IA Instructions Vary in Length and Are Complex

The IA32 instruction set is cumbersome to deal with. A single instruction can be anywhere from one to 15 bytes in length. Attempting to decode and dispatch such instructions at a very high clock rate is counter productive. The Pentium® Pro was the first IA32 processor to translate legacy x86 IA Instructions into primitive, fixed-length μops prior to dispatch and execution.

IA instructions are prefetched from memory and are placed into the unified L2 Cache. As blocks (i.e., lines) of IA instructions are obtained from the L2 Cache, the processor parses them to identify the boundaries between instructions. It then decodes each of the variable-length IA instructions into fixed-length μops. Decoding and dispatching fixed-length instructions at very high clock rates is significantly more efficient than attempting the same with the raw, IA32 instructions.