Computer Hardware User's Guide

Other Registers
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3.2 Other Registers
3.2.1 Program-Counter (PC) Register
The program counter (PC) is a 32-bit register containing the address of the
next instruction fetch. While the program-counter register is not part of the
CPU register file, it can be modified by instructions that modify the program
flow.
3.2.2 Instruction Register (IR)
The instruction register (IR) is a 32-bit register that holds the instruction op-
code during the decode phase of the instruction. This register is used by the
instruction decode control circuitry and is not accessible to the CPU.