The Read-Only Memory

When you first switch on your computer, its memory is - at best - empty but, more likely, it contains 0's and 1's distributed randomly.

The first task required of the CPU is to test then clear the memory - the problem is how does the computer 'tell' the CPU to do this in what is really a "chicken and egg" situation?!

The answer is to provide the computer with some 'instant' instructions which have already been programmed by the computer's manufacturer into a special Read-only memory.

When the CPU first receives power (or when you press the Reset button), the CPU resets its Address Bus to be all 0's and switches its read/write line to Read. Provided, the Read-only memory starts at address zero and that very first memory address contains an instruction code, the CPU should be able to start up smoothly.