5.1.1 Computer memory
Points: (Full course and Short Course)
|Candidates are expected to be able
- define the difference between internal memory and
stating the relative benefits of each in terms of speed and permanence.
- Candidates should be able to identify the main components of a
general purpose computer: main/internal Memory
What is ROM and RAM Internal Memory?
All computers have
main/internal memory chips to store programs and data while the computer is
running. There are two types, ROM and RAM. Accessing data or running
software from a memory chip is much faster than from backing storage such as a
- RAM (Random
Access Memory) chips
are described as volatile, the contents are lost
when the computer is switched off. When a computer is started up, the
operating system (i.e. Windows Vista) is copied into the RAM. Any
software that is run (such as a word processor) is also loaded into
RAM and run from there. When a file is loaded (such as a word
processor document) that is also loaded into RAM.
- ROM (Read
chips are described as non-volatile, the contents cannot be changed by a
program or user and are not lost when
the computer is switched off. They are
used to store the programs used to start the hardware running (an example is the
chip which permanently stores the software needed to start a computer's hardware
and operating system).
Both types of memory are referred to as Immediate
Access Storage (IAS) because the hardware can access the data so
quickly compared to data in backing storage.
Comparing internal memory with backing storage:
- It is always faster
to access data from internal memory
than from backing storage.
- Data stored in internal memory is lost when the
computer is turned off but data stored in backing storage is
- When programs are run or data files are loaded the contents are
copied from the backing
storage to the internal memory of
the computer .
- Internal memory is much smaller
than backing storage. It is far too small to hold
all the data/programs that would be
on the backing storage of a typical computer.
How memory works:
Programs and data files are stored as binary numbers.
Binary is made up
of just 0's or 1's unlike decimal numbers (0-9).
To store the 0's and 1's
while the computer is running you need a memory chip. This is made up of millions of tiny
electrical switches called transistors. They can store a
0 or a 1
by the 'switch' being either open or closed. This 0
or 1 is the simplest unit of memory and is
called a ‘bit’ (Binary
Bits are arranged in units of eight to make a byte.
One byte can therefore store eight 0's or 1's
in 256 different combinations. (00101011 and 01110110
would be just 2 possible combinations for example).
One byte is a very
small amount of memory and it is more usual to refer to kilobytes
megabytes (MB) and gigabytes
- 1kB = 1024 bytes (approximately 1 thousand bytes)
- 1MB = 1024KB (approximately 1 million bytes)
- 1GB = 1024MB (approximately 1 thousand million bytes)
- 1KB of memory could store roughly one full A4 page of text.
- 600 MB (on a CD-ROM) could store roughly the text contents of a 10
Memory chips can only store binary numbers so other data such as
sounds, images or text has to be encoded
into binary (digitised).
If you want to store a character from the keyboard
the computer gives it a number code made up of eight bits (1 byte). These
text codes are the same
internationally and are called the ASCII code
Code for Information
The code for the letter ‘a’ is 01100001
(see below). One byte of memory is therefore used to store
the letter 'a' (in code) on a memory chip.
Remember - since computers can only store binary numbers, all computer data has to be in
format. Images, sounds, video etc. all have to be digitised
before they can be processed by a computer.
How the letter ‘a’ is stored in 1
byte of computer memory:
BYTE (8 bits of memory)
|Contents of each bit =>