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.
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 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).
