RAM Basics
 

Bus Speed

The speed at which the system moves data from one place to another. Measured in megahertz (MHz). RAM bus speed should typically equal the motherboard bus speed. You can install RAM with a faster bus speed than the m/b, but the RAM will typically run at the m/b speed.


Bandwidth

The amount of data the RAM can process per unit of time. RDRAM can process 1.8 gigabytes of data/second. Bandwidth depends on the architecture of the module.


Latency'

The amount of time it takes for the processed data to go from one location to its destination on a system. Some types of RAM have high bandwidth, but because of other factors (such as architecture and location on the m/b).


Parity and Error Correction Code

Parity is a method of detecting 'sibgle bit' errors in data. Abyte of data contains 8 bits. Parity memory adds an additional bit to each byte (processing 9 bits of data for a byte). Parity memory uses this method to detect, but not correct memory errors. This is an issue mostly in older computers (of the Pentium - Pentium II class). You do not mix non-parity RAM with parity RAM. In an older system, parity RAM can only function with parity if the BIOS is set to parity checking. You will not need to worry about parity for DDR and RDRAM.

Error Correction Code (ECC) memory corrects single bit errors and detects (but does not correct) multi-bit errors. RAM is still sold in ECC and non-ECC modes. When looking up RAM for any particular system, you will also be told if the RAM is ECC or non-ECC.
Note: Rambus RIMM's will directly state, on the copper cover, whether the module is ECC or Non-ECC