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

[Bob Krzaczek]

Thus spake Ken Hansen:
; You can not *stuff* 256 Meg of RAM in a SPARCbook 3GX
; without *severe* hardware hacking, of they type that
; would *probably* render your laptop no longer
; portable.

Severe?  Sure.  Unportable?  "Nahhhhhhhh" :)

Actually, just before I moved onto a different laptop, I was
considering exactly this project on my 3GX.  It really shouldn't be
*too* hard (of course, I say that with the confidence of someone who
no longer plans on doing the work).

In the event that there's a hw hacker or two on this list, let me
encourage you!

Consider: the microSparc-II cpu in the 3GX has a nice, friendly,
simple DRAM controller integrated right into it.  It should be fairly
straightforward, using Sun's own documentation on the CPU (available
as a PDF from their web site), to trace back the signals from the SIMM
socket and determine which memory control lines (the various RAS, CAS,
and high address lines) are where on the SIMMs.

At that point alone, one could guess what was different about the
Tadpole 64M SIMMs.  Sure, we know they appear as two 32MB SIMMs to the
hardware; but, also bear in mind that a 32MB SIMM is really two 16 MB
SIMMs. So, expect a little weirdness here; at a guess, I'm betting on
seeing some weirdness with an extra RAS line from the CPU stealing an
unused line on the SIMM socket.  Either way, it shouldn't be too hard
to figure out exactly how the 3GX SIMM sockets were wired wrt. to the
DRAM controller.  From that, I one shold be able to infer how the
special 64M SIMMs were set up.

At that point, things can go one of two ways.

If you're lucky.  Super lucky.  More lucky than you have any right to
expect, you might discover that some existing 64MB SIMMs could be
modified to match Tadpole's design.  This would almost certainly
require one of the "chip heavy" 64MB SIMMs; that is, a SIMM with many less
dense chips, rather than a few dense chips.  Sadly, most 64MB SIMMs
are exactly this latter catgeory.

So, the odds are a little against you there.

The alternative, of course, is that you could add SIMMs inside your
3GX.  There's clearance and space inside that could accomodate two
SIMMs, providing you're good with a soldering iron around delicate
circuitry (let me stress that you'll be wiring to signals that come
right off the CPU, so be @#$%^@ careful).  If you're industrious, you
might try your hand at low profile right handle SIMM sockets;
otherwise, you'll need to put some good thought into a mounting
strategy for the extra SIMMs.

Now, before anyone wets themselves: THIS IS ALL THEORETICAL.  Don't
try this if you have any doubts of your own abilities, or are scared
of toasting your laptop.  There are lots of single mistakes you could
make when working on hardware like this that could utterly trash your
laptop.  So, DON'T DO IT unless you're prepared to take responsibility
for your actions.  I have nothing to do with this or what you do.

(ahem) Sorry.  You see a lot of that on the 'net.  Anyway.

Amusing aside: I *loved* my 3GX, and often wondered why someone else
hadn't done something like this before, once we realized how scarce
the 128MB kits for the 3GX are.  Eventually, my impatience at swapping
drove me to plan this project out, and I was ready for some
exploratory surgery (I'm familiar with the inside of a 3GX, having
opened it up in the past to fix a deteriorating keyboard (it turned
out to be a slipping connector) and other things over recent years).
Then, after I had read up on the CPU and learned something of common
72pin SIMM design, I got a new laptop from work.

And that's when I realized why no one else had done this before.

So, IN THEORY, this is all pretty straightforward (sheesh, it's just
memory).  Chances are good that a couple people with the right skills
could hook up and pull this off.  At the least, you could have a 3GX
with four 32MB SIMMs; if you're insanely lucky, maybe more.  So, if
you're good, and have the inclination and the time, think about it.
It really can't be *that* hard.  And you'll be a hero.

(Oh, great. Now I'm all nostaglic for "bat", my old 3GX... I think I
might dust it off and boot it up this afternoon... ;)

Cheers,

// bob

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