To the instruction parsing ability of InstructionStream I add the actual semantics for execution. The execution state is stored in the indexable fields of my subclasses. This includes temporary variables and a stack of values used in evaluating expressions. The actual semantics of execution can be found in my category "system simulation" and "instruction decode". These methods exactly parallel the operation of the Smalltalk machine itself.

The simulator is a group of my methods that do what the Smalltalk interpreter does: execute Smalltalk bytecodes. By adding code to the simulator, you may take statistics on the running of Smalltalk methods. For example,
Transcript show: (ContextPart runSimulated: [3 factorial]) printString.

	controlling
	activateMethod:withArgs:receiver:class: blockCopy: hasSender: pop push: quickSend:to:with:super: return:to: send:to:with:super: terminate terminateTo: top

	debugger access
	depthBelow: errorReportOn: longStack mclass pc release releaseTo: selector sender shortStack singleRelease sourceCode stack stackOfSize: swapSender: tempNames tempsAndValues tempsAndValuesLimitedTo:indent:

	instruction decoding
	doDup doPop jump: jump:if: methodReturnConstant: methodReturnReceiver methodReturnTop popIntoLiteralVariable: popIntoReceiverVariable: popIntoTemporaryVariable: pushActiveContext pushConstant: pushLiteralVariable: pushReceiver pushReceiverVariable: pushTemporaryVariable: send:super:numArgs: storeIntoLiteralVariable: storeIntoReceiverVariable: storeIntoTemporaryVariable:

	objects from disk
	storeDataOn:

	printing
	printDetails: printOn:

	private
	doPrimitive:method:receiver:args: push:fromIndexable: stackPtr stackp: tryNamedPrimitiveIn:for:withArgs: tryPrimitiveFor:receiver:args:

	private-debugger
	cachesStack

	private-exceptions
	findNextHandlerContextStarting findNextUnwindContextUpTo: isHandlerContext isUnwindContext unwindTo:

	system simulation
	completeCallee: quickStep runSimulated:contextAtEachStep: step stepToSendOrReturn

	instance creation
	basicNew: initializedInstance new new: newForMethod:

	private
	carefullyPrint:on:

	simulation
	initialize primitiveFailToken runSimulated:

	accessing	top
	client Answer the client, that is, the object that sent the message that created this context.
	home Answer the context in which the receiver was defined.
	method Answer the method of this context.
	receiver Answer the receiver of the message that created this context.
	tempAt: Answer the value of the temporary variable whose index is the argument, index.
	tempAt:put: Store the argument, value, as the temporary variable whose index is the argument, index.

	controlling	top
	activateMethod:withArgs:receiver:class: Answer a ContextPart initialized with the arguments.
	blockCopy: Primitive. Distinguish a block of code from its enclosing method by creating a new BlockContext for that block. The compiler inserts into all methods that contain blocks the bytecodes to send the message blockCopy:. Do not use blockCopy: in code that you write! Only the compiler can decide to send the message blockCopy:. Fail if numArgs is not a SmallInteger. Optional. No Lookup. See Object documentation whatIsAPrimitive.
	hasSender: Answer whether the receiver is strictly above context on the stack.
	pop Answer the top of the receiver's stack and remove the top of the stack.
	push: Push val on the receiver's stack.
	quickSend:to:with:super: Send the given selector with arguments in an environment which closely resembles the non-simulating environment, with an interjected unwind-protected block to catch nonlocal returns. Attention: don't get lost!
	return:to: Simulate the return of value to sendr.
	send:to:with:super: Simulate the action of sending a message with selector, selector, and arguments, args, to receiver. The argument, superFlag, tells whether the receiver of the message was specified with 'super' in the source method.
	terminate Make myself unresumable.
	terminateTo: Terminate all the Contexts between me and previousContext, if previousContext is on my Context stack. Make previousContext my sender.
	top Answer the top of the receiver's stack.

	debugger access	top
	depthBelow: Answer how many calls there are between this and aContext.
	errorReportOn: Write a detailed error report on the stack (above me) on a stream. For both the error file, and emailing a bug report. Suppress any errors while getting printStrings. Limit the length.
	longStack Answer a String showing the top 100 contexts on my sender chain.
	mclass Answer the class in which the receiver's method was found.
	pc Answer the index of the next bytecode to be executed.
	release Remove information from the receiver and all of the contexts on its sender chain in order to break circularities.
	releaseTo: Remove information from the receiver and the contexts on its sender chain up to caller in order to break circularities.
	selector Answer the selector of the method that created the receiver.
	sender Answer the context that sent the message that created the receiver.
	shortStack Answer a String showing the top ten contexts on my sender chain.
	singleRelease Remove information from the receiver in order to break circularities.
	sourceCode
	stack Answer an Array of the contexts on the receiver's sender chain.
	stackOfSize: Answer an OrderedCollection of the top 'limit' contexts on the receiver's sender chain.
	swapSender: Replace the receiver's sender with coroutine and answer the receiver's previous sender. For use in coroutining.
	tempNames Answer an OrderedCollection of the names of the receiver's temporary variables, which are strings.
	tempsAndValues Return a string of the temporary variabls and their current values
	tempsAndValuesLimitedTo:indent: Return a string of the temporary variabls and their current values

	instruction decoding	top
	doDup Simulate the action of a 'duplicate top of stack' bytecode.
	doPop Simulate the action of a 'remove top of stack' bytecode.
	jump: Simulate the action of a 'unconditional jump' bytecode whose offset is the argument, distance.
	jump:if: Simulate the action of a 'conditional jump' bytecode whose offset is the argument, distance, and whose condition is the argument, condition.
	methodReturnConstant: Simulate the action of a 'return constant' bytecode whose value is the argument, value. This corresponds to a source expression like '^0'.
	methodReturnReceiver Simulate the action of a 'return receiver' bytecode. This corresponds to the source expression '^self'.
	methodReturnTop Simulate the action of a 'return top of stack' bytecode. This corresponds to source expressions like '^something'.
	popIntoLiteralVariable: Simulate the action of bytecode that removes the top of the stack and stores it into a literal variable of my method.
	popIntoReceiverVariable: Simulate the action of bytecode that removes the top of the stack and stores it into an instance variable of my receiver.
	popIntoTemporaryVariable: Simulate the action of bytecode that removes the top of the stack and stores it into one of my temporary variables.
	pushActiveContext Simulate the action of bytecode that pushes the the active context on the top of its own stack.
	pushConstant: Simulate the action of bytecode that pushes the constant, value, on the top of the stack.
	pushLiteralVariable: Simulate the action of bytecode that pushes the contents of the literal variable whose index is the argument, index, on the top of the stack.
	pushReceiver Simulate the action of bytecode that pushes the active context's receiver on the top of the stack.
	pushReceiverVariable: Simulate the action of bytecode that pushes the contents of the receiver's instance variable whose index is the argument, index, on the top of the stack.
	pushTemporaryVariable: Simulate the action of bytecode that pushes the contents of the temporary variable whose index is the argument, index, on the top of the stack.
	send:super:numArgs: Simulate the action of bytecodes that send a message with selector, selector. The argument, superFlag, tells whether the receiver of the message was specified with 'super' in the source method. The arguments of the message are found in the top numArgs locations on the stack and the receiver just below them.
	storeIntoLiteralVariable: Simulate the action of bytecode that stores the top of the stack into a literal variable of my method.
	storeIntoReceiverVariable: Simulate the action of bytecode that stores the top of the stack into an instance variable of my receiver.
	storeIntoTemporaryVariable: Simulate the action of bytecode that stores the top of the stack into one of my temporary variables.

	objects from disk	top
	storeDataOn: Contexts are not allowed go to out in DataStreams. They must be included inside an ImageSegment.

	printing	top
	printDetails: Put my class>>selector and arguments and temporaries on the stream. Protect against errors during printing.
	printOn: Append to the argument, aStream, a sequence of characters that identifies the receiver.

	private	top
	doPrimitive:method:receiver:args: Simulate a primitive method whose index is primitiveIndex. The simulated receiver and arguments are given as arguments to this message.
	push:fromIndexable: Push the elements of anIndexableCollection onto the receiver's stack. Do not call directly. Called indirectly by {1. 2. 3} constructs.
	stackPtr For use only by the SystemTracer
	stackp: Storing into the stack pointer is a potentially dangerous thing. This primitive stores nil into any cells that become accessible as a result, and it performs the entire operation atomically.
	tryNamedPrimitiveIn:for:withArgs: Hack. Attempt to execute the named primitive from the given compiled method
	tryPrimitiveFor:receiver:args: If this method has a primitive index, then run the primitive and return its result. Otherwise (and also if the primitive fails) return PrimitiveFailToken, as an indication that the method should be activated and run as bytecodes.

	private-debugger	top
	cachesStack

	private-exceptions	top
	findNextHandlerContextStarting Return the next handler marked context, returning nil if there is none. Search starts with self and proceeds up to nil.
	findNextUnwindContextUpTo: Return the next unwind marked above the receiver, returning nil if there is none. Search proceeds up to but not including aContext.
	isHandlerContext
	isUnwindContext
	unwindTo:

	system simulation	top
	completeCallee: Simulate the execution of bytecodes until a return to the receiver.
	quickStep If the next instruction is a send, just perform it. Otherwise, do a normal step.
	runSimulated:contextAtEachStep: Simulate the execution of the argument, aBlock, until it ends. aBlock MUST NOT contain an '^'. Evaluate block2 with the current context prior to each instruction executed. Answer the simulated value of aBlock.
	step Simulate the execution of the receiver's next bytecode. Answer the context that would be the active context after this bytecode.
	stepToSendOrReturn Simulate the execution of bytecodes until either sending a message or returning a value to the receiver (that is, until switching contexts).

	examples	top
	tallyInstructions: This method uses the simulator to count the number of occurrences of each of the Smalltalk instructions executed during evaluation of aBlock. Results appear in order of the byteCode set.
	tallyMethods: This method uses the simulator to count the number of calls on each method invoked in evaluating aBlock. Results are given in order of decreasing counts.
	trace: ContextPart trace: [3 factorial]
	trace:on: ContextPart trace: [3 factorial]
	trace:onFileNamed: ContextPart trace: [3 factorial] onFileNamed: 'trace'

	instance creation	top
	basicNew: Primitive. Answer an instance of this class with the number of indexable variables specified by the argument, sizeRequested. Fail if this class is not indexable or if the argument is not a positive Integer, or if there is not enough memory available. Essential. See Object documentation whatIsAPrimitive.
	initializedInstance Answer an instance of the receiver which in some sense is initialized. In the case of Morphs, this will yield an instance that can be attached to the Hand after having received the same kind of basic initialization that would be obtained from an instance chosen from the 'new morph' menu. Return nil if the receiver is reluctant for some reason to return such a thing
	new Answer a new instance of the receiver (which is a class) with no indexable variables. Fail if the class is indexable.
	new: Answer an instance of this class with the number of indexable variables specified by the argument, sizeRequested.
	newForMethod: This is the only method for creating new contexts, other than primitive cloning. Any other attempts, such as inherited methods like shallowCopy, should be avoided or must at least be rewritten to determine the proper size from the method being activated. This is because asking a context its size (even basicSize!) will not return the real object size but only the number of fields currently accessible, as determined by stackp.

	private	top
	carefullyPrint:on:

	simulation	top
	initialize A unique object to be returned when a primitive fails during simulation
	primitiveFailToken
	runSimulated: Simulate the execution of the argument, current. Answer the result it returns.