The processor, registers, buses and the fetch-execute cycle

Last time

Before we start: last time, in Computer performance and ports, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

What a processor really does

Every program you have ever run is, to the processor, a long list of instructions. It fetches one, works out what it means, does it, and moves on to the next. It repeats this billions of times a second, and it still stops to answer a key press. That is the fetch execute cycle, with interrupts.

The Von Neumann model

In the Von Neumann model, the program instructions and the data are stored together in main memory. The processor fetches each instruction in turn, using three buses: the address bus, the data bus and the control bus. The stored program concept means a computer can run any program, simply by loading different instructions.

The buses

The address bus carries the memory address the processor wants to read or write. It is one-way, from the processor. The data bus carries the data or the instruction itself. It is two-way. The control bus carries control signals, such as read, write and the clock.

Registers

The program counter holds the address of the next instruction to fetch. The memory address register holds the address being used. The memory data register holds the data or instruction just fetched. The current instruction register holds the instruction being decoded. The accumulator holds the results of calculations from the arithmetic logic unit. The status register holds flags, such as carry and overflow.