Outline of IPB Memory Slot
IPB Memory Board
49C460CJ 32-bit CMOS Error Detection and Correction Unit
KMM594000A 4Mx9 CMOS DRAM SIMM Memory Module
CY7C9101-30JC CMOS 16-bit ALU Slice Datasheet
74F899QC 9-bit Latchable Transceiver
ECC-P Memory with 4MB 30 Pin
The memory card has (32) 30 pin SIMM sockets. Each 32 MB
(two banks) has own memory controller. Memory was sold
in 32MB increments. That would be two blocks with four
30 pin SIMMs.
32+4=36, not enough for ECC, BUT... 36+36=72... 64 bits
for data, and 8 bits for ECC... Two 32 bit EDCs can be
chained for a 64 bit word.
So... you need one bank of 8 SIMMs each for ECC-P, so
the minimum is 32MB, the next is two banks (64MB), three
banks (96MB), and four banks for a maximum of 128MB.
-- OR --
So... you need two banks of 4 SIMMs each for ECC-P, so
the minimum is 32MB, the next is four banks (64MB), six
banks (96MB), and 8 banks for a maximum of 128MB.
Which one is right?!
Note: I have seen a
configuration screen, with a choice of "ECC: ON | OFF".
My guess is that ECC=P will not run on an odd number of
banks, so if you wanted to run 48MB, ECC would be "OFF".
Previous IBM servers such as the IBM Server 85 were able
to use standard memory to implement what is known as
ECC-P. ECC-P takes advantage of the fact that a
64-bit word needs 8 bits of parity in order to detect
single-bit errors (one bit/byte of data). Since it is
also possible to use an ECC algorithm on 64 bits of data
with 8 check bits, IBM designed a memory controller
which implements the ECC algorithm using the standard
The following shows the implementation of ECC-P. When
ECC-P is enabled via the reference diskette, the
controller reads/writes two 32-bit words and 8 bits of
check information to standard parity memory. Since 8
check bits are available on a 64-bit word, the system is
able to correct single-bit errors and detect double-bit
errors just like ECC memory.
While ECC-P uses standard non-expensive memory, it needs
a specific memory controller that is able to read/write
the two memory blocks and check and generate the check
bits. Also, the additional logic necessary to
implement the ECC circuitry make it slightly slower than
true ECC memory. Since the price difference between a
standard memory SIMM and an ECC SIMM has narrowed, IBM
no longer implements ECC-P.
Memory Management and OS/2
Up to 128 MB of 80 ns ECC-P memory can be installed in the
Server 295. Although OS/2 1.3 can only address up
to 16 MB of this memory per processor, the rest can be
accessed via the Transparent Reserved Memory Manager
When configured in multiprocessor mode, up to 16 MB of
memory can be assigned to each of the FP and the AP
processors, for a total of 32 MB. This allocation
is done using the utility MPSETUP.
The remaining memory can be accessed via RMM, which is
installed as a device driver. RMM enables this
extra memory to be accessed in a number of ways:
o Disk buffers for the PDAs
o HPFS cache
o Reserved memory for some OS/2 applications - for
example in response to DosAllocSeg and DosAllocHuge
o RAM-based virtual disk
o SWAPPER.DAT file used for segment swapping to disk by
RMM memory is allocated by the /TRMM= switch in the
CONFIG.SYS and CONFIG.FP files. Status information
on RMM can be displayed using the RMMSTAT utility.
RMM is needed because of the 16-bit limitations of OS/2
1.3, and results in substantial performance improvements
for some applications because massive amounts of data
can be held in memory rather than on disk.