You should now be beginning to build up a picture of what a computer is: you know it needs input and output devices to communicate with the world outside and a processor to carry out the instructions that are programmed into it. But where are these instructions stored within the computer? The answer is that they are stored within what is called the computer's main memory, along with any data needed to carry them out.
However, the main memory in computers like PCs is much too small to hold all of the programs and associated data that their users need. In addition, main memory does not hold onto its contents when the computer is switched off. So users must be able to call up the programs they want, and also store and read back the files they have generated with these programs, from some form of capacious and retentive memory. This memory is called secondary memory, and there are two types, removable and permanent. With removable secondary memory the user can store files and then ‘remove’ them from the PC, either to ensure there are copies if the computer fails, or to transport the files to another PC. New software can be downloaded from the web or installed from removable secondary memory. Removable secondary memory includes floppy disks, CD-ROMs, memory cards and DVDs. In contrast, permanent secondary memory is ‘permanently’ attached to the PC and is usually only removed if the PC is undergoing some maintenance or repair. A typical example of permanent secondary memory is a computer's hard disk – so called because it consists of one or more rigid magnetic disks rotating about a central axle. It is common practice to copy the files stored on permanent secondary memory onto some removable secondary memory as a backup in case of disk failure.
Although programs and associated data are stored on the hard disk when not actually in use, both the programs and the data must be copied into the computer's main memory before the processor can execute the instructions or use the data.
The list of features in Figure 2 shows that this laptop has 6 GB of main memory and a hard drive that provides 500 GB of secondary memory. Note that I used the term ‘hard disk’, but in the advert the term ‘hard drive’ is used to refer to the permanent secondary memory. These terms are often used synonymously, though in fact there is a subtle difference which will be explained later.
Activity 1 (Exploratory)
What other secondary memory device or devices are used by the PC in the advert shown in Figure 2?
The list of ‘extras’ shows that the laptop also uses SD cards as removable secondary memory.
All computers have main memory, but not all will have secondary memory. In an ‘invisible’ computer such as the central-heating controller, the software is already stored in the main memory when the computer is purchased. The software is said to be already installed. The PC you are using on this course will have come with some software already installed on it – the software the PC needs to start up when you switch it on. But a key difference between an ‘invisible’ computer like the one in the central-heating controller and the PC is that users cannot install any additional software on the ‘invisible’ computer, whereas they can and do install their own choice of software onto a PC. They do this by copying computer programs into the secondary memory. Such programs are then taken into the main memory when the program is run.