Instruction window
An instruction window in computer architecture refers to the set of instructions which can execute out-of-order in a speculative processor.
In particular, in a conventional design, the instruction window consists of all instructions which are in the re-order buffer. In such a processor, any instruction within the instruction window can be executed when its operands are ready. Out-of-order processors derive their name because this may occur out-of-order.
The instruction window has a finite size, and new instructions can enter the window only when other instructions leave the window. Instructions enter and leave the instruction window in program order, and an instruction can only leave the window when it is the oldest instruction in the window, and it has been completed. Hence, the instruction window can be seen as a sliding window in which the instructions can become out-of-order. All execution within the window is speculative until it is committed in order to support asynchronous exception handling like interrupts.
This paradigm is also known as restricted dataflow because instructions within the window execute in dataflow order but the window in which this occurs is restricted.
The instruction window is distinct from pipelining: instructions in an in-order pipeline are not in an instruction window in the conventionally understood sense, because they cannot execute out of order with respect to one another. Out-of-order processors are usually built around pipelines, but many of the pipeline stages are not considered to be part of the instruction window.