Re: Software for MS-DOS 1.25

Hello!

Mia Magnusson wrote:

>> The other alternative would be to write an IBM PC compatible bios, so 
>> you could boot original msdos/ibm/dr versions.
> That would be a far better alternative if someone really wants to put
> an effort to this.

Theoretically it could be done (though the 8088 card has only 4 kB of
ROM - not much can be put there), but...

> Also if the CBM 8088 card can have ram at $B0000 or $B8000 then it
> might be possible to write 8088/6509 code that just copies this to the
> screen ram, thus emulating CGA or MDA text mode. That way I guess that
> most text mode software would work right out of the box.

This is the show stopper. You cannot put any RAM on the card, as it
would (1) require modifying the card hardware extensively and even worse
(2) the RAM would have to be accessible by the 6509 also, so it cannot
really reside on the card.

The only way to have memory at B0000 or B8000 would be to extend the
main RAM to 1 MB. Which can be done, but requires several modifications
to the mainboard. And even then, you could not have 6509 code copying
from there to the screen memory, because 8088 and 6509 cannot have
access to RAM at the same time. So the 6509 is barred from using RAM
unless the 8088 explicitly instructs it to do something, which normally
happens only after some I/O INT 21h calls.

So the short answer is NO.

> I'm not familiar with the 8088 card for CBM II / PET 700 but if there
> is any chance to add an ISA slot then a real PC display card would make
> good PC compatibility far easier to achieve.

It might be theoretically possible to hack the card to attach an ISA
slot to it, because it's just an extension of the 8088 CPU bus. However,
the board tries to access the mainboard RAM everywhere below F0000, so
trying to access video memory on the ISA card would cause bus collision
and crash. By looking at the board schematic, I think that maybe the
board could be hacked to lower this threshold (for example to 80000) by
playing with the SEG_F signal, but I have no idea whether it would be
sufficient, and even if it were, I don't know if the bus signal timing
would be compatible with what ISA cards expect.

So the short answer is a MAYBE strongly leaning towards NO.

Regards,
Michau.


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