Programming languages

How are directions to be expressed to the computer? The computer is not another human being with whom one can speak easily and clearly in common English. For expressing directions to a computer the programmer in practice uses special programming languages.

Because computers can accept letters and numbers, nearly all the programming languages express the directions in some combination of letters and numbers.

The programming languages in use fall into three general categories in terms of their similarity to ordinary English: machine languages, symbolic languages, and automatic coding languages. In terms of their importance for computer utilization, the machine languages are the most basic, for the computers can use them directly. But symbolic and automatic coding languages are more convenient for the programmer use because they are more similar to English.

Some programming languages are used only with a particular model of computer; some are used with more than one model of computer. For the convenience of the programmer, a language that can be used with several different models of computer is the more useful.

Instructions in a machine language are almost always represented by particular combinations of letters and numbers acceptable to a given computer. Programs written in an appropriate machine language can be directly accepted and used by a computer.

Symbolic languages use symbolic addresses in the operands and usually also as the addresses for the instructions. This is in contrast to machine languages, which use absolute addresses. An absolute address is one expressed in machine language. It identifies a specific and physical location of data in storage. An indirect address is an absolute or symbolic address which has as its contents the absolute address (usually) of the operand needed by the instruction. Indirect addresses allow greater flexibility in programming because the programmer by changing the contents of indirect addresses can, in effect, modify a program.

For data description in the symbolic languages, the programmer uses special commands. Being able to use these commands simplifies the process of data description, because often these commands can be used with symbolic addresses as their operands. Thus in many programming languages the programmer can assign addresses in symbolic, relative, or absolute form, depending upon the character of the language and what is most convenient for the programmer at the time.

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