• The DOS/360 initial/simple operating system for the IBM System/360 family of mainframe
computers (it later became DOS/VSE, and was eventually just called VSE).
• The DOS operating system for DEC PDP-11 minicomputers (this OS and the computers it
ran on were nearly obsolete by the time PCs became common, with various descendants and other
replacements).
• DOS for the IBM PC compatible platform
The best known family of operating systems named "DOS" is that running on IBM PCs type
hardware using the Intel CPUs or their compatible cousins from other makers. Any DOS in this
family is usually just referred to as DOS. The original was licensed to IBM by Microsoft, and
marketed by them as "PC-DOS". When Microsoft licenced it to other hardware manufacturers, it
was called MS-DOS. Digital Research produced a compatible variant known as "DR-DOS", which
was eventually taken over (after a buyout of Digital Research) by Novell. This became "OpenDOS"
for a while after the relevant division of Novell was sold to Caldera International, now called SCO.
There is also a free version named "FreeDOS".
DOS consists of an input/output system, a command processor and several utilities. The
utilities are individual program files found on your DOS disk. While part of DOS, these files are not
needed often enough to make it necessary or practical to keep them in the computer's RAM all the
time. FORMAT.COM, the program that formats blank disks, is an example of a DOS utility.
Sometimes these utilities are called external commands (as opposed to internal commands which are included as part of the file COMMAND.COM and remain resident in memory at all times; e.g.,
DIR and COPY).
The command processor is also a file you see on the disk, but once read into the computer's
memory, it usually resides there. Some programs provide their own command processor, and there
are times when the command processor will be overwritten in memory by a program and have to be
reloaded when the program stops executing.
The input/output system consists of two files and a ROM (Read Only Memory) chip. While the two
files are on your disks and are loaded into memory when the computer starts, they are normally
hidden from your view and not available to you for changing.
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