11 Apr
2007
11 Apr
'07
2:01 p.m.
On 10 Apr 2007 at 22:57, Tony Duell wrote:
Correct. I was getting most of my info from the PC/AT Techref, I assume
the chips are much the same.
Have you tried just a new chip in place of a
piggybacked module? I wonder
if the new chips are actually 128K bits -- that is, they're really the 2
old chips of the pigyback on one die, in one package. If you piggybacked
a new chip onto half an old pair, you'd end up with 2 chips 'in parallel'
for half of the memory space, that might not matter (if you never read an
un-wirtten-to location, there will be no contention becasue the same
value will have been written to both chips)
This reminds me of the early revs of the PC AT, which used (IIRC)
stacked 64K DRAMs (the bottom used a different select than the top one, so that they could be just soldered pin-to-pin)
to get a 128K
module.
IIRC, (and I'll check the schematics if you need me to), it's the RAS/
pin that's seprate for the 2 chips in the piggyback.
Later PC/AT mainboards took 41256 RAMs, but for some unknown reason IBM
only arragned the board to take 18 of them (512K RAM), not 1M (or even
640K in a mix of 41256s and 4164s). That seemd crazy to me (the XT/286
board, of course, was a640K board, 2 off 256K SIMMs and 128K DRAM on the
board itself (I forget what they used, it may well have been 4 off 50464s
and 2 off 4164s or something)).
-tony