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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.
The blue instruction, which was due to be fetched during cycle 3, is stalled for one cycle, as is the red instruction after it. Because of the bubble (the blue ovals in the illustration), the processor's Decode circuitry is idle during cycle 3. Its Execute circuitry is idle during cycle 4 and its Write-back circuitry is idle during cycle 5.
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.
Each stage requires one clock cycle and an instruction passes through the stages sequentially. Without pipelining , in a multi-cycle processor , a new instruction is fetched in stage 1 only after the previous instruction finishes at stage 5, therefore the number of clock cycles it takes to execute an instruction is five (CPI = 5 > 1).
increment, decrement in some ISAs, saving operand fetch in trivial cases. Perform bitwise operations, e.g., taking the conjunction and disjunction of corresponding bits in a pair of registers, taking the negation of each bit in a register. Compare two values in registers (for example, to see if one is less, or if they are equal).
Because the machine code of the jump is already read into the PIQ, and probably also already executed by the processor (superscalar processors execute several instructions at once, but they "pretend" that they don't because of the need for backward compatibility), the change of the code will not have any change of the execution flow.
Alternatively, source code may execute within an interpreter written for the language. [2] If the executable is requested for execution, then the operating system loads it into memory and starts a process. [3] The central processing unit will soon switch to this process so it can fetch, decode, and then execute each machine instruction. [4]
In a computer instruction set architecture (ISA), an execute instruction is a machine language instruction which treats data as a machine instruction and executes it.. It can be considered a fourth mode of instruction sequencing after ordinary sequential execution, branching, and interrupting. [1]