Instruction cycle

An instruction cycle (sometimes called a fetch–decode–execute cycle) is the basic operational process of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction dictates, and carries out those actions. This cycle is repeated continuously by a computer's central processing unit (CPU), from boot-up to when the computer is shut down.

In simpler CPUs the instruction cycle is executed sequentially, each instruction being processed before the next one is started. In most modern CPUs the instruction cycles are instead executed concurrently, and often in parallel, through an instruction pipeline: the next instruction starts being processed before the previous instruction has finished, which is possible because the cycle is broken up into separate steps.

Each computer's CPU can have different cycles based on different instruction sets, but will be similar to the following cycle:

The cycle is then repeated.

The cycle begins as soon as power is applied to the system, with an initial PC value that is predefined by the system's architecture (for instance, in Intel IA-32 CPUs, the predefined PC value is 0xfffffff0). Typically this address points to a set of instructions in read-only memory (ROM), which begins the process of loading (or booting) the operating system.

Step 1 of the Instruction Cycle is called the Fetch Cycle. This step is the same for each instruction:

The Control Unit fetches the instruction's address from the Memory Unit.

Step 2 of the instruction Cycle is called the Decode Cycle. The decoding process allows the CPU to determine what instruction is to be performed, so that the CPU can tell how many operands it needs to fetch in order to perform the instruction. The opcode fetched from the memory is decoded for the next steps and moved to the appropriate registers. The decoding is done by the CPU's Control Unit.

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