MS-DOS is an operating system created by Microsoft in 1982 to run on X86 systems and was the dominant operating system on the market until the rise of Windows. Microsoft produced several different versions of the OS which culminated with version 6.22 in 1994, but Windows still carries an updated version of the code as its command-line interface. Many of the operating system's characteristics have survived into modern systems and influence OS design even today.
Command Line Interface
MS-DOS uses a command line interface, forcing the user to input commands at a prompt. The command set for DOS is limited. It consists mainly of navigation and file manipulation commands. Most complex operations are handled by the programs themselves. Since users have to enter commands in sequence, they could create batch files, which are text files listing a series of commands that the OS would execute in order. One common example of this is AUTOEXEC.BAT, a batch file that the OS runs on first boot and contains commands to set up the computing environment.
Because Microsoft intended MS-DOS to run on early PCs without much in the way of RAM, MS-DOS features no ability for programs to multitask. When the user runs a program, that program consumes system resources until it stops, and then another program may access memory. Certain types of programs called TSRs, for Terminate and Stay Resident, can park themselves in memory, but these programs are vulnerable to memory overlap and frequently crashed. The first versions of Windows introduced true multitasking to PCs.
One distinctive characteristic of MS-DOS is its "8+3" file naming system. File names are limited to eight characters, with a three-character suffix denoting the type of file. For instance, TXT and DOC denote text files, while COM and EXE indicate executables and SYS is reserved for system files. Windows has since done away with the eight-character limit, although file suffixes still indicate to the OS how to handle each type of file, and many of the original suffixes from the early days of MS-DOS still exist.
Another prominent characteristic of MS-DOS is its use of letters to refer to drives. Typically, A and B designate floppy drives, while C and anything later in the alphabet refers to hard drives or optical drives. Users have the ability to map any drive letter they wish, although typically they would assign them in alphabetical order. Today, Windows still uses drive letters, and still reserves A and B by default, despite the fact that modern PCs typically do not contain floppy drives.