COMPUTER ORGANISATION

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HIGHER STILL

ComputingComputer Systems

Higher

Marr College Computing Department 2002

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All computers are based upon the same basic design, known as the Von Neumann Architecture.

Principles of Computer Organisation

Computers carry out tasks by executing instructions in machine code programs.

A machine code program is held in main memory as a stored program, a concept first proposed by John Von Neumann in 1945.

A unit, known as the Central Processing Unit (CPU) fetches, decodes and executes the machine

instructions.

Being able to load and execute different stored programs allows the computer to become a general purpose problem solving machine

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Simple Von Neumann diagram of computer system

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Von Neumann Architecture

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The electronic components of a computer are designed to be in only one of two states.

The Two State Machine

For example, a magnetic storage device records data magnetised in one direction or another, transistors conduct or do not conduct.

The binary digits 0 and 1 are used to represent these two states and hence the computer is termed a two-state machine.

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To execute a machine code program it must first be loaded, together with any data that it needs, into main memory

(RAM).

Fetch, decode and execute are repeated until a program instruction to HALT is encountered. This is known as the fetch-execute cycle.

The Fetch-Execute Cycle

Once loaded, it is accessible to the CPU which fetches one instruction at a time, decodes and executes it at electronic speed.

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Task: Identifying system components used in a computing task

• Find a partner to work with

• Power up a PC and load Microsoft Word

• Now think about the sequence of events that occur when we create a new word processing document.

• Try to identify the components of your PC that are involved in this task.

• For example, there will be a need to transfer the word processing program from the hard disk to RAM in order to run the program.

• What happens next and what components do you think are involved?

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Step 1. Double click on the WP icon which causes the application to load from external memory (hard disk) to main memory (RAM).

Step 2. The CPU starts to execute the program in RAM and a new document window appears on the output device (display screen).

Step 3. Text is entered via an input device (keyboard) and transferred to main memory where it is stored and subsequently transferred to the display screen.

Step 4. Editing of text is carried out, whereby the WP application program executed by the CPU manipulates data in main memory.

Step 5. The file is saved to external storage by transferring the contents of main memory to the hard disk.

Solution to task: