1.4 Machine Language, Assembly, and High-Level Language

Programmers write instructions in various programming languages, some directly understandable by computers and others requiring intermediate translation steps. Hundreds of such languages are in use today. These may be divided into three general types: 1. Machine languages. 2. Assembly languages. 3. High-level languages. Machine Languages Any computer can directly understand only its own machine language, defined by its hardware design. Machine languages generally consist of strings of numbers (ultimately reduced to 1s and 0s) that instruct computers to perform their most elementary operations one at a time. Machine languages are machine dependent (a particular machine language can be used on only one type of computer). Such languages are cumbersome for humans. For example, here’s a section of an early machine-language payroll program that adds overtime pay to base pay and stores the result in gross pay: +1300042774 +1400593419 +1200274027 Assembly Languages and Assemblers Programming in machine language was simply too slow and tedious for most programmers. Instead of using the strings of numbers that computers could directly understand, programmers began using English-like abbreviations to represent elementary operations. These abbreviations formed the basis of assembly languages. Translator programs called assemblers were developed to convert assembly-language programs to machine language at computer speeds. The following section of an assembly-language payroll program also adds overtime pay to base pay and stores the result in gross pay: load basepay add overpay store grosspay Although such code is clearer to humans, it’s incomprehensible to computers until translated to machine language. 10 Introduction to Computers and Python High-Level Languages and Compilers With the advent of assembly languages, computer usage increased rapidly, but programmers still had to use numerous instructions to accomplish even the simplest tasks. To speed the programming process, high-level languages were developed in which single statements could be written to accomplish substantial tasks. A typical high-level-language program contains many statements, known as the program’s source code. Translator programs called compilers convert high-level-language source code into machine language. High-level languages allow you to write instructions that look almost like everyday English and contain commonly used mathematical notations. A payroll program written in a high-level language might contain a single statement such as grossPay = basePay + overTimePay From the programmer’s standpoint, high-level languages are preferable to machine and assembly languages. Python is among the world’s most widely used high-level programming languages. Interpreters Compiling a large high-level language program into machine language can take considerable computer time. Interpreter programs, developed to execute high-level language programs directly, avoid the delay of compilation, although they run slower than compiled programs. The most widely used Python implementation—CPython (which is written in the C programming language)—uses a clever mixture of compilation and interpretation to run programs.6