History of IBM Mainframe Operating Systems - System/360 Operating Systems

System/360 Operating Systems

Up to the early 1960s IBM's low-end and high-end systems were incompatible - programs could not easily be transferred from one to another, and the systems often used completely different peripherals (e.g. disk drives). IBM concluded that these factors were increasing its design and production costs for both hardware and software to a level that was unsustainable, and were reducing sales by deterring customers from upgrading. So in 1964 the company announced System/360, a new range of computers which all used the same peripherals and most of which could run the same programs.

IBM originally intended that System/360 should have only one batch-oriented operating system, OS/360. There are at least two accounts of why IBM later decided it should also produce a simpler batch-oriented operating system, DOS/360: because it found that OS/360 would not fit into the limited memory available on the smaller System/360 models; or because it realized that the development of OS/360 would take much longer than expected, and introduced DOS/360 as one of a series of stop-gaps to prevent System/360 hardware sales from collapsing - the others were BOS/360 (Basic Operating System, for the smallest machines) and TOS/360 (Tape Operating System, for machines with only tape drives).

System/360's operating systems were more complex than previous IBM operating systems for several reasons, including:

• They had to support multiprogramming, otherwise the faster CPUs in the range would have spent most of their time waiting for I/O operations (e.g. disk reads) to complete. This meant that the operating systems had to be the real masters of the systems, to provide whatever services the applications validly requested, and to handle crashes or misbehavior in one application without stopping others that were running at the same time.
• They had to support a much wider range of machine sizes. Memory ranged from 16KB to 1MB and processor speeds from a few thousand instructions per second to 500,000.
• System/360's operating systems had to support a wide range of application requirements, for example: some applications only needed to read through sequential files from start to finish; others needed fast, direct access to specific records in very large files; and a few applications spent nearly all their time doing calculations, with very little reading / writing of files.

This was one of the largest software projects anyone had attempted, and it soon ran into trouble, with huge time and cost over-runs and large numbers of bugs. So the company had to release a series of short-lived stop-gaps because:

• To develop and test the planned operating systems it needed to use System/360 hardware. So it first developed Basic Programming Support(BPS), which it used to develop the tools it needed for developing DOS/360 and OS/360 and the first versions of tools its would supply with these operating systems - compilers (FORTRAN and COBOL), utilities including Sort, and above all the Assembler it needed to build all the other software.
• Competitors took advantage of the delays to announce systems aimed at what they thought were the most vulnerable parts of IBM's market.

IBM released 4 stop-gap operating systems to prevent sales of System/360 from collapsing:

• BOS/360 (Basic Operating System), which loaded from a card reader and supported tape drives and a few disks. This system was supplied to beta test customers and may have been an early version of DOS/360.
• TOS/360, which was designed to provide an upgrade path for customers who had IBM 1401 computers with tape drives and no disks.
• DOS/360, which was built by the developers of BOS/360 and TOS/360 (IBM's small business computers division) and went on to become a mainstream operating system whose descendant z/VSE is still widely used.
• PCP (Primary Control Program), which was a very early option of OS/360 that didn't support multiprogramming.

When IBM announced the S/360-67 it also announced a timesharing operating system, TSS/360, that would use the new virtual memory capabilities of the 360/67. TSS/360 was so late and unreliable that IBM canceled it. By this time the alternative operating system CP-67, developed by IBM's Cambridge Scientific Center, was running well enough for IBM to offer it "without warranty" as a timesharing facility for a few large customers. CP-67 would go on to become VM/370 and eventually z/VM.

The traumas of producing the System/360 operating systems gave a boost to the emerging discipline of software engineering, the attempt to apply scientific principles to the development of software and the management of software projects. Frederick P. Brooks, who was a senior project manager for the whole System/360 project and then was given specific responsibility for OS/360 (which was already long overdue), wrote an acclaimed book, The Mythical Man-Month, based on the problems encountered and lessons learned during the project, two of which were:

• Throwing additional resources (especially staff) at a struggling project quickly becomes unproductive or even counter-productive because of communication difficulties. This is the "Mythical Man-Month" syndrome which gave the book its title.
• The successor to a successful system often runs into difficulties because it gets overloaded with all the features people wished had been in the earlier system. Brooks called this the "second-system effect", and cited OS/360 as a very comprehensive example.