1. A method of emulating the instructions of a target computer in the instructions of a host computer to operate said host computer in accordance with said target computer instructions, said target computer having the capability of modifying its own instructions and data during operation, said method comprising the steps of: storing said target instructions in a target memory segment; sequentially withdrawing said target instructions from said target memory segment and executing said target instructions in said host computer; constructing a template of said host instructions for each of said target instructions executed; storing each of said templates for reuse each time the corresponding target instruction is to be exectued; limiting access to those target instructions in said target memory segment which have been stored in one of said templates to permit only read operations to be performed thereon; and detecting a target instruction which attempts a write operation on one of the target instructions which is in a stored template.