MS-DOS provides general, device-independent access to the resources of a computer. The typical MS-DOS computer is a personal or laptop computer based on the 8086 family of microprocessors. The computer operates in real mode and provides devices for mass storage and for input and output—devices such as disk drives, keyboard, screen, and parallel and serial ports.
From a programmer's perspective, the heart of MS-DOS is its system functions, which provide access to the computer's devices and to a wide range of other services, from memory management to national language support.
Programs that use MS-DOS system functions are device-independent—that is, they need no device-specific code to use a given device. Instead, they rely on MS-DOS and its device drivers to handle all device-specific operations.
Even though the number and capabilities of MS-DOS system functions grow with each new version, programs written for the current version can often run with earlier versions as well. A program should always check the version of MS-DOS with which it is running and use this information to determine which MS-DOS features and system functions it can use.