Computer Structure

What does a processor look like? Intel Pentium 4

This shows the size of a Pentium 3 processor

The heart of a computer is the central processing unit or CPU The heart of a computer is the central processing unit or CPU. This device contains all the circuitry that the computer needs to manipulate data and execute instructions. The CPU is amazingly small given the immense amount of circuitry it contains.

RAM RAM (Random access memory consists of a series of memory locations, each with its own unique address

The CPU is composed of four basic components: registers, buses, the ALU, and the Control Unit. To better understand the basic components of the CPU, we will consider each one in detail.

In order for a CPU to accomplish meaningful work, it must have two inputs: instructions and data. Instructions tell the CPU what actions need to be performed on the data. We have already seen how data is represented in the computer, but how do we represent instructions? The answer is that we represent instructions with binary codes just like data. In fact, the CPU makes no distinction about the whether it is storing instructions or data in RAM. This concept is called the stored-program concept.

Registers: these components are special memory locations that can be accessed very quickly. Three registers are shown: the Instruction Register (IR), the Program Counter (PC), and the Accumulator.

Buses: these components are the information highway for the CPU Buses: these components are the information highway for the CPU. Buses are bundles of tiny wires that carry data between components. The three most important buses are the address, the data, and the control buses. ALU: this component is the number cruncher of the CPU. The Arithmetic / Logic Unit performs all the mathematical calculations of the CPU. The ALU can add, subtract, multiply, divide, and perform a host of other calculations on binary numbers.

Control Unit: this component is responsible for directing the flow of instructions and data within the CPU.

John Von Neumann architecture Backing Storage Memory Input Devices Output Devices Processor (CPU)

The Organisation of a Simple Computer

Memory Read Operation Steps Effect Processor sets up address bus with required address Pinpoints desired memory location Read line on Control Bus is activated Tells the memory location that it is to be read from Data bus transfers data from the memory location to the CPU (data register) Data is transferred (copied) from memory location to CPU

Memory Write Operation Steps Effect Processor sets up address bus with required address Pinpoints desired memory location Processor sets up data register with value to be written to memory Data ready to be sent to memory Write line on Control Bus is activated Tells the memory location that it is to be written to Data bus transfers data to required memory location Data is transferred to required memory location

John Walsh Higher Book – pages 24 - 30 Background Reading John Walsh Higher Book – pages 24 - 30