A computer constructed from actual physical devices is termed an actual computer or hardware computer. From the programming point of view, it is the instruction set of the hardware that defines a machine. An operating system is built on top of a machine to manage access to the machine and to provide additional services. The services provided by the operating system constitute another machine, a virtual machine.
A programming language provides a set of operations. Thus, for example, it is possible to speak of a Pascal computer or a Scheme computer. For the programmer, the programming language is the computer; the programming language defines a virtual computer. The virtual machine for Simple consists of a data area which contains the association between variables and values and the program which manipulates the data area.
Between the programmer's view of the program and the virtual machine provided by the operating system is another virtual machine. It consists of the data structures and algorithms necessary to support the execution of the program. This virtual machine is the run time system of the language. Its complexity may range in size from virtually nothing, as in the case of FORTRAN, to an extremely sophisticated system supporting memory management and inter process communication as in the case of a concurrent programming language like SR. The run time system for Simple as includes the processing unit capable of executing the code and a data area in which the values assigned to variables are accessed through an offset into the data area.
User programs constitute another class of virtual machines.