Abstract: An emulator is comprised of a host processor, an emulator, assist unit, and a memory which are closely coupled together over a co-processor bus. Stored in the memory is a user program which is a sequence of instructions from a user instruction set that is to be emulated, and a control program which is a mixture of host processor instructions and emulator assist unit instructions. In operation, the host processor reads and executes the hosts instructions, and it reads and passes the emulator assist unit instructions to the emulator assist unit for execution in that unit. By this means, the host processor and the emulator assist unit share the emulation tasks; and those tasks which are most difficult for the host are performed by the emulation assist unit.