Category : Miscellaneous Language Source Code
Archive   : HERCULES.ZIP
Filename : HERCBIOS.DOC

***** HERCBIOS *****
BIOS PATCH FOR THE HERCULES BOARD

Dave Tutelman 1986



The accompanying program is a front end to the INT 10 (VIDEO)
functions of the DOS BIOS, so that the important functions work
on a Hercules graphics board or its clones. (It was developed on
a SuperComputer clone of a Hercules) It is a
terminate-and-stay-resident program, that is installed by being
called; it is NOT a DOS driver. If you want it installed at boot,
include it in AUTOEXEC.BAT, not CONFIG.SYS.

WHAT IT'S GOOD FOR

The major strength of this program is that it will allow you to
write programs for the Hercules board that run in graphics mode,
and still write text easily on the graphics screen. With it, you
can program in those higher-level language processors that use
the DOS or BIOS calls for display, using their standard I/O calls
to write text to the graphics screen. (For a list of known
compatible and incompatible languages, see the section later in
this manual.)

A second use of this program is to allow the running of existing
graphics programs that use the BIOS calls for ALL screen display.
It will NOT allow most commercial graphics programs written for
the the Color Graphics Adapter (CGA) to run on the Hercules
board. That is because most graphics programs write directly to
the video memory instead of using the BIOS calls. The only
existing graphics program that this has been tested against is
PC-LISP; that is the only graphics program I've encountered that
uses the BIOS exclusively.


HOW IT WORKS

HERCBIOS is a terminate-and-stay-resident program that intercepts
all calls to Interrupt 10H, the BIOS video services. It will
either process the interrupt or pass the call on to the real
BIOS, depending on whether something specific to the Hercules
board needs to be done. Specifically, HERCBIOS handles the
interrupt itself if (1) the board is in graphics mode, or (2) the
BIOS call is a request to enter graphics mode.

Two graphics modes are recognized and processed by HERCBIOS:

Mode 6 - IBM Hi-res mode: This uses part of the 720x348
Hercules raster as a 640x200 IBM-compatible graphics screen.
It will work with programs for the IBM CGA and its clones,
provided they use the BIOS services for their graphics
display. (Note - such programs are rare.)

Mode 8 - Hercules-specific mode: This uses the full

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Hercules raster.

Actually, both modes are quite capable of putting a pixel
anywhere on the Hercules raster. The major difference is that
Mode 6 draws characters in an 8x8 pixel box (like the CGA), while
Mode 8 uses the finer resolution of the Hercules board to improve
legibility by using a 12x8 pixel box for characters. In either
mode, more characters than 25x80 will fit on the screen. Mode 6
supports 43x90 characters on the screen (but 25x80 inside the
640x200-pixel sub-screen); Mode 8 supports 29x90 characters.

The functions implemented by HERCBIOS are:

Fn 0 - Set mode (6, 7, or 8)
Fn 2 - Set cursor position
Fn 3 - Read cursor position
Fn 5 - New display page
Fn 9 - Write character with attribute
Fn 10 - Write character
Fn 12 - Write pixel
Fn 13 - Read pixel
Fn 14 - Teletypewriter-style character write
Fn 15 - Get video status

Check your System Programmers' Guide for the use of these BIOS
functions.


A number of properties of the alphanumeric display are not
supported by the hardware when you enter graphics mode. For
instance, the cursor is not shown in graphics mode, nor are all
of the character attributes. HERCBIOS does its best to emulate
the alphanumeric mode, but it cannot implement a cursor or the
blinking or bold attributes. The table below shows the "best
shot" that HERCBIOS takes at character attributes:

CODE USUALLY MEANS IBM MODE HERC MODE
00 invisible invisible invisible
01 underline [normal] underline
07 normal normal normal
0F hi-intens [rev video] [rev video]
70 rev video rev video rev video


Anything else displays as normal

The teletypewriter-style output protects the bottom line on the
screen as an unscrolled line, for status messages, function key
labels, etc. This is non-standard, but I like it. (And we do have
more rows than the CGA display. It's the 43rd line that isn't
scrolled.)







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MAKING AND INSTALLING THE PROGRAM
Making the .COM File from Assembler Source

HERCBIOS was originally developed on ASM 1.0. The version
included with this uses MASM 4.0. I don't know for sure whether
it will assemble with other versions of assembler.

The commands for making HERCBIOS.COM from the source are in the
MAKEFILE included with the distribution. I run it with NDMAKE, an
excellent MS-DOS shareware MAKE from Don Knellers, but it should
be easy to adapt to your own favorite MAKE. If you make it by
hand, the commands are:

masm hercbios;
masm gchar;
masm graph;
link hercbios gchar graph, hercbios.exe;
exe2bin hercbios.exe hercbios.com
del hercbios.exe

If you have a machine whose processor is an iAPX 286, 186, or
188, you may be able to get some increased performance and a
smaller .COM file by editing one character in the header file
hercbios.h. Simply remove the ";" that comments out the
definition of iAPX286. That will allow some non-8088 instructions
to assemble, as well as changing some code that was optimized for
speed (at the expense of storage and beauty) on the 8088. (This
option is known to assemble and link, but has not been tested; I
have no access to a 286 machine with Hercules graphics.)


Installing HERCBIOS.COM

Once you have HERCBIOS.COM, store it where your PATH will find it
and add the line

HERCBIOS

to your AUTOEXEC.BAT file somewhere after the PATH command. This
will cause it to be installed during boot, so it will be there
whenever you run your graphics programs. (Its presence won't
affect operation of your computer in alphanumeric mode, since it
passes on to the normal BIOS anything that's not in graphics
mode.)

I am including a couple of demonstration/test programs in this
distribution, so that you can:
- See how to write programs for HERCBIOS.
- Test to assure that it runs with your computer and monitor.
The programs can be run in their executable form and their source
can be examined.






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COMPATIBILITY AND INCOMPATIBILITY

HERCBIOS has been tested on a Hercules board in an IBM PC-XT, a
Hercules-compatible board I built from a SuperComputer bare
board, and a Leading Edge XT clone. The current version works
with all of these, but I have a homebrew monitor that has trouble
syncing to the higher sweep rate of the monochrome display. If
you have trouble with the stability of your image, try fiddling
with the parameters for the 6845 display chip. They are in the
file HERCBIOS.ASM, in the "db" statement defining vid_parm_table
at the end of Function 0 (Set Video Mode). I have left in (but
commented out) the set of parameters that works on my homebrew
monitor.

I have written programs using HERCBIOS in a number of languages.
Here are some of the caveats I'd like to pass on:

- Things are fine using INT 10h calls in assembler. (No big
surprise.)

- Turbo Pascal works with HERCBIOS, with one caveat (at least
for releases 1 and 2). The Pascal cursor function GoTOXY will
home the cursor if presented with x>80 or y>25. To make full
use of the 29x90 or 43x90 screen, you will have to write your
own version of GoTOXY, using Turbo's machine language escape
to issue the INT 10h.

- I've written a little in Microsoft C 3.0. No problems so far.

- The TESTPIX program was written in deSmet C 2.4. It worked
fine, with one caveat. The console I/O routine getchar()
seems to write to display memory (perhaps as part of keyboard
echo). This can interfere with what is displayed on the
Hercules board display page 1. (I had no problems on page
0.)

- Forget about using it with BASICA or GWBASIC. Microsoft BASIC
graphics routines write directly to display memory,
completely bypassing the BIOS.


USE AND ENJOY!

Bug reports to:
Dave Tutelman
16 Tilton Drive
Wayside, NJ 07712


Currently receive EMail at ...!mtuxo!mtuxt!dmt


Flames to:
/dev/null



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