[SPARCbook] What is the Max Ram for 3GX....???

[Bob Krzaczek]

Thus spake Thomas Roehr:
; Just pick up a couple of those sim adapters that let you plug 2 or 4
; sims into one slot. They have jumpers to configure the memory
; layout. No battery, but you can ahve 256 MB's of ram. They are only
; about $20 each.

Hmm.  Interesting thought, and applicable to the 128MB option (256MB
being ruled out by Ian Spray's previous post); I initially ruled the
idea out on the grounds that they'd only let you configure your
"virtual SIMM" in ways that mimic'd a valid SIMM[1].  But now it
occurs to me... do you mean that they actually let you map some of the
addressing lines to places they don't usually go? I'm such a cynic, I
never thought to ask. ;-)

If they're so cheap and obviate my previous post, I expect *someone*
really ought to have tried this by now.  

// boba


Footnotes: 
[1]  I probably didn't do such a good job explaining why Tadpole 64MB
SIMMs aren't standard.  Let me try again:  SIMMs in this range come in
just three sizes 4, 16, and 64MB.  No, really, that's all.  Now 8, 32,
and 128MB commodity SIMMs are really two smaller SIMMs sharing the
same signals with an extra select line to differentiate between
them. We know the DRAM controller in the uSparc-II cannot handle a
SIMM larger than 32MB (which is handled as two normally-addressed 16MB
SIMMs with an extra select signal).  So, I believe a 64MB SIMM for a
3GX has a second select signal, on top of the one that would normally
be used in your average 32MB SIMM.  End result: there's an extra
signal, I'm guessing an extra RAS[2] in addition to the CAS lines
already provided, doing this job.  If you really want your head to
spin, think of this: a 128MB 3GX has two 64MB SIMMs plugged into it,
each of which masquerades in some proprietary way as two 32MB SIMMs,
each of which masquerades in a standard way as two 16MB SIMMs.  Fun.

[2]  Purely a guess[3] on my part, but: the DRAM controller can see a
maximum of 8 32MB SIMMs, and there are 8 RAS select signals coming off
the controller. Hmm.

[3]  A shady guess which, fortunately, is replaced by *fact* and
*knowledge* as soon as someone pins out the SIMM connectors on a 3GX.
It's that easy; pin them out, map them back to the RAM controller, and
you've got the answer.


-- 
// Bob Krzaczek, RIT Center for Imaging Science               <krz at cis.rit.edu>