The instructions constituting a program to be executed by a computer are
loaded in sequential locations in its main memory. To execute this program,
the CPU fetches one instruction at a time and performs the functions
specified. Instructions are fetched from successive memory locations until
the execution of a branch or a jump instruction.
The CPU keeps track of the address of the memory location where the next
instruction is located through the use of a dedicated CPU register, referred
to as the program counter (PC). After fetching an instruction, the contents
of the PC are updated to point at the next instruction in sequence.
For simplicity, let us assume that each instruction occupies one memory
word. Therefore, execution of one instruction requires the following three
steps to be performed by the CPU:
- Fetch the contents of the memory location pointed at by the PC. The contents
of this location are interpreted as an instruction to be executed. Hence,
they are stored in the instruction register (IR). Symbolically this can
be written as:
IR = [ [PC] ]
- Increment the contents of the PC by 1.
PC = [PC] + 1
- Carry out the actions specified by the instruction stored in the IR.