PC and IR are fundamental components in a computer's Central Processing Unit (CPU), crucial for instruction execution. Here's a breakdown of each:
Program Counter (PC)
- The Program Counter (PC) is a register that keeps track of the address of the next instruction to be executed.
- Think of it as a pointer that tells the CPU where to find the next instruction in memory.
- After an instruction is fetched, the PC is typically incremented to point to the subsequent instruction in memory, unless a jump or branch instruction alters the flow of execution.
Instruction Register (IR)
- The Instruction Register (IR) holds the current instruction being executed.
- Once an instruction is fetched from memory (the address of which was stored in the PC), it is placed into the IR.
- The instruction decoder then interprets the instruction stored in the IR, determining what operation needs to be performed.
Table Summary
| Feature | Program Counter (PC) | Instruction Register (IR) |
|---|---|---|
| Function | Points to the next instruction in memory. | Holds the current instruction being executed. |
| Contents | Memory address of the next instruction. | The actual instruction code that needs to be executed. |
| Update Timing | Updated before instruction execution (usually incremented). | Updated after instruction is fetched from memory. |
Analogy
Imagine a cookbook and a cook:
- The PC is like your finger pointing to the next recipe (instruction) you want to cook.
- The IR is like the recipe card you're currently reading and following to prepare a dish.
Importance
The PC and IR work together to enable sequential instruction execution, which is the foundation of how computers execute programs. Without them, the CPU would not know which instruction to execute next or what operation to perform.
