in computer architecture 1 what do we mean when we say in or

Solution

In general, we can say that instruction issue occurs when instruction moves from the decode stage of the pipeline to the first execute stage of the pipeline. In theory, the processor is trying to look ahead of the current point of execution to locate instructions that can be brought into the pipeline and executed. Three types of orderings are important in this regard:

W can group instruction issue policies into the following categories:

In order issue with in order completion:

The simplest instruction issue policy is to issue instructions in the exact order that would be achieved by sequential execution (in-order issue) and to write results in that same order (in-order completion). Not even scalar pipelines follow such a simple-minded policy. However, it is useful to consider this policy as a baseline for comparing more sophisticated approaches.

Out of order issue with out of order completion:

With in-order issue, the processor will only decode instructions up to the point of a dependency or conflict. No additional instructions are decoded until the conflict is resolved. As a result, the processor cannot look ahead of the point of conflict to subsequent instructions that may be independent of those already in the pipeline and that may be usefully introduced into the pipeline.

To allow out of order issue, it is necessary to decouple the decode and execute stages of the pipeline. This is done with a kind of buffer referred to as an instruction window. With this organization, after a processor has finished decoding an instruction, it is placed in the instruction window.

Till this buffer is not full, the processor can continue to fetch and decode new instructions.When a functional unit becomes available in the execute stage, an instruction from the instruction window may be issued to the execute stage. In this issue, instructions are dynamically schdueled to the processor.