5.1.1 Computer 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 hard drive.

• 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 Only Memory) 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 BIOS 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 retained.
• 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 Digit).

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 (KB), megabytes (MB) and gigabytes (GB).

• 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 volume encyclopaedia.

Encoding Data:

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 (American Standard Code for Information Interchange).

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 this digital 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: