Edexcel GCSE Computer Science
Revision Guide (Topic 3 – Computers)
Specification:
, 3.1.1
Components of a Computer (the Von-Neumann Architecture:
RAM (Random-Access Memory) or main memory
Consists of a collection of storage locations, which all have their own
memory address.
These are written in binary.
RAM is normally measured in gibibytes (GiB).
Data is read/written to memory.
Saving the data to memory is called writing. The reverse is
called reading.
RAM is volatile - when the power is turned off, the content is lost.
This is why we need secondary non-volatile storage (e.g. Hard
disk drive).
CPU (Central Processing Unit)
The CPU consists of:
The CU (Control Unit) - decodes instructions received from main
memory and coordinates the actions of the other parts of the CPU to
execute them.
ALU (Arithmetic Logic Unit) - performs arithmetic and logic
operations on data.
Arithmetic = 5 + 2, Logic = is 5 > 2?
Registers - provide fast, temporary storage for instructions,
intermediate results and data. Some registers are general purpose.
Others have a designated function
E.g. the instruction registers hold instructions being executed
currently,
The accumulator holds the results of calculations the ALU
performs.
Buses
Connect the CPU with memory. There are types of bus:
Address bus - carries the address of the memory location. It is
unidirectional. From the CPU to RAM.
Data bus - Holds the data being read/written to memory. Is
bidirectional.
Control bus - Carries command signals from CU to other
components, so they know whether to read/write data. Is
bidirectional.
A bus width - the number of connections on a bus. Each connection
represents a binary digit, so a greater bus width means larger number
values can be communicated.
Clock
Sends out electronic signals at regular intervals to all the other
components, to synchronise their actions.
Revision Guide (Topic 3 – Computers)
Specification:
, 3.1.1
Components of a Computer (the Von-Neumann Architecture:
RAM (Random-Access Memory) or main memory
Consists of a collection of storage locations, which all have their own
memory address.
These are written in binary.
RAM is normally measured in gibibytes (GiB).
Data is read/written to memory.
Saving the data to memory is called writing. The reverse is
called reading.
RAM is volatile - when the power is turned off, the content is lost.
This is why we need secondary non-volatile storage (e.g. Hard
disk drive).
CPU (Central Processing Unit)
The CPU consists of:
The CU (Control Unit) - decodes instructions received from main
memory and coordinates the actions of the other parts of the CPU to
execute them.
ALU (Arithmetic Logic Unit) - performs arithmetic and logic
operations on data.
Arithmetic = 5 + 2, Logic = is 5 > 2?
Registers - provide fast, temporary storage for instructions,
intermediate results and data. Some registers are general purpose.
Others have a designated function
E.g. the instruction registers hold instructions being executed
currently,
The accumulator holds the results of calculations the ALU
performs.
Buses
Connect the CPU with memory. There are types of bus:
Address bus - carries the address of the memory location. It is
unidirectional. From the CPU to RAM.
Data bus - Holds the data being read/written to memory. Is
bidirectional.
Control bus - Carries command signals from CU to other
components, so they know whether to read/write data. Is
bidirectional.
A bus width - the number of connections on a bus. Each connection
represents a binary digit, so a greater bus width means larger number
values can be communicated.
Clock
Sends out electronic signals at regular intervals to all the other
components, to synchronise their actions.
