Re: ROM 3 and the Octoram

[dempson@actrix.gen.nz (David Empson)]

In article <2APR199400202742@jetson.uh.edu>,
Fretz, David <st3b1@jetson.uh.edu> wrote:
> I know there was some discussion of the ROM 3 a week ago, but a user
> at my user group seems to thimk that the ROM 3 will work with an
> Octoram with 7 Megs  instead of the usual 8.  Can anyone confirm
> this?

I haven't ever had more than 4 MB in my OctoRAM, and I've only used it
in my ROM 01 (I also have a ROM 03).

I did a partial circuit analysis of the OctoRAM a few years ago.  If
you have more than four SIMMs installed, it uses one of the bits in
the bank address (on the data bus) to work out which group of four
SIMMs should be selected.

With 1MB SIMMs, it uses D6 (address bit 22, $400000).

With four or less SIMMs, it works fine in both machines.

In a ROM 01 and ROM 03 with eight SIMMs, I think the banks in each SIMM
are as follows:

         ROM 01        ROM 03
SIMM 0   02-11         10-1F
SIMM 1   12-21         20-2F
SIMM 2   22-31         30-3F
SIMM 3   32-3F         not addressed
SIMM 4   42-51         50-5F
SIMM 5   52-61         60-6F
SIMM 6   62-71         70-7F
SIMM 7   72-7F,40-41   40-4F

This is only a theory - I haven't had a close enough look to confirm
this, and I haven't tried it myself.  If I'm right, then you should be
able to leave out SIMM 3 in a ROM 03, and still have 8 MB in total.
You cannot do this in a ROM 01, or you'll end up with a "hole" in the
memory.

Note that the OctoRAM is not DMA-compatible if you have more than 4
SIMMs installed, unless you have a RamFAST SCSI card, or set RAM5 to
fill all memory beyond $400000.

Also note that the ROM 03 has a bug in its RAM5 firmware which
prevents the RAM disk from being created if you have a total of 8 MB
of RAM.  System 6.0.1's RAM5 driver gets around this.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand