A high-level language virtual machine executes portable intermediate code through a runtime implemented for the host platform.
The Java Virtual Machine (JVM) is an example. Its bytecode defines a virtual instruction set rather than a particular physical CPU’s instruction set.
Compilation and Distribution
| Stage | Conventional Native Compilation | Language VM |
|---|---|---|
| Input | High-level source program. | High-level source program. |
| Compilation | Compiler front end produces intermediate code, then the back end produces target object code. | Compiler produces portable bytecode for a virtual instruction set. |
| Distribution | Distribute the target-specific binary. | Distribute the portable bytecode. |
| Loading | The OS loader prepares the executable memory image. | The VM loader prepares the program for the runtime. |
| Execution | The CPU executes native instructions. | An interpreter or compiler executes bytecode using host instructions. |
The distribution point determines portability. Native binaries target a particular platform. Portable bytecode requires a compatible VM implementation on each target platform.
Interpretation and Compilation
- Interpretation
The runtime executes bytecode operations through its interpreter. - Just-in-time compilation
The runtime translates code into native instructions during execution. - Runtime services
The VM can supply memory management, checks, and other language services.
Portability Limits
- A portable virtual instruction set reduces dependence on the physical ISA.
- Native libraries and OS-specific interfaces can restrict portability.
- Runtime availability and supported bytecode versions constrain deployment.
- Translation and runtime services have costs, while runtime optimization can improve execution.
System and Language VMs
| Property | System VM | Language VM |
|---|---|---|
| Interface | Virtual hardware platform. | Virtual instruction set and runtime services. |
| Workload | Guest OS and its applications. | Programs targeting the language runtime. |
| Portability Requirement | Compatible virtual hardware and guest environment. | Compatible runtime and application dependencies. |
| Example | An OS running under a hypervisor. | Java bytecode running on a JVM. |
