Mixing Memory Modules; Error Correcting Code Operation - HP X Class 500/550MHz Reference Manual

System Board
Main Memory Bus

Mixing Memory Modules

Unbuffered and Registered ECC modules are not compatible with each other, and cannot
be mixed. If they have been mixed, this will be detected in the basic pre-boot tests when
you boot the system for the first time. The BIOS checks that the inserted DIMMs are
compatible with one another and, if it detects an error, reports it to MaxiLife. The error
message (shown below) will then be displayed on the LCD status panel.
Table 2-7. Memory Module Compatibility
Memory Module Compatibility
Unbuffered ECC
Registered ECC (512 MB)

Error Correcting Code Operation

Error correcting code (ECC) memory 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 host bridge 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).
50
Error!
RAM type
Adding Unbuffered ECC
Compatible
Not Compatible
Adding Registered ECC
(512 MB)
Not Compatible
Compatible
Chapter 2