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The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch–execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage.
In computer engineering, instruction pipelining is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions ...
In computer science, instruction scheduling is a compiler optimization used to improve instruction-level parallelism, which improves performance on machines with instruction pipelines. Put more simply, it tries to do the following without changing the meaning of the code:
Once again, assuming a uniform distribution of branch instruction placements, 0.5, 1.5, and 3.5 instructions fetched are discarded. The discarded instructions at the branch and destination lines add up to nearly a complete fetch cycle, even for a single-cycle next-line predictor.
In the control logic, the combination of cycle counter, cycle state (high or low) and the bits of the instruction decode register determine exactly what each part of the computer should be doing. To design the control logic, one can create a table of bits describing the control signals to each part of the computer in each cycle of each instruction.
The instruction unit (I-unit or IU), also called, e.g., instruction fetch unit (IFU), instruction issue unit (IIU), instruction sequencing unit (ISU), in a central processing unit (CPU) is responsible for organizing program instructions to be fetched from memory, and executed, in an appropriate order, and for forwarding them to an execution unit (E-unit or EU).
Image credits: JessTheTwilek #7. I was training a new employee (male) and part of the training was sitting in with me when I met with clients. First day, first meeting, I explained to the client ...
The first machine to use out-of-order execution was the CDC 6600 (1964), designed by James E. Thornton, which uses a scoreboard to avoid conflicts. It permits an instruction to execute if its source operand (read) registers aren't to be written to by any unexecuted earlier instruction (true dependency) and the destination (write) register not be a register used by any unexecuted earlier ...