Re: Software for MS-DOS 1.25

Den Fri, 20 Oct 2017 12:46:46 +0200 skrev Michał Pleban
<lists@michau.name>:
> Mia Magnusson wrote:
> 
> > No, three banks of 256k would give that. Two banks of 64k and two
> > banks of 256k would give exactly 640k. If the two banks of 64k
> > isn't socketed it might be worth the effort to look into the
> > possibility to use a total of 5 banks instead of 4 banks, i.e.
> > 64+64+256+256+256k, maybe with one bank not visible from the CBM-II
> > side.
> 
> It depends on how the memory is added.
> 
> If it is just added to the 8088 card (easier), then two banks of 256k
> are sufficient, because combined with 256k on the mainboard they give
> 768k (640k for DOS plus 128k which can be used for video buffer).
> 
> If it is added to the mainboard in place of one 64k bank, then indeed
> 2 x 256k will be too small and 3 banks must be used.

Yes, that was what I were thinking about.

> > I came to think of that I'm not sure if REP CMPSW is feasable.
> > Would it overwrite all flags from a previous pass?
> 
> Nothing a PUSHF/POPF pair cannot solve, I presume?

Well, AFAIK you can't REP more than one instruction. The question is
will the 8088 throw away the result of one CMP when the next one runs
in the REP loop? Or is it possible to break out of the REP loop if CMP
sets the condition codes to "the data differs"?

> > If we anyway need to add some hardware for memory expansion, it
> > could contain some detection for writes to graphic memory.
> 
> This could be done easily, with some latch that is triggered when
> segment B000 is accessed and cleared when the 8088 code finished
> updating the screen.

Yes.

> > I don't know which way is the best to go. Maybe do something with
> > the segment F signal mapping out most of the CBM-II access from 8088
> > memory, and remap the CBM-II memory from C0000 upwards. That way the
> > CBM-II memory could be switched out when the 8088 is running normal
> > DOS programs, and switch it in when it runs special communication
> > code in it's BIOS. Maybe a REP MOVW (or whatever it's called in
> > 8088 syntax) could just copy the PC text mode screen to CBM-II
> > memory and then the memory could be switched over to the 6509 and
> > then let the 6509 process it?
> 
> I am inclined to let the 8088 do all the processing because it is much
> faster, then just hand out the data to the 6509 to copy them from DRAM
> to video RAM.

Yes, the only reason for letting the 6509 do the job is if the 6509
could do the conversion while otherwise idle.

> > Maybe this would reach the point where it would be better to make a
> > completely new 8088 board as it otherwise would need both some
> > board to sit between the 8088 board and the CBM-II board (to remap
> > memory), and also some board to sit on top of the 8088 board adding
> > more memory and the appropriate adress decoding and change of
> > "segment F" signal.
> 
> It indeed starts looking like a new improved board would be better.
> But let's start with what we have :-)

:)

Maybe the improved board could be an add-on to another Commodore 8-bit
machine. For example the 8296 has faster memory than it actually needs
as it in one 1MHz half cycle does two fetches for the video hardware.
That would mean that with a reprogrammed PLA the CPU could use the
memory at 2MHz :)


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