Program counter

The program counter, or PC (also called the instruction pointer[1], or instruction address register[2], or just part of the instruction sequencer[3] in some computers) is a processor register that indicates where the computer is in its instruction sequence. Depending on the details of the particular computer, the PC holds either the address of the instruction being executed, or the address of the next instruction to be executed.

In most processors, the instruction pointer is incremented automatically after fetching a program instruction, so that instructions are normally retrieved sequentially from memory, with certain instructions, such as branches, jumps andsubroutine calls and returns, interrupting the sequence by placing a new value in the program counter.

Such jump instructions allow a new address to be chosen as the start of the next part of the flow of instructions from the memory. They allow new values to be loaded (written) into the program counter register. A subroutine call is achieved simply by reading the old contents of the program counter, before they are overwritten by a new value, and saving them somewhere in memory or in another register. A subroutine return is then achieved by writing the saved value back in to the program counter again.