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Click here to follow the execution of a data processing instruction.

Executing an instruction step-by step

Let’s begin at the beginning. Assume that the program counter initially contains the 8-bit binary value 00000111 (we will assume 8-bit arithmetic in this example for the sake of simplicity).

At the first clock pulse, the program counter is incremented to point at the next instruction; that is, it changes fro m 00000111 to 00001000. Note the incremented is by 1 and not 4 because this has an 8-bit instruction set. The PCs of real 32-bit processors are incremented by 4.

Here we show the PC’s timing. At the clock pulse (the rising edge of the clock) the value changes to 00001000 for the duration of the current cycle.

This is the address of the instruction we are going to execute.