4.2. CPU AND MEMORY REQUIREMENTS
A computer's CPU (central processing unit) processes data in 8-bit chunks. Those chunks, as we learned in the previous section, are commonly referred to as bytes. Because a byte is the fundamental unit of processing, the CPU's processing power is often described in terms of the maximum number of bytes it can process at any given time. For example, Pentium and PowerPC microprocessors currently are 64-bit CPUs, which means they can simultaneously process 64 bits, or 8 bytes, at a time.
Each transaction between the CPU and memory is called a bus cycle. The number of data bits a CPU can transfer during a single bus cycle affects a computer's performance and dictates what type of memory the computer requires. Most desktop computers today use 168-pin DIMMs, which support 64-bit data paths. Earlier 72-pin SIMMs supported 32-bit data paths, and were originally used with 32-bit CPUs. When 32-bit SIMMs were used with 64-bit processors, they had to be installed in pairs, with each pair of modules making up a memory bank. The CPU communicated with the bank of memory as one logical unit.
Interestingly, RIMM modules, which are newer than DIMMs, use smaller 16-bit data paths; however they transmit information very rapidly, sending several packets of data at a time. RIMM modules use pipelining technology to send four 16-bit packets at a time to a 64-bit CPU, so information still gets processed in 64-bit chunks.