How compatible were "MS-DOS Compatible" machines?

[Jim Leonard]

Fred Cisin wrote:
>> That was also an issue with the AT&T PC 6300/Olivetti M24 if you were
>> using a monochrome monitor.  It was monochrome CGA, but the monitor
>> would do things like _underline_ the text based on the attribute.
> 
> That was a normal feature of MDA

Of MDA, yes, but this was "monochrome CGA" which was supposed to display 
16 different shades of green, ie. behave like CGA and not MDA.  That's 
why I listed it as an oddity.

> INT 10h, function 0Fh returns in AL with the numeric value of what video
> mode is in use.  2 = B&W CGA     3 = color CGA     7 = MDA
> IFF sufficiently compatible (Toshiba T300 was NOT), then 2,3 mean B800h
> and 7 means B000h

Right, but this assumes IBM PC compatibility.  I was asking if there is 
a separate call for "where is the base of screen memory" (it appears 
there is not).

> In the early days, it was possible to call BASIC subroutines.  But, I
> don't think that they kept the same addresses for long.

Wow -- while that's cool, what was a useful application for that? 
Borrowing the use of the floating-point routines, perhaps?  ROM has a 
faster access time than RAM, but would it have been all that much faster 
than doing it yourself, or would it have just been merely convenient?

> Working with the "NON-PC-compatible" MS-DOS machines, such as DEC Rainbow,
> TI Proffessional, Toshiba T300, Sanyo 550?, HP 150?, Tandy 2000,
> Victor 9000, etc. would require major additional work for EACH one that
> you support.
> Unless you are willing to fall back to ONLY MS-DOS calls (MOV AH,2
> MOV DL, 7  INT 21h  to beep), etc. stick with "clones".

After reading your reply (and others) I think that's what I'm going to 
do.  Full source (and on request, compiler) will be distributed so it's 
not like people can't fix it for their platform.

Since the keyboard is the only thing different amongst the various 
clones I want to support, I've decided against my own keyboard handler, 
and will be doing this:

- BIOS Int 16h for keyboard
- DOS calls for file access, of course
- Direct writing to screen (with optional snow handling)
- Using the timer to hook to the speaker port to general square-wave 
tones without tieing up the CPU

Thanks for your advice.
-- 
Jim Leonard (trixter at oldskool.org)            http://www.oldskool.org/
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