Program Counter

The program counter (PC), also called the instruction pointer, is a processor register that holds the memory address of the next instruction to be executed in a program. It is the physical mechanism that implements control flow: incrementing the PC fetches the next sequential instruction, while branch instructions overwrite the PC with a new address, redirecting execution. The PC is the single point of sequentiality in an otherwise parallel machine — every out-of-order execution engine, every superscalar pipeline, and every speculative fetcher exists to keep the PC advancing as quickly as possible while hiding the latency of memory and computation from its critical path.

The PC is not merely a register. It is the locus of determinism in a computer. All other state — caches, registers, flags — can be reconstructed from memory and the PC. This is why checkpointing and fault tolerance systems save the PC first: it is the thread that, when pulled, unravels the entire computation back to a known point. The modern CPU's instruction pipeline is essentially a conveyor belt of program counters, each stage decoding, executing, or retiring an instruction that some earlier PC initiated.