L40SX foggy memories
Louis Ohland
Why is the sky blue? What happens if the earth stops spinning? Why
does the LS40SX only support _ONE_ 4MB SIMM?
I suppose with the 90s a fading memory, nobody can ever tell us with
certainty why this happens...
- pull !@#$ William R. Walsh
> Why is the sky blue? What happens if the earth stops spinning? Why
> does the LS40SX only support _ONE_ 4MB SIMM?
Who says it does? Experiences or IBM? What I'm asking in a roundabout way is
whether or not anyone has ever tried it.
By the way, Louis, I will look in my L40 tonight for the IBM FRU numbers on
the genuine IBM 8MB SIMM it has inside.
William
Louis Ohland
> >Why does the LS40SX only support _ONE_ 4MB SIMM?
>
> Who says it does? Experiences or IBM?
Both. IBM says it. Experience shows it.
Rick Ekblaw
>> >Why does the LS40SX only support _ONE_ 4MB SIMM?
I found this old reference over the weekend:
From ASKQ item #ISDM8 on HONE:
Item ISDM8 Document ID Q534089
TITLE: 910515 TWO MEMORY QUESTIONS ABOUT THE 8543.
1. The reason you cannot put two 4mb SIMMs on the planar of the
new 8543 laptop is because of the way the planar was designed. The
system memory consists of four banks of memory, banks 0 through 3.
The size of each memory bank is 2mb or 8mb depending on what SIMMs
you have installed. A 4mb SIMM is assigned to two 2mb banks.
Therefore, if you have the 2mb standard memory assigned to bank 0
as 2mb, and a 4mb SIMM assigned to banks 1 and 2 as 2mb each, then
if you have another 4mb SIMM there is only one more bank available
and a 4mb SIMM must have two banks available for it to work.
2. Only the SIMMs specifically designed for the 8543 can be used
on this machine. They are specially designed low-power CMOS
memory modules.
Rick Ekblaw
Louis Ohland
Rick, how the heck is there FOUR banks on a L40SX? I'm not belittling
your abilities in cut n pasting, but help my disbelief
Bank 0 - 2MB soldered on planar
I >assume<
Bank 1 - SIMM 1 (rightmost SIMM socket)
Bank 2 - SIMM 2 (leftmost SIMM socket)
So where is bank 3?
Louis Ohland
The brown monster is expanding my awareness. (still can't pilot a
Guildliner, this'll have to do)
Based on my detailed knowledge of IBM's memory schemes, here's MY guess
on the 8MB configuration-
With 2x 8MB SIMMs, there is a 2MB chunk left out of the second 8MB
SIMM, based on the P75 info, the planar mem >under most circumstances<
cannot be disabled. I fear to hazard a guess on how the L40SX disables
base memory, and in what size (1MB increments?) but even contemplating
screwing around with the planar memory is a S.H.I.T. that I'm not going
to take.
So- with 2x 8MB, and 2MB planar, the system (obviously) enables the
2MB planar (it can't do otherwise) and 14MB of the 2x 8MB SIMMs (16MB
limit). The fact that the last 2MB can even be accessed is interesting.
How or why >if the memory bank is 8MB< can the L40SX access a NON-8MB
bank (the last 2MB of SIMM2)?
Rick Ekblaw
Ah, grasshopper, you *assume* a direct physical mapping, and that's where it all
falls apart. Banks 0 to 3 in the ASKQ response are "logical" banks in the memory
controller. So "logical" bank 0 is always mapped to the "physical" 2MB built-in
RAM, and each of the other banks is mapped to either a 2MB or 8MB physical RAM
"chunk". This is where the 4MB SIMM throws in the monkey wrench -- it has one
physical 4MB "chunk" which is mapped to 2 "logical" 2MB banks. So you wind up
with only a few supported memory configurations:
2M: 2M standard, no additional memory (only 1 bank assigned)
4M: 2M standard, 1 2MB SIMM (only 2 banks assigned)
6M: 2M standard, 2 2MB SIMMs (only 3 banks assigned)
6M: 2M standard, 1 4MB SIMM (only 3 banks assigned)
8M: 2M standard, 1 2MB SIMM, 1 4MB SIMM (all 4 banks assigned)
10M: 2M standard, 1 8MB SIMM (only 2 banks assigned)
12M: 2M standard, 1 2MB SIMM, 1 8MB SIMM (only 3 banks assigned)
14M: 2M standard, 1 4MB SIMM, 1 8MB SIMM (all 4 banks assigned)
18M: 2M standard, 2 8MB SIMMs (only 3 banks assigned)
The big question is: does the memory controller actually assign a bank to either
a 2MB or 8MB range, or does it assign a bank to each "memory chunk with an x18
configuration" that it finds, and treats the 4MB SIMM as the "odd case" where two
banks have to be assigned because it is configured as 1Mx36? If the first case is
true, then even the hypothetical 2Mx18 4MB low-power SIMM would consume two
banks, and you could still only have 1 4MB SIMM installed. If the second case is
true, then you could have 2 2Mx18 4MB low-power SIMMs installed, and have a
second valid 10MB configuration.
Rick Ekblaw
Louis Ohland
> Ah, grasshopper, you *assume* a direct physical mapping, and that's where it all falls apart. Banks 0 to 3 in the ASKQ response are "logical" banks in the memory controller. So "logical" bank 0 is always mapped to the "physical" 2MB built-in RAM,
I stick with my theory that it uses a 2MB or 4MB bank, never allows
either-or (but that poses an interesting conundrum later).
The 2MB is always counted. Either the bank is 2MB, or if there is two
8MBs, then an 8MB bank is used. Try to wrap your mind on the bank size
of 2MB (planar) plus an 8MB and a 4MB. Bet it would be 2MB banks.
arrggghhh...
How would it support... blech... forget it.