What is the role of the Program Counter (PC) and Stack Pointer (SP)?

 

The Program Counter (PC) and Stack Pointer (SP) are two essential registers in a microprocessor or microcontroller. They serve critical but distinct roles in instruction execution and memory management:

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1. Program Counter (PC)

Definition:

The Program Counter holds the memory address of the next instruction to be executed.

Role:

• Controls the flow of program execution.

• After each instruction is fetched from memory, the PC is automatically incremented to point to the next instruction.

• In the case of jumps, branches, or function calls, the PC is modified to point to a new instruction address.

Example Behavior:

If the PC contains 0x08000000, and a 32-bit instruction is executed, the PC updates to 0x08000004 (next instruction address).

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2. Stack Pointer (SP)

Definition:

The Stack Pointer holds the memory address of the top of the stack — a special area of RAM used for temporary data storage.

Role:

• Manages the stack, which is used to store:

  • Return addresses

  • Local variables

  • Function parameters

  • Register values during interrupts or function calls

• Grows downward or upward in memory, depending on the CPU architecture (STM32, ARM = downward).

• Modified automatically during function calls (PUSH, CALL) and returns (POP, RET).

Example Behavior:

If the SP contains 0x20001000, and a 4-byte value is pushed, SP becomes 0x20000FFC.

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 Summary Table

Register	Full Name	Main Purpose	Automatically Modified?	Memory Direction
PC	Program Counter	Holds address of next instruction	✅ (each cycle or on jump)	Forward (instruction address sequence)
SP	Stack Pointer	Points to top of stack in RAM	✅ (during push/pop, calls)	Typically downward

 In Context: Function Call Example

c

void foo() {
  int x = 42;
}

• PC: Fetches address of foo, executes it step-by-step.

• SP: Stores return address and allocates space for variable x.