History of IBM Mainframe Operating Systems - Early Time-sharing and Virtual Machine Systems

Early Time-sharing and Virtual Machine Systems

For an explanation of time-sharing, virtual memory or virtual machines, see the Technical notes near the end of this article.

MIT's Fernando Corbató produced the first experimental time-sharing systems, such as CTSS, from 1957 to the early 1960s, using slightly modified IBM 704 and IBM 7090 mainframes; these systems were based on a proposal by John McCarthy. In the 1960s IBM's own laboratories created experimental time-sharing systems, using standard mainframes with hardware and microcode modifications to support virtual memory: IBM M44/44X in the early 1960s; CP-40 from 1964 to 1967; CP-67 from 1967 to 1972. The company even released CP-67 without warranty or technical support to several large customers from 1968 to 1972. CP-40 and CP-67 used modified System/360 CPUs, but the M44/44X was based on the IBM 7044, an earlier generation of CPU which was very different internally.

These experimental systems were too late to be incorporated into the System/360 series which IBM announced in 1964, but encouraged the company to add virtual memory and virtual machine capabilities to its System/370 mainframes and their operating systems in 1972:

• The M44/44X showed that a partial approach to virtual machines was not good enough, and that thrashing could severely reduce the speed of virtual memory systems. Thrashing is a condition in which the system runs very slowly because it spends a lot of its time shuffling virtual memory pages between physical memory and disk files.
• IBM learned from CP-40 and CP-67: how to make the thrashing problem manageable; that its other virtual memory and virtual machine technologies were sufficiently fast and reliable to be used in the high-volume commercial systems which were its core business. In particular IBM's David Sayre convinced the company that automated virtual memory management could consistently perform at least as well as the best programmer-designed overlay schemes.

In 1968 a consulting firm called Computer Software Systems used the released version of CP-67 to set up a commercial time-sharing service. The company's technical team included 2 recruits from MIT (see CTSS above), Dick Orenstein and Harold Feinleib. As it grew, the company renamed itself National CSS and modified the software to increase the number of paying users it could support until the system was sufficiently different to warrant a new name, VP/CSS. VP/CSS was the delivery mechanism for National CSS' services until the early 1980s, when it switched to IBM's VM/370 (see below).

Universities produced three other S/360 time-sharing operating systems in the late 1960s:

• The Michigan Terminal System (MTS) was developed in 1967 by a consortium of universities led by the University of Michigan. All versions ran on IBM mainframes which had virtual memory capability, starting with the S/360-67. MTS remained in use until 1999.
• McGill University in Montreal started developing MUSIC (McGill University System for Interactive Computing) in 1969. MUSIC was enhanced several times and eventually supported text searches, web publishing and email as well as software development. MUSIC became an IBM Systems Product (MUSIC/SP or Multi-User System for Interactive Computing / System Product) in 1985. The last official version was released in 1999.
• ORVYL and WYLBUR were developed by Stanford University in 1967-68 for the IBM S/360-67. They provided some of the first time-sharing capabilities on IBM S/360 computers.