Date   : Wed, 06 May 1987 23:49:00 -0800
From   : Ken Wallewein <kenw%noah.arc.cdn%ubc.csnet@RELAY.CS.NET>
Subject: A problem with my parallel output port (HELP, please)

  Patching the stuff in is (should be) trivial. It's a matter of reassembling 
your BIOS with the appropriate device handler stuff included. Since you appear
to know 8080 assembler and how to handle a parallel port, the rest should be
easy. I ASSUME you have the BIOS source. Without it you're stuck.
  At the very beginnning of BIOS are a bunch of CALL instructions. These are
the standard BIOS entry points for all the things the BIOS does. I don't have
my books here, so I can't tell you exactly which one it is. Your listing 
should have it commented/labeled as LSTOUT or some such. The character to
be printed will be expected in a register, probably C. Your LSTOUT routine
should wait until the printer's not busy, send the character, and return.
I don't think return status is important here.
  As I said, you really need your BIOS listing. Read the existing code to
see what it does, following the path from the entry point at the top. It also
helps to have access to a CP/M Internals book. One of the best (I can't 
remember the title) was written by Donald Cortesi of Dr. Dobb's Journal.
All you really need are the BIOS entry point and register-handling conventions.
You MIGHT want to worry about the USTAT byte for device redirection (via STAT),
but I never bother with that. Good luck.
/kenw