DOS

DOS, short for "Disk Operating System",[1] is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.

Related systems include MS-DOS, PC-DOS, DR-DOS, FreeDOS, PTS-DOS, ROM-DOS, Caldera DOS, Novell DOS and several others.

In spite of the common usage, none of these systems were simply named "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.

User interface

DOS systems utilize a command line interface. Programs are started by entering their filename at the command prompt. DOS systems include several programs as system utilities, and provides additional commands that don't correspond to programs (internal commands).

In an attempt to provide a more user-friendly environment, numerous software manufacturers wrote file management programs that provided users with menu- and/or icon-based interfaces. Microsoft Windows is a notable example, eventually resulting in Microsoft Windows 9x becoming a self-contained program loader, and replacing DOS as the most-used PC-compatible program loader. Text user interface programs included Norton Commander, Dos Navigator, Volkov Commander, Quarterdesk DESQview, and SideKick. Graphical user interface programs included Digital Research's Graphical Environment Manager (originally written for CP/M) and GEOS.

Eventually, the manufacturers of major DOS systems began to include their own environment managers. MS-DOS/IBM DOS 4 included DOS Shell;[36] DR-DOS 5, released the next year, included ViewMAX, based upon GEM.

Limitations

Several limitations plague the DOS architecture. The original 8088 microprocessor could only address 1 megabyte of physical RAM. With additional hardware devices being mapped into this range, the highest amount of available memory was 640 kilobytes, known as conventional memory. Due to DOS's structure, this was assumed to be the maximum, and DOS could not address more than this. An early workaround was expanded memory; later, extended memory was developed with the 80286. While these provided usable memory to applications, they still had to start in conventional memory, thereby using part of the existing 640 KB. With the 80386 microprocessor's redesigned protected mode, DOS extenders and the DOS Protected Mode Interface were able to provide additional memory to applications, as well as multitasking.

DOS also has an upper limit to the size of hard disk partitions. This has two causes. First, many DOS-type systems never had support for any file system newer than FAT16, which, by design, does not allow partitions larger than 2.1 gigabytes.[43] Additionally, DOS accesses the hard disk by calling Interrupt 13, which utilizes the cylinder-head-sector system of mapping the disk. Under this system, only 8 gigabytes are visible to the operating system.[44] Newer operating systems accomplished disk access via software means, e.g. 32-bit disk access.

Using FAT16 (and FAT12 for floppy disks) required use of the 8.3 filename. Filenames in DOS can not be longer than eight characters, and the filename extension cannot be longer than three. Win95's patented VFAT hack worked around this in a unique way.