The Free On-line Dictionary of Computing (30 December 2018):

Microsoft Disk Operating System
Microsoft DOS
MS-DOS

    /M S doss/ (Or "MS-DOS", "PC-DOS",
   "MS-DOG", "mess-dos") Microsoft Corporation's clone of
   the CP/M disk operating system for the 8088 crufted
   together in 6 weeks by hacker Tim Paterson, who is said to
   have regretted it ever since.

   MS-DOS is a single user operating system that runs one
   program at a time and is limited to working with one megabyte
   of memory, 640 kilobytes of which is usable for the
   application program.  Special add-on EMS memory boards
   allow EMS-compliant software to exceed the 1 MB limit.
   Add-ons to DOS, such as Microsoft Windows and DESQview,
   take advantage of EMS and allow the user to have multiple
   applications loaded at once and switch between them.

   Numerous features, including vaguely Unix-like but rather
   broken support for subdirectories, I/O redirection and
   pipelines, were hacked into MS-DOS 2.0 and subsequent
   versions; as a result, there are two or more incompatible
   versions of many system calls, and MS-DOS programmers can
   never agree on basic things like what character to use as an
   option switch ("-" or "/").  The resulting mess became the
   highest-unit-volume operating system in history.  It was
   used on many Intel 16 and 32 bit microprocessors and IBM
   PC compatibles.

   Many of the original DOS functions were calls to BASIC (in
   ROM on the original IBM PC), e.g. Format and Mode.  People
   with non-IBM PCs had to buy MS-Basic (later called
   GWBasic).  Most version of DOS came with some version of
   BASIC.

   Also know as PC-DOS or simply DOS, ignoring the fact that
   there were many other OSes with that name, starting in the
   mid-1960s with IBM's first disk operating system for the
   IBM 360.

   [Jargon File]

   (2007-05-21)