Cromemco 256KZ Bank Addressing ?
dave04a at dunfield.com
Wed Apr 6 13:46:43 CDT 2005
I've got a Cromemco System-3 which I've been wrestling with
on and off for quite a few months now ... I've got it to the
point where it will boot and run CDOS, however Cromix hangs
I've "boiled it down" to just the ZPU, 16FDC, and the RAM
cards which are a pair of 256KZs, with the 948 and 949
addressing ROMs in them (sw.1 is ON for the 948 card and
OFF for the 949 card as described in the documentation).
I want to run a comprehensive memory test... I written
such a test and can "load" it by having a PC "type" the
HEX code into RDOS's SM command - then run it with 'G' -
that all works fine, and I have verified that the default
32k that comes up with RDOS appears to be OK.
Now I want to test all of the possible banks in the 256KZs
and so far I can not find any documentation as to how the
bank select works. From the 16KZ docs I see that the normal
bank select register is 40h, and this does do "something"
to the 256KZs ... I've determined that by writing individual
bits 1, 2, 3 or 4 I can swap in four distinct 32k blocks into
the upper half, while nibbles with other than one bit set all
seem to map to the same block which is different from the
other 4 ... This makes 5x32k = only 160k.
I've also determined that writing individual bits 4, 5, 6 or 7
causes the system to die, which I'm guessing is because it is
swapping the lower 32k (where my program is running)... It it
works the same as the lower-nibble/upper-block, then that would
give a total of 10x32k = 320k that I can access from the 512k
physical memory on the cards ... ?
Can anyone provide me with details of exactly how the bank
select works with dual 256KZ cards installed, and how to map
in the entire 512k (obviously a section at a time)?
Unfortunately I don't have the 947 address ROM which would
have shipped with single cards, so I can't test the cards by
thenselves... Anybody got a spare?
Also, I note that RDOS will not come up if the ZPU is set for
2MHZ ... It appears to only run at 4MHZ - is this normal?
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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