1. An apparatus for emulating in software the hardware and operations of a target computer system including: A) a central processing unit which is part of a host system; B) a mass memory which is a part of a host system; C) target system memory contained within said mass memory of the host system; D) a signal handler which catches control of the software emulation program following a host system hardware detected exception condition; E) a mechanism in the signal handler for returning control to the software emulation program; F) a mechanism in the software emulation program for taking control from the signal handler, and recovering from the exception such that the emulation performs in the manner of the legacy hardware system, G) said recovery producing results in the same manner of the legacy hardware system as though the checks for potential exception had been made as part of the code of the software emulator prior to the host system hardware exception and the exception conditions avoided.