Re: Software for MS-DOS 1.25

There is a nice article (I have part II so I assume part one is the month
before this) on the subject of porting DOS to non IBM systems in the
Micro/Systems Journal March/April 1986 "Implementing PC-DOS on Non-IBM
Compatible Computers" by Christopher Cochran & Kristofer Sweger.

I read separately that Commodore internally used the program Move-it to
port CP/M 86 to the B and presumably MS DOS files to the B that way as
well.  They used a 1:1 interleave

On Sun, Oct 8, 2017 at 8:49 AM, Michał Pleban <lists@michau.name> wrote:

> 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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>


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