DOS is an acronym for Disk Operating System (OS). A disk operating system is software that organizes and controls how computers read, write and interact with disks (floppy, hard, CD-ROM) and communicate with a computer's various input/output devices: keyboards, pointing devices, scanners, microphones, serial and parallel ports; screens, printers, modems, etc. The first two DOS variants came out concurrently, with the introduction of IBM's PC in August 1981: IBM PC-DOS, and Microsoft MS-DOS. There are now many DOSes: 4DOS (a DOS shell, not an OS), DR-DOS (Novell DOS), FreeDOS, MS-DOS, OpenDOS, PC-DOS, PTS-DOS, REAL-32, RTXDOS, RxDOS, TSX-32, and more, even real-time DOS. All DOSes have monolithic architectures, though FreeDOS 32 will be componentized. On this page, links are arranged in three groups and levels: 1) Top group: issues spanning multiple unrelated DOSs. 2) Middle group: graphic user interfaces for DOS. 3) Bottom group: specific DOSs.