Program counter

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Program counter

The program counter (PC), also known as the instruction pointer (IP) in x86 architecture, is a processor register that indicates where a computer is in its program sequence. The program counter holds the address of the next instruction to be executed in the instruction cycle.

Function

The primary function of the program counter is to keep track of the memory address of the next instruction to be fetched from the memory and executed. During the fetch-decode-execute cycle, the program counter is incremented after fetching the instruction, so it points to the next instruction in the sequence.

Operation

1. **Fetch**: The CPU fetches the instruction from the memory location pointed to by the program counter. 2. **Decode**: The fetched instruction is decoded to determine the operation to be performed. 3. **Execute**: The decoded instruction is executed, and the program counter is updated to point to the next instruction.

Branching and Jumping

When a branch or jump instruction is executed, the program counter is updated to point to a new memory address specified by the instruction, rather than simply incrementing to the next sequential address. This allows for control flow changes in the program, such as loops and conditional execution.

Interrupts

During an interrupt, the current value of the program counter is saved to a specific location, so the CPU can return to the correct place in the program after handling the interrupt. This is crucial for maintaining the correct sequence of operations in the presence of asynchronous events.

Related Concepts

• Instruction register
• Stack pointer
• Memory management unit
• Control unit
• Pipeline (computing)

See Also

• Central processing unit
• Instruction cycle
• Fetch-decode-execute cycle
• Branch predictor
• Microarchitecture