Dos, the Disk Operating System

Turning the personal computer on is not enough; it must have instructions for the tasks it is to perform. These instructions come in the form of software. The primary tasks and functions the personal computer performs come from the instructions in systems programs.

You have already learned that there are specific systems software instructions embedded in chips that make up ROM (read only memory). These instructions help the computer give itself a checkup when you switch the power on, to make sure everything is in working order. Most of these instruc­tions occur without our being aware of them. However, one that you often can see when you first turn the power on is the computer checking its RAM.

The other aspect of systems software, the one we can actually interact with, is called the operating system. The operating system for the personal computer is called DOS, for disk operating system. DOS must be loaded into the personal computer's main memory before it can be used. There are also a number of instructions given to the computer by the operating system. Many are unavailable to you, the knowledge worker. One that is very important that you can't see is the Basic Input/ Output System (BIOS). It contains instructions specific to the computer you are using; for example, there is a BIOS that only works on an IBM PC.

When you load the operating system into the computer's memory, it is called a boot or booting the computer. The term comes from the old saying, "Pulling himself up by his own bootstraps," meaning someone who is able to do something on his own. Starting a program in the early days of computers was called bootstrapping, shortened to boot. Once booting is completed, you can issue DOS instructions and use application programs such as word processing.

DOS awakens the hardware and allows you to control the personal computer. It is responsible for managing all the input and output tasks, such as assuring that when you type a character on the keyboard, that same exact character appears on the screen and is stored in memory. Think of yourself as the personal computer's manager; DOS makes sure all your instructions are carried out accurately and promptly.

Dos Files

DOS is actually a collection of programs, each of which performs specific tasks. Programs and data are stored in files. The file is a group of related records and the primary unit of data storage in DOS-based computers (as well as most others). It is very similar to the file folder in a common filing cabinet. Just as a file folder might contain memos with instructions or informational letters and reports, so a DOS file can contain either program instructions or data. There are files that are unique to DOS; files that both DOS and an application uses; and files created by the user while working with an application. DOS uses files itself, performs operations on applications files, and allows you to create your own files.

Dos Filenames

Each file in DOS has a unique filename, which can be up to eight characters long, followed by an optional period or dot and three-character filename extension. DOS requires an .EXE or .COM extension for a program file, but you can give your files any extension (or no extension) you want.

There are five different types of files used in DOS itself, and they are distinguished from one another by their filename extensions. Of the five different types,.COM, .EXE, and .SYS are the most commonly used. Batch (.BAT) files contain instructions that perform a special grouping of commands and are often created by the user. .CPI files are used by skilled programmers.

The two types of files you'll work with most commonly in DOS are .COM and .EXE files because they perform the widest variety of tasks. For example, PRINT.COM will print a file, and DOS executable (.EXE) files are used to start almost all application programs.

.COM A command file that issues a specific command

.EXE An executable program file, more complex than

a .COM file

.SYS An operating system file

.BAT A batch file, which starts other programs

.CPI A Code Page Information file