IBM M44/44X
The IBM M44/44X was an experimental computer system from the mid-1960s, designed and operated at IBM's Thomas J. Watson Research Center at Yorktown Heights, New York. It was based on an IBM 7044 (the 'M44'), and simulated multiple 7044 virtual machines (the '44X'), using both hardware and software. Key team members were Dave Sayre and Rob Nelson. This was a groundbreaking machine, used to explore paging, the virtual machine concept, and computer performance measurement. It was purely a research system, and was cited in 1981 by Peter Denning as an outstanding example of experimental computer science.

The term virtual machine probably originated with the M44/44X project, from which it was later appropriated by the CP-40 team to replace their earlier term pseudo machine.

Unlike CP-40 and later CP/CMS control programs, M44/44X did not implement a complete simulation of the underlying hardware (i.e. full virtualization). CP-40 project leader Robert Creasy observed:

The M44/44X "was about as much of a virtual machine system as CTSS – which is to say that it was close enough to a virtual machine system to show that 'close enough' did not count. I never heard a more eloquent argument for virtual machines than from Dave Sayre."

M44/44X "implanted the idea that the virtual machine concept is not necessarily less efficient than more conventional approaches" – a core assumption in the CP/CMS architecture, and one that ultimately proved very successful.

References
 R. J. Creasy, "The origin of the VM/370 time-sharing system", IBM Journal of Research & Development, Vol. 25, No. 5 (September 1981), pp. 483-490,– perspective on CP/CMS and VM history by the CP-40 project lead, also a CTSS author
 Peter J. Denning, "Performance Modeling: Experimental Computer Science at its Best", Communications of the ACM, President's Letter (November 1981) – an influential survey paper, citing the following M44/44X papers:

 L. Belady, "A study of replacement algorithms for virtual storage computers," IBM Systems Journal Vol. 5, No. 2 (1966), pp. 78-101

 L. Belady and C. J. Kuehner, "Dynamic space sharing in computer systems," Communications of ACM Vol. 12 No. 5 (May 1969), pp. 282-288

 L. Belady, R. A. Nelson, and G. S. Shedler, "An anomaly in the space-time characteristics of certain programs running in paging machines," Communications of the ACM Vol. 12, No. 6 (June 1969), pp. 349-353

 – describes the M44/44X, reports performance measurements related to memory and paging, and discusses performance impact of multiprogramming and time-sharing
 R. A. Nelson, "Mapping Devices and the M44 Data Processing System," Research Report RC 1303, IBM Thomas J. Watson Research Center (1964)– about the M44/44X
 D. Sayre, On Virtual Systems, IBM Thomas J. Watson Research Center (April 15, 1966)– an early virtual machine paper describing multiprogramming with the M44/44X.
 Melinda Varian, VM and the VM community, past present, and future, SHARE 89 Sessions 9059-9061, 1997– the outstanding source for CP/CMS and VM history

Citations

M44 44X
Virtualization software
VM (operating system)