VMware

VMware software provides a completely virtualized set of hardware to the guest operating system.[89] VMware software virtualizes the hardware for a video adapter, a network adapter, and hard disk adapters. The host provides pass-through drivers for guest USB, serial, and parallel devices. In this way, VMware virtual machines become highly portable between computers, because every host looks nearly identical to the guest. In practice, a system administrator can pause operations on a virtual machine guest, move or copy that guest to another physical computer, and there resume execution exactly at the point of suspension. Alternatively, for enterprise servers, a feature called vMotion allows the migration of operational guest virtual machines between similar but separate hardware hosts sharing the same storage[90] (or, with vMotion Storage, separate storage can be used, too). Each of these transitions is completely transparent to any users on the virtual machine at the time it is being migrated.

VMware Workstation, Server, and ESX take a more optimized path to running target operating systems on the host than that of emulators (such as Bochs) which simulate the function of each CPU instruction on the target machine one-by-one, or that of dynamic recompilation which compiles blocks of machine-instructions the first time they execute, and then uses the translated code directly when the code runs subsequently (Microsoft Virtual PC for macOS takes this approach). VMware software does not emulate an instruction set for different hardware not physically present. This significantly boosts performance, but can cause problems when moving virtual machine guests between hardware hosts using different instruction sets (such as found in 64-bit Intel and AMD CPUs), or between hardware hosts with a differing number of CPUs. Software that is CPU agnostic can usually survive such a transition, unless it is agnostic by forking at startup, in which case, the software or the guest OS must be stopped before moving it, then restarted after the move.