DOS consists of an input/output system, a command processor and several utilities. The utilities are individual program files found on your DOS disk. While part of DOS, these files are not needed often enough to make it necessary or practical to keep them in the computer's RAM all the time. FORMAT.COM, the program that formats blank disks, is an example of a DOS utility. Sometimes these utilities are called external commands (as opposed to internal commands which are included as part of the file COMMAND.COM and remain resident in memory at all times; e.g., DIR and COPY).

The command processor is also a file you see on the disk, but once read into the computer's memory, it usually resides there. Some programs provide their own command processor, and there are times when the command processor will be overwritten in memory by a program and have to be reloaded when the program stops executing.

The input/output system consists of two files and a ROM (Read Only Memory) chip. While the two files are on your disks and are loaded into memory when the computer starts, they are normally hidden from your view and not available to you for changing.

Input/Output System

This most primitive of the DOS systems has two parts:

• BIOS (Basic Input/Output System). These are the fundamental routines that control the keyboard, video display and other peripherals. The BIOS is comprised of a ROM on the computer's main circuit board and the file IBMBIO.COM (or IO.SYS), one of the two hidden files on your disk.
• Operating System. This is the main file-handling system for the computer. Actually, two systems exist: one for disk-based files and one for non-disk peripheral devices. They are in hidden file IBMDOS.COM (or MSDOS.SYS). (IBMBIO and IBMDOS are IBM names; MS-DOS uses IO.SYS and MSDOS.SYS.)

The two systems are necessary because non-disk peripherals demand their data as strings of characters, while disks move information in large groups, known as blocks.

Command Processor

The command processor (COMMAND.COM on your disk) performs three tasks:

• It handles critical interrupts...that is, COMMAND.COM takes care of all demands for attention by parts of the computer. The user typing the Control-Break program break command is an example of an interrupt.
• It handles critical errors...that is, COMMAND.COM takes care of problems. For example, if you leave the disk drive door open during a disk operation COMMAND.COM is responsible for the error message you will see.
• It performs end-of-program housekeeping...that is, COMMAND.COM takes care of making the computer's memory available for other programs and reloading parts of itself if the program wrote over them.

COMMAND.COM also places the C> prompt on the screen and interprets any command(s) you might type. In short, the command processor tells the rest of DOS what to do.

Starting a Computer