The Interpreter
We knew that the published Blue Book interpreter description would suffice to get us started. Moreover, we were spared from the tedium of transcription by Mario Wolczko, who had already keyed in the code for use as an on-line reference source for a Smalltalk implementation project at the University of Manchester.
The interpreter is structured as a single class that gets translated to C along with the Object Memory and BitBlt classes. In addition, a subclass (Interpreter Simulator) runs all the same code from within a Smalltalk environment by supporting basic mouse, file, and display operations. This subclass was the basis for debugging the Squeak system into existence. All of this code is included in the Squeak release and it can run its own image, albeit at a snail's pace (every memory access, even in BitBlt, runs a Smalltalk method). Having an interpreter that runs within Smalltalk is invaluable for studying the virtual machine. Any operation can be stopped and inspected, or it can be instrumented to gather timing profiles, exact method counts, and other statistics.
Although we have constantly amended the interpreter to achieve increasing performance, we have stayed pretty close to the Blue Book message interface between the Interpreter and the Object Memory. It is a testament to the original design of that interface that completely changing the Object Memory implementation had almost no impact on the Interpreter.
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