Inside the Machine

The Pentium 4's Instruction Window

Before I discuss the nature of the Pentium 4’s instruction window, note that I’m using the terms instruction pool and instruction window somewhat interchangeably. These two terms represent two slightly different metaphors for thinking about the set of queues and buffers positioned between a processor’s front end and its back end. Instructions collect up in these queues and buffers—just like water collects in a pool or reservoir—before being drained away by the processor’s back end. Because the instruction pool represents a small segment of the code stream, which the processor can examine for dependencies and reorder for optimal execution, this pool can also be said to function as a window on the code stream. ...