Confusion with instruction register and counter register

Question

I have difficulty in understanding the workings of instruction register and counter register in avr micro-controller like an atmega128p.

Is there a way to explain what they do in a simple way?

Answer 1

I suppose you are interested in the difference between the Instruction Register(IR) and Program Counter (PC).

Their working principles are pretty simple.

• IR: Holds the instruction that will be or is currently executed
• PC: Holds the address of the instruction that should be loaded into IR from flash memory in the next clock cycle.

Note that AVRs normally use a 2-stage pipeline (fetch + execute) therefore the IR is double buffered (inside a clock cycle the next instruction is loaded from flash and the current instruction is executed).