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

[Thomas Roehr]

Some of them are listed with jumpers allow mapping address lines. I have not done
it becuase I am usually doing something else, and because I only have two of the
32MB simms.  You can get them from MemoryX for about $150.00. This isn't any
different then the hack I did to my first PC to get 768Megs into a 256 Meg spot.

Tom

to Bob Krzaczek wrote:

> 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>
>
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