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The instruction unit (I-unit or IU), also called, e.g., instruction fetch unit (IFU), instruction issue unit (IIU), instruction sequencing unit (ISU), in a central processing unit (CPU) is responsible for organizing program instructions to be fetched from memory, and executed, in an appropriate order, and for forwarding them to an execution unit (E-unit or EU).
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 pipelined computer, instructions flow through the central processing unit (CPU) in stages. For example, it might have one stage for each step of the von Neumann cycle: Fetch the instruction, fetch the operands, do the instruction, write the results. A pipelined computer usually has "pipeline registers" after each stage.
The term "latency" is used in computer science often and means the time from when an operation starts until it completes. Thus, instruction fetch has a latency of one clock cycle (if using single-cycle SRAM or if the instruction was in the cache). Thus, during the Instruction Fetch stage, a 32-bit instruction is fetched from the instruction memory.
The POWER8 [22] [23] core has 64 KB L1 data cache contained in the load-store unit and 32 KB L1 instruction cache contained in the instruction fetch unit, along with a tightly integrated 512 KB L2 cache. In a single cycle each core can fetch up to eight instructions, decode and dispatch up to eight instructions, issue and execute up to ten ...
When these instructions are encountered again, the trace cache allows the instruction fetch unit of a processor to fetch several basic blocks from it without having to worry about branches in the execution flow. Instructions will be stored in the trace cache either after they have been decoded, or as they are retired.
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 simple central processing unit (CPU), the PC is a digital counter (which is the origin of the term "program counter") that may be one of several hardware registers.The instruction cycle [8] begins with a fetch, in which the CPU places the value of the PC on the address bus to send it to the memory.