Process Level VM

Process Level VM provides application witha virtual ABI environment. Process VM exhibits in various form

(1) Replication - e.g. multiprogramming - most operating system can support mulitple user processes. In other words, the operating system provides a replicated process-level VM for each of the concurrently executing applicaitons. The host OS can be same of different. The most straightforward emulatio is interpretation. The interpretor emulates the source ISA. The process can be relatively slow as each source instruction may require tens of native target instructions.

(2) Emulation - For better performance, binary translation is used. Block of source instructions are converted to target instruction set, which can be cached and reused. Interpretation has relatively low start up overhead but high execution time. On the other hands, dynamics binary translatir has high initial overhead but is fast for repeated execution. some VM use a staged emulation strategy combined with profiling (i.e. collect statistics regarding the program's behaviour). Initially, a block of source instruction is interpreted and profile track how frequently the block is executed. Binary translation is used for block with repeated execution.

(3) Optimization - In addition to emulation, the target code can be optimized. This leads naturrally for VM where soruce and target instruction set are the same and optimization is the the primary purpose of the VM.

(4) Platform independence - Emulation translates from a specific source ISA to a specific target ISA. A virutalized ISA can be used for ultimate protability. The VM environment does not dorectly corrrespond to any real platform. Rather, it is designed for ease of protability and to match the features of a high level language. The HLL VM focuses on minimizing hardware-specific and OS-specific features which would compromise protability. Examples of HLL VM are Java VM and MS CLI (.Net). These VM uses bytecodes (each instruction is encoded as a sequence of bytes) which are stack based (to eliminate register requirement). Memory size is conceptually unbounded with garbage collection as an assumed part of the implementation.