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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 number of dependent steps varies with the machine architecture. For example: The 1956–61 IBM Stretch project proposed the terms Fetch, Decode, and Execute that have become common. The classic RISC pipeline comprises: Instruction fetch; Instruction decode and register fetch; Execute; Memory access; Register write back
The data hazard is detected in the decode stage, and the fetch and decode stages are stalled - they are prevented from flopping their inputs and so stay in the same state for a cycle. The execute, access, and write-back stages downstream see an extra no-operation instruction (NOP) inserted between the LD and AND instructions.
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 a typical fetch-decode-execute cycle, each step of a macro-instruction is decomposed during its execution so the CPU determines and steps through a series of micro-operations. The execution of micro-operations is performed under control of the CPU's control unit , which decides on their execution while performing various optimizations such ...
Both CPUs evaluate branches in the decode stage and have a single cycle instruction fetch. As a result, the branch target recurrence is two cycles long, and the machine always fetches the instruction immediately after any taken branch. Both architectures define branch delay slots in order to utilize these fetched instructions.
Nearly all CPUs follow the fetch, decode and execute steps in their operation, which are collectively known as the instruction cycle. After the execution of an instruction, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter. If a ...
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]