The document describes the internal and external hardware components of a computer. It discusses the CPU, main memory including RAM and ROM, input/output controllers and ports, system buses including the address bus, control bus, and data bus. Peripherals and secondary storage are also summarized as external components that connect via input/output controllers.

1. Internal and External Hardware
Components of a Computer

2. 3 Box model
CPU Main Memory I/O
Control Bus
System Bus
Address Bus
Data Bus

3. The Processor
The most complex and expensive
component of the computer. Transistor
count is currently 2.6 billion (2011). The
processor executes programs and
supervises the rest of the system at the
same time. Transistors are devices that
amplify and switch electronic signals and
power. A voltage or current applied to one
pair of the transistors terminals changes the
currents flowing through another pair of
terminals.

4. Main Memory
Main memory is data stores that can be directly
addressed by the CPU. It’s used to store
program instruction and data and uses the
system bus to communicate with the CPU. Main
memory is made up of RAM and ROM chips.
Today Ram is measured in Gb and large
programs can run or multiple programs can run
at the same time. The larger the memory the
more/bigger the program you can run. Main
memory is data stored in different addresses
and so the more memory you have the more
addresses you have.

5. RAM
Random access memory or RAM is used for any
memory that is readable and writable so the process
uses it to store data and load it back. However RAM is
volatile, meaning the contents of the RAM is
forgotten when the computer is turned off.

6. ROM
Read only memory or ROM remembers data when
the computer is turned off making it non-volatile.
ROM can provide random access like RAM but it can’t
be written to. This makes it useful to store fixed
programs on.

7. I/O Controllers and Ports
I/O controllers regulate the power between the devices
and the processor. Can handle different types of
devices now whereas before the processor wouldn’t
recognise new devices and whole new CPU would have
to be installed.
Generic ports are now used for all devices e.g. USB
because all computers have them and everyone can
plug in devices. This is the method of performing I/O
between the CPU and the device.

8. Connection between devices
In the 3 box model each components is connected by a
bus, known as the system bus. The different buses are
made up of wires that connect to each component of
the computer system and passes signals between them.
Signals either represent an address, data or control
information and so the system bus is made up of the
address bus, the data bus and the control bus.

9. Address Bus
Unidirectional and has 32 wires connecting to the
address memory which gives 110 locations. It carries
signals from the CPU to the main memory and I/O
devices. The CPU could request some data from the
main memory and it does this by sending the address of
the data to the main memory and then the main
memory returns the data along the data bus.

10. Control Bus
Bidirectional and has more than 16 wires (usually 32)
and is transports control signals between the 3
components of the box model. It’s used to carry
important information such as messages to say when a
device has finished a job or when a device has been
plugged in. It controls interrupt signals as well which
allow devices to signal that they have finished a
request. The CPU temporarily suspends its current
program, services the device and then resumes its
program. Ctrl + Alt + Del is the highest priority of
interrupt.

11. Data Bus
Bidirectional and has 32 wires used to transport data
and instruction between all the components in the 3
box model. The larger the data bus is the more data
that can be transported at one time. E.g.
11111111 = 8 digit’s which means the data bus is 8 bit.

12. Peripherals
Peripherals are any device that
isn’t connected to the CPU.
I/O devices aren’t connected to
the CPU – meaning they are
peripheral. The system uses them
to get information in or out. E.g.
a keyboard gets information in
and a VDU gets information out.

13. Secondary Storage
Secondary storage is not directly connected to the
processor making it a peripheral device but it is
permanent storage. An example of secondary storage
is a magnetic hard disk. Any secondary storage is
connected to the processor and the main memory by
an I/O controller and data is transferred by using the
system bus.