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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 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 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.
It separates the fetch and decode stages from the execute stage in a pipelined processor by using a buffer. The buffer's purpose is to partition the memory access and execute functions in a computer program and achieve high-performance by exploiting the fine-grain parallelism between the two. [41]
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).
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 ...
Pipelined MIPS, showing the five stages: instruction fetch, instruction decode, execute, memory access and write back. The first MIPS microprocessor, the R2000, was announced in 1985. It added multiple-cycle multiply and divide instructions in a somewhat independent on-chip unit.