Computer
Systems
Nat 4/5 Computing Science
Computer Structure:
Lesson 1:
Processor Structure
Lesson Aims
 By
the end of this lesson:
 Pupils at will be able to:
 Label a computer system in terms of a 5 block
diagram
 Describe the phrase ‘word length’
 Describe the purpose of a processor
 List and describe the parts of a processor:
 Control Unit, ALU, registers
 Describe what a bus is and does
Nat 4/5
A Basic Computer System
Backing
Storage
Input
Devices
Processor
Output
Devices
Memory
 This
is known as the 5 block diagram.
Nat 4/5
The 5 Block Diagram
 Input

Devices
These are devices that enter data INTO the
computer. i.e. Keyboard and mouse
 Output

Devices
These are devices that generate output FROM
the computer i.e. printer.
 Memory

There are two types of memory. RAM and ROM
(more on this later)
 Backing

Storage
This is required to store permanent copies of our
files.
Nat 4/5
The Processor
 The

 It
processor is the ‘brain’ of the computer.
It is the piece of equipment that actually
performs the instructions required.
can be shown in the following diagram:
Processor
Control Unit
Arithmetic and Logic Unit (ALU)
Registers
Nat 4/5
Processor Parts
Control Unit
This controls the order of execution and the timing of
instructions
Arithmetic and Logic Unit (ALU)
This performs the calculations for the processor. It also
carries out the Logical decisions.
Registers
These are temporary stores of information, at best they will
store a few bytes of information.
Nat 4/5
32 bit and 64 bit computing?
 You
often hear a processor being described
as 32 bit or 64 bit.
 This
is usually referring to the ‘word length’ of
the computer
 This
is the largest binary number that the
processor can manipulate in one operation.
Nat 4/5
What does a bus do?
Processor
Main Memory
Data Bus
Control Unit
RAM
Address Bus
ALU
ROM
Registers
 A bus
is a collection of wires which can
carry data
Nat 4/5
Buses
 Data
Bus
 The
data bus is used to carry data to and
from the CPU and Memory.
 It will hold a binary word.
 Address
 This
Bus
carries the address of the memory
location that is being used
Nat 4/5
Summary

The CPU (Central Processing Unit) is the
heart of the computer

The CPU has 3 main parts
1.
Control Unit

2.
Arithmetic and Logic Unit (ALU)

3.
Performs the arithmetical and logical functions
Registers


Controls the timing and order of execution
Temporary stores of information
Word length is the length of the biggest binary
number that the processor can process in one
operation