Devices On The Main Memory Bus; Main Memory - HP Kayak XU Technical Reference Manual

NOTE
Error Correcting Code
Operation

Devices on the 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.

Main Memory

There are four 168-pin DIMM slots on the system board for installing main
memory; slots A, B, C and D. All HP Kayak XU PC Workstation models are
supplied with one memory module (either 32 MB, 64 MB or 128 MB ECC
SDRAM) in one of the four DIMM slots, leaving three slots free for memory
upgrades. Memory upgrades are available in single 32 MB, 64 MB or 128 MB
memory modules. The maximum memory size of 512 MB is reached by using
four 128 MB DIMMS.
If ECC and non-ECC memory modules are both installed, ECC will be
invalidated for all memory modules.
The error correcting code (ECC) memory of the HP Kayak XU 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. 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).
2 System Board
Devices on the Main Memory Bus
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