Current Instruction Register

Acronym is CIR.

An essential part of a central processing unit (CPU). Also known as the Instruction Register.

The current instruction register holds the instruction that is currently being decoded and acted upon. It is part of the control unit section of the CPU.

As part of the Fetch-Decode-Execute cycle, the end of the Fetch cycle loads the instruction register with the command to be carried out. The output of the instruction register is made available to other parts of the CPU so that they are all set up correctly to execute the instruction.

In complicated modern CPUs it is common to have an instruction pipeline to speed things up. The pipeline consists of a whole set of linked registers with each register handling the decoding of the next few instructions to be carried out. When ready, the output of the first register in the pipeline is passed to its neighbour and so on until the end of the pipeline is reached whereupon the instruction is executed.

2021-09