Virtual Machine

=> A virtual machine is a tightly isolated software container that can run its own operating systems and applications as if it were a physical computer. A virtual machine behaves exactly like a physical computer and contains it own virtual (ie, software-based) CPU, RAM hard disk and network interface card (NIC).



An operating system can’t tell the difference between a virtual machine and a physical machine, nor can applications or other computers on a network. Even the virtual machine thinks it is a “real” computer. Nevertheless, a virtual machine is composed entirely of software and contains no hardware components whatsoever. As a result, virtual machines offer a number of distinct advantages over physical hardware.

IMPLEMENTATION
Modes:
- virtual user mode and virtual monitor mode,
- Actual user mode and actual monitor mode

Time
- Whereas the real I/O might have taken 100 milliseconds, the virtual I/O might take less time (because it is spooled) or more time (because it is interpreted.)
- The CPU is being multi-programmed among many virtual machines, further slowing down the virtual machines in unpredictable ways.

BENEFITS
In general, VMware virtual machines possess four key characteristics that benefit the user:

Compatibility: Virtual machines are compatible with all standard x86 computers
Isolation: Virtual machines are isolated from each other as if physically separated
Encapsulation: Virtual machines encapsulate a complete computing environment
Hardware independence: Virtual machines run independently of underlying hardware

Two advantages
To provide a robust level of security
- no direct sharing of resources.
- Two solutions
To allow system development to be done easily
- A perfect vehicle for OS research and development.
- difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.
- Wine for Linux.

EXAMPLES
~ Compiled Java programs are platform-neutral bytecodes executed by a Java Virtual Machine (JVM).
~ JVM consists of
- class loader
- class verifier
- runtime interpreter
~ Just-In-Time (JIT) compilers increase performance