Data Integrity - HP Vectra XW Optimization Manual

2 Technical Reference
Main Memory

Data Integrity

Many Personal Workstations protect the integrity of memory data by
using parity bits stored with each data byte. Parity is calculated by
adding up all the data bits in a byte. If the total is an even number, the
parity bit is set to 0, while if the total is an odd number this will be 1.
Parity is generated by the system's memory controller when writing
data to memory and this parity is checked when the data is read back.
Any difference between the parity written and that read indicates that
one of the bits in the corresponding data byte has changed.
The advantage of parity is that it enables the memory controller to
detect if memory data has been changed. However, it does not enable
the memory controller to correct the error because the precise bit is
not identified. Furthermore, if two bits of data are changed within the
same byte of data, no error would be detected.
ECC Code
Your HP Vectra XW uses a different system to guarantee the integrity of
your memory data. This system, called Error Correcting Code (ECC),
enables the memory controller to detect and automatically correct any
single-bit data error that occurs.
ECC Code is generated using a mechanism called 8-bit hamming code.
This takes larger, disjointed groups of data bits and performs the same
addition used for parity to produce an ECC bit. With eight bytes of
data, eight ECC bits will be generated, which means that the groups of
bits used to generate the ECC bits overlap.
The groups of data bits used to calculate the ECC bits are carefully
selected, so that a change in any one data bit will change a unique
combination of the ECC bits. If a single-bit data error occurs, your
system's memory controller decodes the ECC bits to identify exactly
which data bit has been changed.
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