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, 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. IBM was forced to quickly develop four additional systems:
- BPS/360 for machines with at least 8 KB of core memory and a punched card reader,
- BOS/360 for machines with at least 8 KB memory and a disk drive,
- DOS/360 for machines with at least 16 KB memory and a disk drive,
- TOS/360 for machines with at least 16 KB memory and a tape drive.
When OS/360 was finally released, a year late, it required at least 64 KB of memory. DOS was designed to use little memory, and could run on 16 kB machines, a configuration available on the low-end S/360 model 30. Unlike OS/360, DOS/360 was initially a single-job system which did not support multitasking. A version with multitasking, supporting up to three memory partitions, requiring 32 kB of memory was later released. Despite its limitations, DOS/360 became the most widely used operating system for processors with less than 256 KB of memory, 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.
Read more about this topic: DOS/360 And Successors
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