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# 3.1 Memory in ‘invisible’ computers

Although programs and associated data are stored in secondary memory 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. Secondary memory devices usually make up the bulk of the memory in desktop, laptop and mainframe computers, and other devices such as mobile phones or tablets, but may be completely absent in embedded computer systems, such as control systems for appliances like washing machines and microwave ovens.

Figure 4 A central heating controller.

This means that while all computers have main memory, not all will have secondary memory. In an embedded computer system, such as the central-heating controller in Figure 4, the computer is ‘invisible’ and its software is already stored in the main memory when the computer system 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.

## Activity 1

What other secondary memory device or devices are used by the PC in the advert shown in Figure 1? Make a note in the box below.

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### Discussion

The list of ‘extras’ shows that the laptop also uses SD cards as removable secondary memory.

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