Cambridge O Level Computer Science · Syllabus 2210 · Hardware
Current Instruction Register
What is Current Instruction Register?
The CPU register that stores the instruction currently being decoded and executed, so the control unit can read it and work out which operation is required and on what.
This definition is part of the Hardware chapter in Cambridge O Level Computer Science.
Questions students ask about Current Instruction Register
What is the difference between the PC, MAR, MDR and CIR registers?
The program counter (PC) holds the address of the next instruction to be fetched. The memory address register (MAR) holds the address of the memory location currently being accessed, supplying it to memory on the address bus. The memory data register (MDR) temporarily stores the data or instruction transferred to or from memory over the data bus. The current instruction register (CIR) holds the instruction currently being decoded and executed. PC and MAR hold a "where"; MDR and CIR hold a "what".