Well,
Looking at the REXCPM board, it is possible to go from 3 signals on the
jumper to 4.
This would allow for A15 to be brought in, and to identify the special case
where /BANK3 is zero but the address bus is 8000-9FFF.
At first blush I can't think of a good way to solve this other than by
adding
AH, I see your point.
I stand corrected, A15 is not on the socket.
[image: image.png]
So, maybe your suggestion works; is A15 needed at all?
Yes, I think it is needed.
How would you tell the difference between these two addresses, from the
perspective of REXCPM?
vs 8000.
Both have /BANK3
Couple of comments.
Agree with your rule, but REXCPM is even more fancy.
It supports 32k bank switching in to 7FFF and 2 banks of 16k switched
at 8000 to BFFF and C000 to . So REXCPM has to differentiate a bit
based on which bank is indicated.
All this bank switching to support RAM dis
Yes, that helps quite a bit. Thank you.
One more question: If you are already handling situations 1 & 3 — bank3 is
low and bank 6 is high for both, so A15 must be checked — why not reduce
the scenarios to a single rule:
- If either bank3 or bank6 is low, address is 0 to , rexcpm responds.
when bank3 is low, address to 7fff, bank6 is high, rexcpm responds.
When bank6 is low, address a000 to , bank3 is high, rexcpm responds
When bank3 is low, address 8000 to 9fff, bank6 is high, rexcpm responds.
All of these scenarios are unique. Cpm gets 64k when option rom and optikn
ra
I'm a little confused.
It sounds like you're selecting on /BANK3 = 0 (option ROM selected) AND
/BANK6 = 0 (expansion RAM selected). But, if I'm reading figure 4-6 from
the service manual correctly, /BANK3=0 AND /BANK6=0 will always be false
because no two banks are ever selected at the same time.
I've been thinking about it too.
/BANK6 is perfect for selecting that ram bank for address A000 to . No
issue.
I believe that , when option rom is selected, it is actually selected for
all addresses from to 9FFF.
Now about A15. It is actually present in the socket because the AD bus
If you can get A15 without a blue wire, it does not seem that you would
need the new select signal based on /BANK6. Requests for 8000 to 9FFF
appear to already go to Bank 3, so you'd just need to detect addresses in
that range in the OPTION ROM. It looks like the service manual schematic
for bank s
(Sorry for the random seeming reply. I had thought I had sent that earlier.)
On Wed, May 25, 2022 at 4:49 PM B 9 wrote:
> Let me see if I understand correctly. The main problem is that CPM
> requires the entire 64K of address space to be RAM. For the Model 100,
> REXCPM somehow is able to replac
Let me see if I understand correctly. The main problem is that CPM requires
the entire 64K of address space to be RAM. For the Model 100, REXCPM
somehow is able to replace both the builtin ROM (0-7FFF) and RAM
(8000-) with its own System RAM. That method will not work for the
Model 200 in which
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