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Random memory trivia for PS/2s not specific to a singular system.
Memory in A1-B1 or J1-J3 fail in 90 / 95 systems
ECC and FPM Support
Loading SIMM Pairs
Support for Greater Than 16MB
Support for Greater Than 64MB
IBM Presence Detect SIMMs
8x36 E Memory? Whazzat?
Memory in A1-B1 or J1-J3 fail in 90 / 95 systems
One may encounter failing memory in A1/B1 or J1/J3
(possibly in other systems). PS/2s test the first SIMM or
pair of SIMMs very rigorously, because if the low memory
(first 1MB) fails, the system stops. If that first MB is
undependable, you may get data corruption as a result of
altered drivers or messed up O/S code.
If you are loading up your 90 or 95, and get a message
that one or both SIMMs in the first pair are bad, SIMMply
swap in another (matching) SIMM or pair from a later pair
(eg. A2/B2) and run advanced diags on them. You must run
advanced diags to clear the error from the previous SIMM
configuration.
Note: The memory
controller in the 90 and 95 can handle pairs of SIMMs with
mixed speeds but same sizes, but accesses to that pair
will be at the speed of the slowest SIMM.
ECC
and FPM Support
All PS/2 systems support Fast Page Mode (FPM)
memory. Older systems may use 30 pin SIMMs, later systems
use 72 pin SIMMs.
Some of the later systems contain memory
controllers that support FPM -OR- ECC. But either memory
type is exclusive of the other
In the case of the Model 90 or 95, the first complex to
support ECC is the Model M 486DX-50 (only double deck
complex). It supports EITHER
64MB of FPM -OR- 64MB of ECC. Do not mix FPM and ECC in
any combination, either in pairs (A1/B1 FPM and A2/B2 ECC)
or by banks (A1-A4 FPM and B1-B4 ECC). Mixing pairs will
result at best with the first matched pair being enabled,
the next (opposing) pair deallocated, or in the case of
banks, nothing because the system memory controller can't
get the first pair to pass the BIST.
The later Type 4 complexi all support either 64MB of
parity -OR- 256MB of ECC.
Loading
SIMM Pairs
In systems with an even number of SIMM sockets (and
no soldered-on planar memory like a 9556), load SIMMs in
matched pairs (size and speed). Load the largest pair
first, then smaller and slower pairs in the following SIMM
sockets.
For systems with soldered planar memory with an odd number
of SIMM sockets, you are facing either no support for
interleaved memory, or only a single pair of SIMMs in
interleaved memory support. Each system accesses memory
according to it's design.
The first system to support interleaved memory is the
Model 70-486. (IIRC)
Support for Greater Than 16MB
All PS/2 systems with a 286 or 386 have a maximum
of 16MB addressable memory, due to the 24 bit memory
address. This also includes 386 variants like the 486SLC.
For 24 bit address systems (either with an odd number of
SIMMs (three) or soldered-on memory and SIMM sockets), you
get 16MB of total memory if you exceed 16MB of SIMM.
As an example, the 9533 comes with 4MB soldered on-planar,
and two SIMM sockets. Now if you load two 8MB SIMMs, you
will get 16MB of total RAM. But wait, 4+8+8=20, right?
Nope, everything over 16MB cannot be addressed.
Support for
Greater Than 64MB
In the beginning, memory addressing was limited to
24 bits. With the advent of the 486, 32 bit memory
addressing was possible. Unfortunately, not everybody
updated their programs or driver support to incorporate
support for 32 bit memory addressing. Part of this problem
is the code to determine memory size.
IBM did update their code to support 32 bit addressing,
and provided drivers that kept memory addressing below
16MB. But again, proper support for memory size varied.
In the case of Windows 9x, the limit of 64MB was finally
overcome with HIMEMUPD.EXE
this is simple to apply and allows you to see and use as
much as your system can recognize.
IBM Presence
Detect SIMMs
Most IBM PS/2s that require 72 pin SIMMs with
Presence Detect, which is four lines on a SIMM that are
either open or shorted (pins 71-68). IBM SIMMs with PD are
usually 36 bits wide (32bits data plus 4bits parity) or 39
bits wide (32bits data plus 7bits ECC). In the case of
ECC, there is a further line (36?) that is grounded.
My children, do not be led astray by RS/6000 SIMMs.
The RS/6000 32MB SIMMs are 40 bits wide. They do NOT work
in a 95A, even when you alter the PD bits to agree with
the PS/2 PD scheme.
8x36
E Memory
As I have scoured Flea Buy for 32MB 8x39 ECC, my
attention was captured by some 8x36 E SIMMs... Huh, what?
Another oddity, these were for the PC Server 720, which
uses some odd configurations... Again, PS/2 ECC memory
controllers expect 39 bit wide ECC, NOT 36 bit wide ECC.
Ed. Tom: These were probably the
ECC-On-SIMM (EOS) modules.
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