Transcript Computer Organisation

COMPUTER ORGANISATION
HIGHER STILL
I
Computing
Computer Systems
Higher
Marr College Computing Department 2002
Principles of Computer Organisation
All computers are based upon the same basic design, known as the Von
Neumann Architecture.
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 Two State Machine
The electronic components of a computer are designed to be in only
one of two states.
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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The Fetch-Execute Cycle
To execute a machine code
program it must first be loaded,
together with any data that it
needs, into main memory
(RAM).
Once loaded, it is accessible to
the CPU which fetches one
instruction at a time, decodes
and executes it at electronic
speed.
Fetch, decode and execute are repeated until a program instruction to HALT
is encountered. This is known as the fetch-execute cycle.
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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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Solution to task:
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.
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