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Do not confuse memory with backing storage. One name for memory is ‘internal memory’ because it is inside the computer, closely connected to the CPU. Backing storage is outside the processor. Module 3 will look further at the differences between memory and storage devices.
The size of a computer’s memory (and backing storage – see Module 3) is normally measured in kilobytes (kB), megabytes (MB) or gigabytes (GB). Table 1.1 shows how these relate to each other. Table 1.1 Common units for measuring the size of memory.
Unit
Question 2 ROM and RAM are two different types of memory found in computers. a What does ROM stand for? b What does RAM stand for?
Abbreviation
Size
kilobyte
kB
1 kB = 1024 bytes
megabyte
MB
1 MB = 1024 kB
gigabyte
GB
1 GB = 1024 MB
terabyte
TB
1 TB = 1024 GB
c What is the main difference between ROM and RAM?
Measuring the size of memory Computers store and process data using binary numbers. For example, a computer might store the letter B as 10000011. Data is stored in a computer as a series of 0s and 1s. The computer does not understand things like ‘a’ or ‘hello’ or ‘23’. It can only understand electrical signals being turned on or off. These signals can stand for 0 (no signal being sent) or 1 (an electrical signal). Everything else has to be made up of combinations of these electrical signals or no electrical signals. We can’t keep talking about electrical signals or no signals so we need a name for a single unit (a 0 or a 1). It is called a bit, which stands for binary digit. By putting a number of these bits together the computer can represent many different types of data. For example 10000010 might stand for ‘A’ and 10000011 might stand for ‘B’. We shall be studying this in some depth later in this book. Notice that the examples of A and B that were given both contain 8 bits. This is not an accident. Quite often 8 bits are grouped together like this. A group of bits like this is called a byte. Computer memory is measured in bytes. One byte is commonly made up of eight bits. A computer’s memory will contain a large number of these bytes otherwise it could not store very much data. Generally, the more bytes that can be stored, the more useful the memory will be.
SAQs 3 Complete the following sentences by using the correct words from this list: memory, megabytes, eight, gigabytes. a A school computer has 512 ……… of memory. b A byte is a unit of computer ……… and it consists of ……… bits. 4 Put the following in order of size, smallest first: 1 terabyte, 2048 kB, 1.2 GB, 16 bits, 1.5 MB.
Backing storage A storage device is used to store programs and data when the processor is turned off. The programs and data must be put back into the processor’s memory when needed for use. This is why there are two arrows on Figure 1.5, to show programs and data going into and out of the storage device.
Figure 1.7
Inside a PC.
Types and components of computer systems 5