The complexity of modern processors requires that their
development be paralleled by the development of a simulator
to guide design decisions.  In addition to facilitating the 
development of the processor, a simulator can also facilitate 
the development of compiler techniques to exploit the increased 
speed and functionality of the new processor.  Finally, since 
successful processors are succeeded by newer and better 
processors, an established simulator can be invaluable in 
aiding the development of the successor processor.  However, 
the advantages that a simulator provides to a developing 
architecture require that the simulator be easy to extend 
and modify.

In this paper, we exploit object technology to present the 
design and implementation of a simulator for the 80x86 processor 
family: SIMx86. We begin by describing the design of a simulator 
for the 8088 processor. We demonstrate the extensibility of our 
design by extending it to include the 8086 processor. We further 
demonstrate extensibility by incorporating debugging facilities 
into our simulator model.  To evaluate the performance of our 
SIMx86 simulator, we compare execution times with an existing
simulator, the SimpleScalar sim-fast simulator.  Our experiments 
indicate that SIMx86 is competitive with the sim-fast simulator.  
In most cases, the sim-fast simulator was about three times as 
fast as SIMx86.  However, the ease of modification, extension 
and maintenance provided by our design offsets the performance 
gains provided by the traditional approach to simulator 
construction.