DOS

In spite of the common usage, none of these systems were named simply "DOS" (a name given only to an unrelated IBM mainframe operating system in the 1960s). A number of unrelated, non-x86 microcomputer disk operating systems had "DOS" in their name, and are often referred to simply as "DOS" when discussing machines that use them (e.g. AmigaDOS, AMSDOS, ANDOS, Apple DOS, Atari DOS, Commodore DOS, CSI-DOS, ProDOS, and TRS-DOS). While providing many of the same operating system functions for their respective computer systems, programs running under any one of these operating systems would not run under others.

All DOS-type operating systems run on machines with the Intel x86 or compatible CPUs, mainly the IBM PC and compatibles. Initially, DOS was not restricted to these, and machine-dependent versions of DOS and similar operating systems were produced for many non-IBM-compatible x86-based machines. In particular, DOS-C's predecessor DOS/NT ran on Motorola 68000 CPU's.

The DOS kernel provides various functions for programs, like displaying characters on-screen, reading a character from the keyboard, accessing disk files and more.

DOS by default provides a primitive ability for shell scripting, via batch files (with the filename extension .BAT). These are text files that can be created in any DOS text editor, such as the MS-DOS Editor. They are executed in the same fashion as compiled programs, and run each line of the batch file as a command. Batch files can also make use of several internal commands, such as goto and conditional statements. gosub and simple arithmetic is supported in some third-party shells but can also be faked via strange workarounds; however, no real form of programming is usually enabled.

There are reserved device names in DOS that cannot be used as filenames regardless of extension; these are used to send application output to hardware peripherals. These restrictions also affect several Windows versions, in some cases causing crashes and security vulnerabilities.

In DOS, drives are referred to by identifying letters. Standard practice is to reserve "A" and "B" for floppy drives. On systems with only one floppy drive DOS assigns both letters to the drive, prompting the user to swap disks as programs alternate access between them. This facilitates copying from floppy to floppy or having a program run from one floppy while accessing its data on another. Hard drives were originally assigned the letters "C" and "D". DOS could only support one active partition per drive. As support for more hard drives became available, this developed into first assigning a drive letter to each drive's active primary partition, then making a second pass over the drives to allocate letters to logical drives in the extended partition, then a third pass to give any other non-active primary partitions their names (where such additional partitions existed and contained a DOS-supported file system.) Lastly, DOS allocates letters for optical disc drives, RAM disks, and other hardware. Letter assignments usually occur in the order the drivers are loaded, but the drivers can instruct DOS to assign a different letter; drivers for network drives, for example, typically assign letters nearer the end of the alphabet.

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