Here's a short story:

DOS is a family of disk operating systems primarily consisting of MS-DOS and a rebranded version under the name IBM PC DOS which were introduced in 1981, as well as some later compatible systems from other manufacturers: DR-DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998). MS-DOS dominated the x86-based IBM PC compatible market between 1981 and 1995. 

Dozens of other operating systems also use the acronym "DOS", including the mainframe DOS/360 from 1966. Others are Apple DOS, Apple ProDOS, Atari DOS, Commodore DOS, TRSDOS, and AmigaDOS. 

IBM PC DOS (and the separately sold MS-DOS) and its predecessor, 86-DOS, resembled Digital Research's CP/M—the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 based microcomputers. DOS instead ran on Intel 8086 16-bit processors. Starting with MS-DOS 1.28 and PC DOS 2.0 the operating system incorporated various features inspired by Xenix, Microsoft's variant of Unix. 

When IBM introduced the IBM PC, built with the Intel 8088 microprocessor, they needed an operating system. Seeking an 8088-compatible build of CP/M, IBM initially approached Microsoft CEO Bill Gates (possibly believing that Microsoft owned CP/M due to the Microsoft Z-80 SoftCard, which allowed CP/M to run on an Apple II). IBM was sent to Digital Research, and a meeting was set up. However, the initial negotiations for the use of CP/M broke down; Digital Research wished to sell CP/M on a royalty basis, while IBM sought a single license, and to change the name to "PC DOS". Digital Research founder Gary Kildall refused, and IBM withdrew. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: what oprocessor did IBM use int heir PC?
Guess: 16-bit

Question: who did IBM initially approach?
Guess: Bill Gates

Question: why did they talkk to him?
Guess: possibly believing that Microsoft owned CP/M

Question: what card might have given that impression?
Guess: the Microsoft Z-80 SoftCard

Question: what did that do?
Guess: allowed CP/M to run on an Apple II

Question: what does DOS stand for?
Guess: disk operating systems

Question: name 2 main versions?
Guess: MS-DOS and  IBM PC DOS

Question: when were they introduced?
Guess: in 1981

Question: which market did MD-DOS dominate?
Guess: the x86-based IBM PC compatible market

Question: what years?
Guess: 1981 and 1995

Question: what year was FreeDOS launched?
Guess: 1998

Question: what launched in 1988
Guess: DR-DOS

Now answer the following question in the same format.

Question: which  mainframe OS uses the acronym?

DOS/360