Re: Software for MS-DOS 1.25

Den Sun, 08 Oct 2017 14:49:50 +0200 skrev Michał Pleban
<lists@michau.name>:
> > 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.

Well, a memory upgrade would be a nice add-on for a CBM-II anyway,
wouldn't it?

The problem seems to be that the 8088 card interacts with the main
board ram directly. Any memory expansion then needs to be dram using
the on board and 8088 card dram access circuits, otherwise it can't
share memory.

> > 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.

If you add memory to the CBM-II then there wouldn't be any bus
conflicts as long as you once and for all have written the same data to
all "duplicate" memory areas.

But the SEG_F signal seems rather easy to change. The existing circuit
should be kept for the on board rom but another circuit detecting
anything above $3FFFF should generate SEG_F to U25 which propagates to
BUSEN2.

With some adapter it would probably be rather simple to replace the
existing bios with a 64k byte eprom or even eeprom/flash memory if that
would be usable.


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