Implementation
Conventional emulators like Bochs emulate the microprocessor, executing each guest CPU instruction by calling a software subroutine on the host machine that simulates the function of that CPU instruction. This level of abstraction allows the guest machine to run on host machines with a different type of microprocessor, but is also very slow.
A more efficient approach consists in software debugger technique. Some parts of the code are executed natively on the real processor; on 'bad' instructions, there are software interrupts that break execution of the guest operating system code and that particular instruction is emulated.
twoOStwo, as well as VMware Workstation, Virtual PC for Windows and QEMU with the kqemu add-on, take an even more optimized approach and run code directly when this is possible. This is the case for user mode and virtual 8086 mode code on x86.
The drawback is that the guest OS has to be compatible with the host CPU. So unlike an emulator, one cannot use twoOStwo to run Mac/PowerPC software on an Intel x86 processor. Another drawback is that it is not normally possible to efficiently nest virtual machines. Finally, although twoOStwo virtual machines run in user mode, twoOStwo itself requires installing various device drivers in the host operating system.
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