DOS/360 and successors

Not to be confused with DOS.

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.

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. 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; its successor z/VSE is still widely used as of 2006.

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

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:

• 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 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, 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 data sets were normally read one record at a time from beginning to end.
• In indexed (ISAM) files a specified section of each record was defined as a key which could be used to look up specific records.
• In direct access (BDAM) files, the application program had to specify the physical location on the disk of the data it wanted to access. BDAM programming was not easy and most customers never used it themselves; but it was the fastest way to access data on disks and many software companies used it in their products, especially database management systems such as ADABAS, IDMS and IBM's DL/I.

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:

• The first version could run only one program at a time. A later enhancement allowed 3 at the same time, in one of 3 "partitions" whose size was set by each customer when DOS/360 was installed.
• The JCL it used for submitting jobs was designed to be easy for the low-end machines to process, and as a result programmers did not find it easy to read or write.
• There was no spooling sub-system to improve the efficiency of punched card and printer use. In the late 1960s Software Design, Inc, an independent software company, started selling a spooler called GRASP.
• DOS/360 had no relocating loader, so users had to link edit a separate executable version of each program for each partition in which the program was likely to be run.
• Executable programs were stored in the Core Image Library, which did not reclaim space when programs were deleted or replaced by newer versions. When the Core Image Library became full, it had to be compressed by one of the utility programs, and this could halt development work for as much as half a day.
• Its application programming interface was different from that of OS/360. Programs written in high level languages such as COBOL needed small modifications before they could be used with OS/360 and Assembler programs needed larger changes.

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 MVT and its 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.

External links

• Disk Operating System — Entry in the FOLDOC
• Online Demo of DOS-VS/POWER Rel 34

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.