This is an interrupt-driven device driver for the serial port(s) on an
IBM PC. I tried to follow the guidelines in the DOS tech ref manual as best
I could. It seems to work fine in my application (a PC with COM1 going to
a Cray, COM2 going to a text-to-speech/telephone-management box; dial it up
from a touch-tone phone, "beep in" your number and password, it logs into the
Cray and reads you your mail), but certainly hasn't been exhaustively tested.
    Its biggest problem is something I suspect is a DOS bug: The byte count
returned from the 3F DOS function always seems to be short by 1, meaning you
can't distinguish between a "nothing there" return and a "one byte returned"
return. I claim to be able to point to the goof in disassembled DOS that's
doing this, but the oh-so-helpful-before-we-bought-our-80-PCs people at IBM
don't return my calls. Anyway, to get around this problem, I've had to precede
each "get-a-character" call with an "input status" call to see if there's
anything there.
    I am posting this in hopes that people who know more than I about 8088
assemblers, device drivers, etc. will point out bugs or possible improvements.

  -  Peter. (pearson@lll-mfe.arpa)