DOS - Design

Design

All MS-DOS-type operating systems run on machines with the Intel x86 or compatible CPUs, mainly the IBM PC and compatibles. Machine-dependent versions of MS-DOS were produced for many non-IBM-compatible x86-based machines, with variations from relabelling of the Microsoft distribution under the manufacturer's name, to versions specifically designed to work with non-IBM-PC-compatible hardware. For as long as application programs used DOS APIs instead of direct hardware access, they could thereby also run on non-IBM-PC compatible machines. In 1985, Digital Research also had a version of Concurrent DOS 68K for use on Motorola 68000 CPUs, and the original FreeDOS kernel DOS-C derived from DOS/NT, also for Motorola CPUs, in the early 1990s. While these systems resembled the DOS architecture, applications were not binary compatible due to the incompatible instruction sets of these non-x86-CPUs. However, applications written in high-level languages could be ported easily.

DOS is a single-user, single-tasking operating system with basic kernel functions that are non-reentrant: only one program at a time can use them and DOS itself has no functionality to allow more than one program it execute at a time. The DOS kernel provides various functions for programs (an application program interface), like character I/O, file management, memory management, program loading and termination.

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 text 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 with the DR DOS COMMAND.COM as well as some with third-party shells like 4DOS; however, no real form of programming is usually enabled.

The operating system offers an application programming interface that allows development of character-based applications, but not for accessing most of the hardware, such as graphics cards, printers, or mice. This required programmers to access the hardware directly, usually resulting in each application having its own set of device drivers for each hardware peripheral. Hardware manufacturers would release specifications to ensure device drivers for popular applications were available.