Editing DOS/360 and successors (section)

==History ==
When developing a new hardware generation of unified [[System/360]] (or S/360) computers, IBM had originally committed to delivering a single operating system, [[OS/360 and successors|OS/360]], also compatible with low-end machines; but hardware was already available and the OS/360 project fell further and further behind schedule, as described at length by [[Fred Brooks]] in [[The Mythical Man-Month]]. IBM was forced to quickly develop four additional systems: 
* [[IBM Basic Programming Support|BPS/360]] for machines with at least 8&nbsp;KB of [[core memory]] and a [[punched card]] reader,
* [[BOS/360]] for machines with at least 8&nbsp;KB memory and a [[disk drive]],
* DOS/360 for machines with at least 16&nbsp;KB memory and a [[disk drive]],
* TOS/360 for machines with at least 16&nbsp;KB memory and a [[tape drive]].<ref group=NB>DOS/360 and TOS/360 had the same code base; the difference was whether it used disk libraries or tape libraries.</ref>
When OS/360 was finally released, a year late, it required at least 64&nbsp;KB of memory. DOS was designed to use little memory, and could run on 16&nbsp;KB machines, a configuration available on the low-end [[IBM 2030|S/360 model 30]]. Unlike OS/360, DOS/360 was initially a single-job system which did not support [[Computer multitasking|multitasking]].  A version with multitasking, supporting up to three [[Memory management (operating systems)#Partitioned allocation|memory partitions]], requiring 32&nbsp;KB of memory was later released. Despite its limitations, DOS/360 became the most widely used operating system for processors with less than 256&nbsp;KB of memory{{citation needed|date=September 2017}} because: System/360 hardware sold very well; DOS/360 ran well on System/360 processors which medium-sized organizations could afford; and it was better than the "operating systems" these customers had before.

DOS/360 was the operating system which filled the time gap between the announcement of the System/360 and the availability of the intended operating system, OS/360. As a result of the delay, a number of customers implemented DOS systems and committed significant investments to run them.  IBM expected that DOS/360 users would soon upgrade to OS/360, but as a result of those investments, they were reluctant to commit to such conversion. IBM then needed to continue to offer DOS/360 as an additional operating system. The [[Hacker's Jargon File]] incorrectly states that [[GECOS]] (also known as GCOS) was copied from DOS/360, which was not the case, however the [[Xerox Data Systems]] [[Xerox Operating System]] (XOS) was intentionally similar to DOS to simplify program porting.