Computer programs, known as software, are instructions that tell a computer what to do. Computers do not understand human languages, so programs must be written in a language a computer can use. There are hundreds of programming languages, and they were developed to make the programming process easier for people. However, all programs must be converted into the instructions the computer can execute.

Machine Language

A computer’s native language, which differs among different types of computers, is its machine language—a set of built-in primitive instructions. These instructions are in the form of binary code, so if you want to give a computer an instruction in its native language, you have to enter the instruction as binary code. For example, to add two numbers, you might have to write an instruction in binary code, like this:

1101101010011010

Assembly Language

Programming in machine language is a tedious process. Moreover, programs written in machine language are very difficult to read and modify. For this reason, assembly language was created in the early days of computing as an alternative to machine languages. Assembly language uses a short descriptive word, known as a mnemonic, to represent each of the machine-language instructions. For example, the mnemonic add typically means to add numbers and sub means to subtract numbers. To add the numbers 2 and 3 and get the result, you might write an instruction in assembly code like this:

add 2, 3, result

Assembly languages were developed to make programming easier. However, because the computer cannot execute assembly language, another program—called an assembler—is used to translate assembly-language programs into machine code. Writing code in assembly language is easier than in machine language. However, it is still tedious to write code in assembly language. An instruction in assembly language essentially corresponds to an instruction in machine code. Writing in assembly requires that you know how the CPU works. Assembly language is referred to as a low-level language, because assembly language is close in nature to machine language and is machine dependent.

High-Level Language

In the 1950s, a new generation of programming languages known as high-level languages emerged. They are platform independent, which means that you can write a program in a high-level language and run it in different types of machines. High-level languages are English-like and easy to learn and use. The instructions in a high-level programming language are called statements. Here, for example, is a high-level language statement that computes the area of a circle with a radius of 5:

area = 5 * 5 * 3.14159;

There are many high-level programming languages, and each was designed for a specific purpose. A program written in a high-level language is called a source program or source code. Because a computer cannot execute a source program, a source program must be translated into machine code for execution. The translation can be done using another programming tool called an interpreter or a compiler.

• An interpreter reads one statement from the source code, translates it to the machine code or virtual machine code, and then executes it right away. Note that a statement from the source code may be translated into several machine instructions.
• A compiler translates the entire source code into a machine-code file, and the machine-code file is then executed.