The concept of allowing the computer user to modify the virtual machine being emulated by a microprogram controlled computer is not new. Schemes for implementing this typically include the definition of a standard virtual machine (the base machine ) in read-only control storage, and the provision of some writable memory to contain user-written microroutines for the duration of the user job. Alternation of a computer instruction set by the temporary addition of microroutines to the system repertoire for emulation of new instructions is known as dynamic user microprogramming .
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