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In computer architecture, cycles per instruction (aka clock cycles per instruction, clocks per instruction, or CPI) is one aspect of a processor's performance: the average number of clock cycles per instruction for a program or program fragment. [1] It is the multiplicative inverse of instructions per cycle.
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 architecture, instructions per cycle (IPC), commonly called instructions per clock, is one aspect of a processor's performance: the average number of instructions executed for each clock cycle. It is the multiplicative inverse of cycles per instruction. [1] [2] [3]
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 selection is the stage of a compiler backend that transforms its middle-level intermediate representation (IR) into a low-level IR. In a typical compiler, instruction selection precedes both instruction scheduling and register allocation; hence its output IR has an infinite set of pseudo-registers (often known as temporaries) and may still be – and typically ...
Generally speaking, however, complex instructions inflate the number of clock cycles per instruction because they must be decoded into simpler micro-operations actually performed by the hardware. After converting X86 binary to the micro-operations used internally, the total number of operations is close to what is produced for a comparable RISC ...
It often takes a well-defined number of clock cycles to execute. In other instruction sets, there is no explicit NOP instruction, but the assembly language mnemonic NOP represents an instruction which acts as a NOP; e.g., on the SPARC, sethi 0, %g0. A NOP must not access memory, as that could cause a memory fault or page fault.
The clock rate of a CPU is limited by the time it takes to execute the slowest sub-operation of any instruction; decreasing that cycle-time often accelerates the execution of other instructions. [46] The focus on "reduced instructions" led to the resulting machine being called a "reduced instruction set computer" (RISC).