Question

What is the difference between direct and interpreted virtual machines?

• How are they similar?

Answer 1

Direct Virtual Machines

A Direct virtual machine is an emulated computer system created using the software which uses physical system resources, such as the CPU, RAM, and disk storage, etc which is isolated from other software on the computer. Virtual machines can easily be created, modified, or destroyed without affecting the host computer. They provide similar functionality to physical machines, but they do not run directly on the hardware. In place of that, a software layer exists between the hardware and the virtual machine. The software that manages one or more VMs is called a "hypervisor" and the VMs are called "guests" or virtualized instances. Each guest can interact with the hardware, but the hypervisor controls them.

Interpreted Virtual Machines

They are the variation of the idea of the compiler is the interpreter. Instead of translating the program/code into machine code, it creates a fictitious machine that understands high-level language.  

The interpreter can also be considered as a processor, which instead of executing machine instructions, executes high-level instructions.

The similarity between direct and interpreted virtual machines:

• An interpreter takes the code and executes it.
• An emulator is an interpreter specifically for the low-level machine code of some specific device.
• A virtual machine is an emulator specifically for a hypothetical/idealized device rather than a physically pre-existing one.

Both interpreter and compiler translate between high-level programming language to machine code of a CPU. The difference is compiler stores machine code on disk, and the interpreter executes it immediately. There are different pointer-calculation challenges associated with both. Interpreters are also starting to look like compilers these days thanks to JIT technology. Conceptually interpreters and emulators are the same things. Perhaps one could say emulators and VMs are special cases of interpreters. Emulators and VMs typically are associated with an instruction set and their store models a regular machine architecture through registers or a stack. Read about Lisp machines to get a better intuition for this perspective. In particular SECD and the followup abstract machines.