Program Counter (PC) - The Instruction Tracker

Program Counter (PC) - The Instruction Tracker

What It Is:

The Program Counter is a special register that holds the memory address of the NEXT instruction to be fetched and executed. It's like a bookmark keeping your place in a book.

How It Works:

• During FETCH: The address in PC is copied to the MAR
• After FETCH: The PC automatically increments (adds 1) to point to the next sequential instruction
• During branches: JUMP, BRANCH, or CALL instructions can change the PC to a different address (for loops, IF statements, functions)
• On program start: PC is set to the address of the first instruction

Key Characteristics:

• Holds an address (not data or instructions)
• Increments during the FETCH stage of the FDE cycle
• Size depends on the address bus width (e.g., 32-bit PC can address 2³² locations)
• Essential for sequential execution and program flow control

Real-world analogy: Like line numbers in a script. An actor (CPU) reads line 5, performs the action, then automatically looks at line 6 next. Unless the script says "GOTO line 20" (a branch instruction), the actor continues sequentially.