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.[1]

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."[2]

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.[3]


  • 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
  • R. W. O'Neill. Experience using a time sharing multiprogramming system with dynamic address relocation hardware. Proc. AFIPS Computer Conference 30 (Spring Joint Computer Conference, 1967). pp. 611–621. doi:10.1145/1465482.1465581.
    – 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


  1. ^ Denning, op. cit.
  2. ^ Creasy, op. cit. – relationship between M44/44X and CP-40
  3. ^ L. Talkington, "A Good Idea and Still Growing", White Plains Development Center Newsletter, Vol. 2, No. 3 (March 1969) [quoted in Varian, op. cit., p. 10, Note 26]