1. A method for emulating on a computer system the operation of a guest computer system, comprising the steps of: providing an emulation program that runs as an application on the operating system of the host computer system, the emulation program emulating for at least one guest application program the operation of a guest operating system; receiving at the emulation program blocks of software instructions from the guest application program; and determining, for each block of instructions, whether (a) the block of instructions includes user level instructions, in which case the instructions are passed to the processor of the computer system for execution, or (b) the block of instructions include supervisor level instructions, in which case the block of instructions is translated and the translated block of instructions is passed to the processor of the computer system for execution.