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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 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.
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 other words, this register is used to access data and instructions from memory during the execution phase of instruction. MAR holds the memory location of data that needs to be accessed. When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes ...
When a next-line predictor points to aligned groups of 2, 4, or 8 instructions, the branch target will usually not be the first instruction fetched, and so the initial instructions fetched are wasted. Assuming for simplicity, a uniform distribution of branch targets, 0.5, 1.5, and 3.5 instructions fetched are discarded, respectively.
As the Execution Unit is executing the current instruction, the bus interface unit reads up to six (or four) bytes of opcodes in advance from the memory. The queue lengths were chosen based on simulation studies. [9] An exception is encountered when the execution unit encounters a branch instruction i.e. either a jump or a call instruction. In ...
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).
Each instruction processes one data item, but there are multiple execution units within each CPU thus multiple instructions can be processing separate data items concurrently. Superscalar CPU design emphasizes improving the instruction dispatcher accuracy and allowing it to keep the multiple execution units in use at all times.