DOS/360 and successors

History of IBM mainframe
operating systems

On early mainframe computers:

On S/360 and successors:

  • DOS/360 and successors 1966
    • DOS/VS 1972
    • DOS/VSE 1980s
    • VSE/SP late 1980s
    • VSE/ESA 1991
    • z/VSE 2005

Disk Operating System/360, also DOS/360, or simply DOS, was an operating system for IBM mainframes. It was announced by IBM on the last day of 1964, and it was first delivered in June 1966.[1]

DOS/VS was further development, released in 1972, as the virtual memory mechanism became available on new System/370 series hardware.

In 1979 IBM offered a feature on the 4341 called ECPS:VSE.[2] that provided a single-level storage for the I/O channels, not just the processor, and introduced DOS/VSE as the software support, although it could also run on a System/370 without that feature. DOS/VSE was the last free version of DOS.

For a brief period IBM offered program products that installed on top of DOS/VSE, but it then replaced both the add-on products and DOS/VSE with VSE/SP. VSE/SP supported only 24-bit addresses, despite cutomer requests to provide an XA (31 bit) version. IBM eventually provided a 31-bit version, VSE/ESA, but it would only run in S/370-ESA mode, not in the older XA mode.

IBM later released z/VSE, which is the most current version since 2005. In its time DOS was the most widely used operating system in the world;[3] its successor z/VSE is still widely used as of 2006.[4]

Although their names are similar, there are no technical similarities between DOS/360 of the 1960s and the x86-DOS of the 1980s.

Contents

DOS/360

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:

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 a single-job system, i.e. it did not support multitasking (although a version with limited multitasking, requiring 32 kB of memory, was later released). It used three memory partitions, but had little protection against programs overwriting other partitions. The three partitions were not specialised, but frequently one was used for spooling punched cards to disk, one for batch job execution, and one for spooling from disk to printers. Despite its limitations, DOS/360 became the most widely used operating system for processors with less than 256 KB of memory,[5] 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.

Technical details

DOS/360 had a good set of utility programs, an Assembler, and compilers for FORTRAN, COBOL and eventually PL/I. And it supported a useful range of file organizations with access methods to help in using them:

Sequential and ISAM files could store either fixed-length or variable-length records, and all types could occupy more than one disk volume.

DOS/360 also offered BTAM, a data communications facility which was primitive and hard to use by today's standards. But BTAM could communicate with almost any type of terminal, which was a big advantage at a time when there was hardly any standardization of communications protocols.

But DOS/360 had significant limitations compared with OS/360, which was used to control most larger System/360 machines:

DOS/VS

DOS/VS is the successor to DOS/360, introduced to support the virtual memory capabilities of System/370. It added virtual storage capability through a fixed page table which strictly mapped the real storage with fixed page table entries. Since the DOS/VS system maintained the small system memory scheme limited to 16 megabytes for all partitions combined, user programs and online systems were small and less complex than those found on systems using MFT, MVT and their successors.

DOS/VS increased the number of partitions (separate simultaneous programs) from 3 (named Background, Foreground 1 and Foreground 2) to 7 (BG and F1 through F6).

DOS/VS was succeeded by DOS/VSE.

Notes

  1. ^ DOS/360 and TOS/360 had the same code base; the difference was whether it used disk libraries or tape libraries.

References

  1. ^ E.W. Pugh, L.R. Johnson, and John H. Palmer, IBM's 360 and early 370 systems, MIT Press, Cambridge MA and London, ISBN 0-262-16123-0
    – extensive (819 pp.) treatment of IBM's offerings during this period
  2. ^ IBM (January 1979), IBM 4300 Processors Principles of Operation for ECPS:VSE Mode, First edition, SA22-7070-0, http://www.bitsavers.org/pdf/ibm/43xx/GA22-7070-0_4300_ECPS_VSE_Mode_Principles_Of_Operation_Jan79.pdf. 
  3. ^ Pugh, op. cit., p. 331 – most widely used OS
  4. ^ Jerry Johnston, VSE: A Look at the Past 40 Years (2005)
  5. ^ Ray Saunders, "MVS... And Before OS/360 ?"

External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.