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CTSS influenced all interactive systems created since then: Many CTSS design-
ers went on to MIT's Project MAC to create MULTICS, which in turn inspired
the development of UNIX. Project MAC was responsible for some of the most
influential research in artificial intelligence, theory of computation, and operating
systems. The Association for Computing Machinery (ACM), the preeminent com-
puting science organization, presented Professor Corbató with the 1990 Turing
Award in recognition of his contribution to computing systems.
Another major event of that decade was IBM's introduction of System/360,
a family of architecturally identical computer systems with models at different
price and performance points. IBM wanted System/360 sites to run a single batch-
oriented operating system called OS/360, without provision for time sharing.
System/360 lacked address relocation hardware to support virtual memory—at
the time, a new and controversial feature. While this batch-oriented system be-
came very successful in the commercial space, its lack of support for address re-
location and time sharing were among the reasons it was not selected for Project
MAC. The loss of this highly visible project spurred IBM to create a system that
could be used to understand time sharing and virtual memory systems.
IBM studied virtual memory with an experimental system called CP-40. This
system ran on a customized System/360 model 40 and used an associative memory
to translate virtual memory addresses to real memory addresses. CP-40 intro-
duced a concept its developers named a “virtual machine,” and was the parent
of all subsequent hypervisor-based virtualization. CP-40 proved that a physical
computer could concurrently run multiple virtual computers, each of which could
run a different operating system. The hypervisor was also referred to as a “virtual
machine monitor” or “host,” while the virtual machines running under it were
commonly called “guests.”
CP-40 was based on technical prerequisites for virtualization that have remained
consistent over the subsequent decades. These requirements were described by
Popek and Goldberg ( Communications of the ACM, Volume 12, Number 7, July
1974) as follows:
Equivalence : Execution in a guest environment is “essentially identical” to
execution running on bare metal.
Resource control : Guest environments only have access to a physical ma-
chine's resources that are explicitly granted to it by the hypervisor.
Efficiency : A dominant fraction of the instructions executed by the guest are
executed directly by hardware at native speed, without intervention by the
virtual machine monitor.
These properties are provided by hardware architectures that support a system
mode of execution in which all instructions can be executed, and a user mode
 
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