A Level Computing for AQA Teacher’s Resource CD-ROM 4 CHAPTER: Functional characteristics of a processor

A Level Computing for AQA Teacher’s Resource CD-ROM © 2005 Bob Reeves, Dave Fogg/Hodder Murray The stored program concept Very early computers had no storage capacity, so programs and data could not be stored at all. The invention of the microchip allowed computer programs to be stored for the first time. Computer programs are a combination of instructions and data. Both are stored in memory which is the stored program concept. Instructions and data are passed between memory and the processor. The processor carries out the instructions on the data and stores the result back in memory. Memory is made up of millions of addressable cells into which data and instructions are stored and retrieved.

A Level Computing for AQA Teacher’s Resource CD-ROM © 2005 Bob Reeves, Dave Fogg/Hodder Murray Fetch-Decode-Execute This is the cycle that the processor runs through when a program is run: –fetch: the next instruction or data item needed by the program is fetched from memory –decode: the processor interprets the instruction by referencing the instruction set –execute: the processor carries out the instruction, which could involve reading an item of data from memory, performing a calculation or writing data back into memory. The processor will go through this cycle millions of times per second.

A Level Computing for AQA Teacher’s Resource CD-ROM © 2005 Bob Reeves, Dave Fogg/Hodder Murray Registers and Buses As the cycle is repeated millions of times per second, there needs to be a mechanism for keeping track of all of the instructions that are being handled. What seems like a simple instruction in human terms, such as adding two numbers together, may in fact takes several cycles for the processor. In order to handle each cycle, the processor uses a number of different registers and buses. Registers are small areas of storage on the processor where instructions and data are temporarily stored while they are being handled. Different registers carry out different functions during the fetch-decode- execute cycle. Buses are microscopic wires used to transmit data between these registers and between the processor, memory and the Input/Output controllers. There are three types: –data bus –address bus –control bus.

A Level Computing for AQA Teacher’s Resource CD-ROM © 2005 Bob Reeves, Dave Fogg/Hodder Murray Buses The data bus carries data and instructions between the processor, memory and the Input/Output controllers and back again. It also carries data between registers. The address bus carries the memory address of the next instruction needed by the processor or the memory address to store a data item in. It only goes in one direction – from the processor to memory. The control bus transmits signals that control the data and address buses.

A Level Computing for AQA Teacher’s Resource CD-ROM © 2005 Bob Reeves, Dave Fogg/Hodder Murray Input/Output (I/O) ports The processor also communicates with the I/O devices, such as the keyboard, mouse and monitor. These connect via ports to allow signals to be passed to and from the devices. Rather than connect directly to the processor, each device has an I/O controller which acts as an interface between the processor and the device. The I/O controllers have their own circuitry to handle the instructions flowing between the device and the processor.