Virtual Machine Concept

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It is normally difficult to program in L₀ for a low-level machine because it is enormously detailed and consists purely of numbers. Another new language L₁, which is easier to use, may be built based on the L₀. Another improved language L₂ may also be created based on the L₁, and so on.

A virtual machine is a self-contained operating environment that behaves as if it is a separate computer. Virtual machines are often used in the implementation of portable executors for high-level languages. The HLL is compiled into code for the virtual machine (an intermediate language) which is then executed by an interpreter written in assembly language or some other portable language like C. Examples are Java Virtual Machine, POPLOG, SNOBOL Implementation Language, and Smalltalk. This design has two advantages:

• System independence: A Java application will run the same in any JVM, regardless of the hardware and software underlying the system.


• Security: Because VM has no contact with operating system, there is little possibility of a Java program damaging other applications of files.

Level	Virtual Machine	Comments
5	High-level language	Examples are C, C++, and Java.
4	Assembly language	Assembly language uses short mnemonics such as ADD, SUB, and MOV that are easily translated to the machine language.
3	Operating system	The low-level software which handles the interface to peripheral hardware, schedules tasks, allocates storage, and presents a default interface to the user.
2	Instruction set architecture (ISA)	Machine language. Each machine-language instruction is executed by several microinstructions.
1	Microarchitecture	A technique for implementing the instruction set of a processor as a sequence of microcode instructions.
0	Digital logic	Physical machine hardware