What is a Computer

1
What is a Computer?
A programmable electronic device that can
store, retrieve and process data.
(N. Dale D. Orshalick, Introduction to PASCAL
and Structured Design, D.C. Heath
Co., Lexington MA, 1983, p. 2)
2
Components of a Computer
3
Categories of Computer Hardware

• Central Processing Unit (CPU)
• Storage
• Primary Cache, RAM
• Secondary Hard disk, removable (e.g., CD)
• I/O
• Input Devices
• Output Devices

4
Central Processing Unit (CPU)

• The Central Processing Unit (CPU), also called
  the processor, is the brain of the computer.

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5
CPU Parts

• The CPU consists of three main parts
• Control Unit
• Arithmetic/Logic Unit
• Registers do not need to know anything about
  registers for this course

6
General Purpose Computer

• For the purpose of the OCR exam, a general
  purpose computer consists of
• Control Unit
• Arithmetic/Logic Unit
• RAM

7
CPU Parts Control Unit
Everything that happens to data inside a general
purpose computer has to be coordinated or it just
wont work properly. Organising this coordination
is the job of the Control Unit.
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Control Unit

• The control unit acts as a go-between linking the
  ALU and the system memory.
• The control unit controls the functioning of the
  CPU and data flow.
• It accepts the data from the system memory,
  passes the data to the ALU for performing
  arithmetic and logical operations, and returns
  the processed data back to the system memory.
• The Control Unit does not do, it directs or
  decides what to do next.

9
Arithmetic/Logic Unit
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Arithmetic/Logic Unit

• The Arithmetic/Logic Unit (ALU) performs
  arithmetic and logical operations.
• Arithmetic operations e.g., add, subtract,
  multiply, divide, square root, cosine, etc.
• Logical operations e.g., compare two numbers to
  see which is greater, check whether a true-false
  statement is true, etc.

11
Random Access Memory (RAM)

• RAM is used to store programs and data whilst the
  computer is switched on. As soon as the power is
  switched off, all the data disappears. It is
  known as Volatile memory.
• RAM if very fast compared to a hard disk. A
  typical hard disk can take about 10 milli-seconds
  to get a piece of data, whilst RAM will take
  about 10 nano-seconds. That is one thousand
  million times faster!
• This is why a computer needs both a hard disk and
  RAM. It uses RAM for speed and the hard disk to
  store data whilst switched off.

12
RAM
It is Random because any part of the memory can
be accessed directly by the computer. RAM is
made up of locations, also known as cells.
Each location has a unique address that never
changes.