Languages and programs:How software works and What is software?

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What is software?

So what is computer software? It consists of sets of computer instructions, stored on magnetic disk, semiconductor ROM, or other suitable storage device. To run these instructions, the computer loads them from the storage device into RAM, and they are then executed by the CPU. In the case of computers based upon conventional microprocessors, the instructions are executed serially, i.e. one after another. In the case of computers based upon the transputer, many of the instructions are executed in parallel, i.e. at the same time.

The computer needs software to tell it how to carry out its various tasks:

• It needs system software to carry out 'system' tasks like formatting disks, reading files on the disk, controlling the screen display, and so on.

• It needs application software to carry out its various applications, such as writing letters, keeping records, and producing drawings or charts.

I shall be describing both of these types of software in subsequent chapters. In this chapter I shall deal mainly with how software is written. The process of writing software is known as coding, and the resulting code is known as a program.

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How software works

An instruction contained in a computer program takes the form of a sequence of bits (binary Os and ls). When the instruction is executed, these bits travel as electrical pulses to the CPU. Here, logic gates contained in special control circuits make these bits close switches in a second circuit along which the computer data flows. In this way the program operates on data.

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This is represented diagrammatically in Figure 4.1. Each type of action that the computer can perform - such as addition, subtraction, or a logical operation- has associated with it a particular set of logic gates. The bits flowing down the control circuit pass each set of logic gates, but only one set will output a current and so close a switch.

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