Last updated: 23/01/2013
Components of a Computer System
Syllabus
Notes
The computer is made up of several sub-components, each doing a special job and working together.
Here is a block diagram of the components and how they interact:
Central Processing Unit
ALU
CU
Control bus
ROM
Data bus
Address bus
RAM
I/O Subsystem
Input
Secondary
Storage
Output
By Mr. Tanti www.teachertanti.com
Page 1 of 3
Last updated: 23/01/2013
Input, Output and secondary storage we have already seen in previous topics. However it should be
noted that the I/O devices and secondary storage media are all treated in the same way by the
computer, which is why primary storage is faster than secondary storage. Let’s see what the other boxes
are.
The CPU
The Central Processing Unit (CPU) is responsible for all processing and execution of programs in the
computer. It is made up of the Control Unit (CU) and the Arithmetic Logic Unit (ALU).
The CU is responsible for controlling the flow of data around the computer and for preparing everything
for the instructions of programs to be executed.
The ALU is there to perform arithmetic (“plus”, “minus”, etc.) and logical (“AND”, “OR”, “equal to”, etc.)
operations in order to execute an instruction. It is often drawn in the shape of a “V” to symbolize that it
takes in two operands at the top and returns a single result at the bottom.
RAM and ROM
The RAM and ROM together store data and programs which are immediately accessible to the CPU.
Sometimes you might see “Memory Unit” (MU) or “Central Memory” instead. It communicates with the
CU through three buses, the data bus, address bus and control bus.
Data, address and control buses
Buses are basically wires used to transmit data from one part of the computer to another. Here is what
each of the three buses in the diagram does:
In the case of RAM, the CPU needs to send new data to store in it and read data from it. In the case of
secondary storage devices, the CPU needs to send data to save in them and read data from them. In the
case of I/O devices, the CPU needs to send data to output and receive inputs. The data bus is there to
send this data to these components and since data must travel both ways, it is drawn as a two
directional arrow.
How does the CPU say where the data that it wants to send should go? Every place where the data can
go has a unique numerical address. RAM and secondary storage devices also have a unique address for
each location in them. The address bus is there for the CPU to send this address to these components
and since only the CPU can send an address (the CPU has no address for other components to refer to),
it is drawn as a one directional arrow.
RAM needs to be told whether the address it received is for it to read and send data or to receive and
store data. The control bus is used to control whether data needs to go to the RAM or to the CPU.
Through it, the CU sends a “read/write signal”, which depending on whether it is a 1 or a 0 will tell the
RAM whether to accept the data in the data bus or to send data to the data bus. Actually the control bus
is used to allow the CU to control a lot of things around the computer but we won’t be going into all
that.
By Mr. Tanti www.teachertanti.com
Page 2 of 3
Last updated: 23/01/2013
The I/O subsystem
The I/O Subsystem is there as an intermediary between the input/output devices, the secondary storage
and the CPU. This subsystem is a circuit which is responsible for controlling the flow of data between
these components and the CPU.
Further reading
Past papers




2006
o
I
2007
o
IIA
o
IIB
2009
o
2010
o
o

11

3f


1a
2a

2c

3a(iii)

2b(i-ii)
I
IIA
IIB
By Mr. Tanti www.teachertanti.com
Page 3 of 3