT520\]] Digital. VT520/VT525 Video Terminal Programmer Information. July 1994.
* [[VT100]]https://vt100.net/docs/vt100-ug/contents.html[[VT100\]] Digital. User Guide VT100. 1981.
