An instruction window in
computer architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...
refers to the set of
instructions which can
execute out-of-order in a
speculative
Speculative may refer to:
In arts and entertainment
*Speculative art (disambiguation)
*Speculative fiction, which includes elements created out of human imagination, such as the science fiction and fantasy genres
**Speculative Fiction Group, a Per ...
processor
Processor may refer to:
Computing Hardware
* Processor (computing)
**Central processing unit (CPU), the hardware within a computer that executes a program
*** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
.
In particular, in a conventional design, the instruction window consists of all instructions which are in the
re-order buffer
A re-order buffer (ROB) is a hardware unit used in an extension to the Tomasulo algorithm to support out-of-order and speculative instruction execution. The extension forces instructions to be committed in-order.
The buffer is a circular buffer ...
(ROB). 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 (if operands to a younger instruction are ready before those of an older instruction).
The instruction window has a finite size, and new instructions can enter the window (usually called ''dispatch'' or ''allocate'') only when other instructions leave the window (usually called ''retire'' or ''commit''). 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 (i.e., side-effects are not applied outside the CPU) until it is committed in order to support asynchronous
exception handling
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
like
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
s.
This paradigm is also known as restricted dataflow
[Patt et al., ''HPS, A New Microarchitecture: Rationale and Introduction.'' In IEEE International Symposium on Microarchitecture (MICRO), 1985.] because instructions within the window execute in dataflow order (not necessarily in program order) but the window in which this occurs is restricted (of finite size).
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 (e.g., front-end instruction fetch and decode stages) are not considered to be part of the instruction window.
See also
*
Superscalar processor
A superscalar processor is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor, which can execute at most one single instruction per clock cycle, a sup ...
References
{{Reflist
Computer architecture
Instruction processing