Programming Essays - Programming Language Implementation

Programming PROGRAMMING A program is a sequence of instructions that tells the hardware of a computer what operations to perform on data. Programs can be built into the hardware itself, or they may exist independently in a form known as software. In some specialized, or dedicated, computers the operating instructions are embedded in their circuitry; common examples are the microcomputers found in calculators, wristwatches, automobile engines, and microwave ovens. A general-pur pose computer, on the other hand, contains some built-in programs (in ROM) or instructions (in the processor chip), but it depends on external programs to perform useful tasks. Once a computer has been programmed, it can do only as much or as little as the software controlling it at any given moment enables it to do. Software in widespread use includes a wide range of applications programs-instructions to the computer on how to perform various tasks. Languages. A computer must be given instructions in a language that it understands-that is, a particular pattern of binary digital information. On the earliest computers, programming was a difficult, laborious task, because vacuum-tube ON-OFF switches had to be set by hand. Teams of programmers often took days to program simple tasks such as sorting a list of names. Since that time a number of computer languages have been devised, some with particular kinds of functioning in mind and others aimed more at ease of use-the user-friendly approach. Machine Language. Unfortunately, the computer's own binary-based language, or machine language, is difficult for humans to use. The programmer must input every command and all data in binary form, and a basic operation such as comparing the contents of a register to the data in a memory-chip location might look like this: 11001010 00010111 11110101 00101011. Machine-language programming is such a tedious, time consuming, task that the time saved in running the program rarely justifies the days or weeks needed to write the program. Assembly Language. One method programmers devised to shorten and simplify the process is called assembly-language programming. By assigning a short (usually three-letter) mnemonic code to each machine-language command, assembly-language programs could be written and debugged-cleaned of logic and data errors-in a fraction of the time needed by machine-language programmers. In assembly language, each mnemonic command and its symbolic operands equals one machine instruction. An assembler program translates the mnemonic opcodes (operation codes) and symbolic operands into binary language and executes the program. Assembly language, however, can be used only with one type of CPU chip or microprocessor. Programmers who expended much time and effort to learn how to program one computer had to learn a new programming style each time they worked on another machine. What was needed was a shorthand method by which one symbolic statement could represent a sequence of many machine-language instructions, and a way that would allow the same program to run on several types of machines. These needs led to the development of so-called high-level languages. High-Level Languages. High-level languages often use English-like words-for example, LIST, PRINT, OPEN, and so on-as commands that might stand for a sequence of tens or hundreds of machine-language instructions. The commands are entered from the keyboard or from a program in memory or in a storage device, and they are intercepted by a program that translates them into machine-language instructions. Translator programs are of two kinds: interpreters and compilers. With an interpreter, programs that loop back to re-execute part of their instructions reinterpret the same instruction each time it appears, so interpreted programs run much more slowly than machine-language programs. Compilers, by contrast, translate an entire program into machine language prior to execution, so such programs run as rapidly as though they were written directly in machine language. The first commercial programmer was probably Grace Hopper (1906-92), an American. After programming an experimental computer at Harvard University, she worked on the UNIVAC I and II computers and developed a commercially usable high-level programming language called FLOWMATIC. To facilitate computer use in scientific applications, IBM then developed a language that would simplify work involving complicated mathematical formulas. Begun in 1954 and completed in 1957, FORTRAN (FORmula TRANslator) was the first comprehensive high-level programming language that was widely used. In 1957, the Association for Computing