Editing Virtual machine (section)

== History ==
{{See also|History of CP/CMS|timeline of virtualization development}}
{{More citations needed section|date=July 2015}}

Both system virtual machines and process virtual machines date to the 1960s and remain areas of active development.

''System virtual machines'' grew out of [[time-sharing]], as notably implemented in the [[Compatible Time-Sharing System]] (CTSS). Time-sharing allowed multiple users to use a computer [[Concurrent computing|concurrently]]: each program appeared to have full access to the machine, but only one program was executed at the time, with the system switching between programs in time slices, saving and restoring state each time. This evolved into virtual machines, notably via IBM's research systems: the [[IBM M44/44X|M44/44X]], which used [[partial virtualization]], and the [[IBM CP-40|CP-40]] and [[SIMMON]], which used [[full virtualization]], and were early examples of [[hypervisor]]s. The first widely available virtual machine architecture was the [[CP-67]]/CMS (see [[History of CP/CMS]] for details). An important distinction was between using multiple virtual machines on one host system for time-sharing, as in M44/44X and CP-40, and using one virtual machine on a host system for prototyping, as in SIMMON. [[Emulator]]s, with hardware emulation of earlier systems for compatibility, date back to the [[IBM System/360]] in 1963,<ref name="Pugh_1995"/><ref name="Pugh_1991"/> while the software emulation (then-called "simulation") predates it.

''Process virtual machines'' arose originally as abstract platforms for an [[intermediate language]] used as the [[intermediate representation]] of a program by a [[compiler]]; early examples date to around 1964 with the [[META II]] compiler-writing system using it for both syntax description and target code generation. A notable 1966 example was the [[O-code machine]], a virtual machine that executes [[O-code]] (object code) emitted by the [[Compiler#Front end|front end]] of the [[BCPL]] compiler. This abstraction allowed the compiler to be easily ported to a new architecture by implementing a new [[Compiler#Back end|back end]] that took the existing O-code and compiled it to machine code for the underlying physical machine. The [[Euler (programming language)|Euler]] language used a similar design, with the intermediate language named ''P'' (portable).<ref name="Wirth_1966"/> This was popularized around 1970 by [[Pascal (programming language)|Pascal]], notably in the [[Pascal-P]] system (1973) and [[Pascal-S]] compiler (1975), in which it was termed [[p-code machine|p-code]] and the resulting machine as a [[p-code machine]]. This has been influential, and virtual machines in this sense have been often generally called p-code machines. In addition to being an intermediate language, Pascal p-code was also executed directly by an interpreter implementing the virtual machine, notably in [[UCSD Pascal]] (1978); this influenced later interpreters, notably the [[Java virtual machine]] (JVM). Another early example was [[SNOBOL4]] (1967), which was written in the SNOBOL Implementation Language (SIL), an assembly language for a virtual machine, which was then targeted to physical machines by transpiling to their native assembler via a [[macro assembler]].<ref name="Griswold_1972"/> Macros have since fallen out of favor, however, so this approach has been less influential. Process virtual machines were a popular approach to implementing early microcomputer software, including [[Tiny BASIC#Implementation in a virtual machine|Tiny BASIC]] and adventure games, from one-off implementations such as [[Pyramid 2000]] to a general-purpose engine like [[Infocom]]'s [[z-machine]], which [[Graham Nelson]] argues is "possibly the most portable virtual machine ever created".<ref name="inform-interpreters"/>

Significant advances occurred in the implementation of [[Smalltalk]]-80,<ref name="Goldberg_1983"/>
particularly the Deutsch/Schiffmann implementation<ref name="Deutsch_1984"/>
which pushed [[just-in-time compilation|just-in-time (JIT) compilation]] forward as an implementation approach that uses process virtual machine.<ref name="Aycock_2003"/>
Later notable Smalltalk VMs were [[VisualWorks]], the [[Squeak Virtual Machine]],<ref name="Ingalls_1997"/>
and [[Strongtalk]].<ref name="Griswold_1993"/>
A related language that produced a lot of virtual machine innovation was the [[Self (programming language)|Self]] programming language,<ref name="Ungar_1987"/> which pioneered [[adaptive optimization]]<ref name="Hoelzle"/> and [[Tracing garbage collection#Generational GC (ephemeral GC)|generational garbage collection]]. These techniques proved commercially successful in 1999 in the [[HotSpot (virtual machine)|HotSpot]] Java virtual machine.<ref name="Paleczny_2001"/> Other innovations include a register-based virtual machine, to better match the underlying hardware, rather than a stack-based virtual machine, which is a closer match for the programming language; in 1995, this was pioneered by the [[Dis virtual machine]] for the [[Limbo (programming language)|Limbo]] language.