National 5 Computing Science Buses

2. Buses
A bus is:
a collection of wires
through which data is transmitted
from one part of a computer to another.

3. Buses
ALU
Control
Unit
Memory
Data
Register
Memory
Address
RegisterOther
Registers
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Address Bus
32 Lines
Data Bus
16 Lines
Control Bus
6 Lines
Each memory location is represented by a unique address.

4. The Address Bus
Memory
Address
Register
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Address Bus
• information is carried from the processor to the main memory
• this informs the main memory which memory location will be read or used to store data
• each wire on the bus carries one bit of information at a time

5. The Address Bus
Memory
Address
Register
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Address Bus
• the number of wires in this bus determines the number of memory locations
8 lines will allow 28
= 256 memory locations
32 lines will allow 232
= 4,294,967,296 memory locations
• increasing the width of this bus, increases the number of memory locations that it is possible to
address

6. The Data Bus
Memory
Data
Register
Processor Main Memory
11110011
Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Data Bus
• information is carried to and from the processor and main memory
• this stores data in a memory location and reads data from a memory location
• each wire on the bus carries one bit of information at a time

7. The Data Bus
Memory
Data
Register
Processor Main Memory
11110011
Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Data Bus
• the description of the computer informs the user of the number of wires in the data bus
a 32 bit computer has 32 wires on the data bus
• increasing the width of the data bus increases the quantity of data that can be carried at one
time and so increases the performance of the computer system

8. The Control Bus
Control
Unit
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Control Bus
Each wire on the bus has its own separate function and is
activated independently of the others
Read
Informs the memory that data is to be sent to the processor from a particular memory location
Write
Informs the memory that data is to be stored in a particular memory location
Clock
Generates a constant pulse which regulates the flow of information
A clock of 600MHz (megahertz) generates a pulse 600,000,000 times a second

9. The Control Bus
Control
Unit
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Control Bus
Each wire on the bus has its own separate function and is
activated independently of the others
Interrupt
A message from a peripheral device causes the processor to stop processing the current task. Current
data is stored in a temporary area called the stack. The processor deals with the interrupt. The data is
then retrieved from the stack and the task is resumed
Reset
Clears all internal processor registers and returns the computer to its initial switched on state

10. The Control Bus
Control
Unit
Processor Main Memory Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Control Bus
Each wire on the bus has its own separate function and is
activated independently of the others
Interrupt
A message from a peripheral device causes the processor to stop processing the current task. Current
data is stored in a temporary area called the stack. The processor deals with the interrupt. The data is
then retrieved from the stack and the task is resumed
Reset
Clears all internal processor registers and returns the computer to its initial switched on state