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2 System Board
Main Memory Bus
Main Memory Bus
The memory bus is 72-bits wide, comprised of 64 bits of data and 8 bits of
ECC. It is connected to the Main Memory and to the PAC (440LX) chip.
There are three 168-pin DIMM slots on the system board for installing main
memory; slots A, B and C. All HP Kayak XA PC Workstation models are
supplied with one memory module (either 16 MB, 32 MB or 64 MB ECC
SDRAM) in one of the three slots, leaving the other slots free for memory
upgrades.
The slots can be filled in any order, but there is a performance advantage in
filling the slots in the order A, B, C. Memory upgrades are available in single
32 MB, 64 MB or 128 MB ECC SDRAM modules. Note that replacement of
the supplied memory module may be necessary to obtain the 384 MB
maximum memory, unless the supplied module was 128 MB.
With non-ECC SDRAM memory modules, a maximum of 192 MB can be
obtained. These memory modules can be either; 16 MB, 32 MB or 64 MB.
NOTE If ECC and non-ECC memory modules are both installed, ECC will be
invalidated for all memory modules.
Error Correcting Code
Operation
The error correcting code (ECC) memory of the HP Kayak XA PC
Workstation allows any single bit error that occurs in any 72-bit line of
memory (64 data bits plus 8 parity bits) to be corrected (automatically and
transparently) by the PAC chip.
The ECC detects single and dual bit errors. It can correct single bit errors
during SDRAM reads. The corrected data is transmitted to the requester
(PCI or CPU) but not written back to the SDRAM. A double bit error would
cause an NMI to be generated, and the PC Workstation to be halted.
If more bits are faulty within any given 72-bit line, the effect is the same as it
would have been without error correction. The effect of executing a faulty
instruction is always unpredictable, and might cause the program to ‘hang’.
The effect of reading a faulty data word is often similarly unpredictable, but
can sometimes be tolerated (for instance, it might merely appear as a
corrupted pixel on a video display).