After updating to Squeak 2.3, Squeak would no longer simulate itself
using the InterpreterSimulator.  I fixed some bugs and it now works for me.

There were two bugs:
1. fetchNextBytecode was added to the bytecode methods in the interpreter.
   However, it was still present in the testStep method in
   InterpreterSimulator.  This resulted in advancing 2 bytes instead of 1.
   I moved it from testStep to testStart.

2. primitiveShortAt had been added to the interpreter which used
   inline (textual) C code to get a short from an address.
   I copied the same method to InterpreterSimulator, changing the
   C code to a call to halfWordAt:, and added halfWordAt: and halfWordAt:put:
   to InterpreterSimulator and InterpreterSimulatorLSB.
   (InterpreterSimulatorLSB is a new class added by Tim Rowledge to 
    simulate Squeak on little-endian systems.  It is available at 

http://sumeru.stanford.EDU/tim/pooters/SqFiles/deltas/VMsimulator.6Nov.cs )

File-ins follow.

Greg Gritton
gritton at ibm.net


!InterpreterSimulator methodsFor: 'testing' stamp: 'GVG 1/21/1999 18:26'!
test

	self testStart.
	[true] whileTrue: [
		self testStep].
	self testEnd
! !

!InterpreterSimulator methodsFor: 'testing' stamp: 'GVG 1/27/1999 19:57'!
testStart

	Transcript clear.
	byteCount _ 0.
	self internalizeIPandSP.
	currentBytecode _ self fetchByte.
! !

!InterpreterSimulator methodsFor: 'memory access' stamp: 'GVG 1/27/1999
20:47'!
primitiveShortAt
	"Treat the receiver, which can be indexible by either bytes or words, as
an array of signed 16-bit values. Return the contents of the given index.
Note that the index specifies the i-th 16-bit entry, not the i-th byte or
word."

	| index rcvr sz addr value |
	index _ self stackIntegerValue: 0.
	rcvr _ self stackValue: 1.
	self success: ((self isIntegerObject: rcvr) not and: [self isWordsOrBytes:
rcvr]).
	successFlag ifFalse: [ ^ nil ].
	sz _ ((self sizeBitsOf: rcvr) - BaseHeaderSize) // 2.  "number of 16-bit
fields"
	self success: ((index >= 1) and: [index <= sz]).
	successFlag ifTrue: [
		addr _ rcvr + BaseHeaderSize + (2 * (index - 1)).
		value _ self halfWordAt: addr.
		self pop: 2.  "pop rcvr, index"
		self pushInteger: value.  "push element value"
	].
! !

!InterpreterSimulator methodsFor: 'memory access' stamp: 'GVG 1/27/1999
20:41'!
halfWordAt: byteAddress
    "Return the half-word at byteAddress which must be even."
	| lowBits |
	lowBits _ byteAddress bitAnd: 2.
	^((self longAt: byteAddress - lowBits)
		bitShift: (lowBits - 2) * 8)
		bitAnd: 16rFFFF
! !

!InterpreterSimulator methodsFor: 'memory access' stamp: 'GVG 1/27/1999
20:43'!
halfWordAt: byteAddress put: halfWord
	| longWord shift lowBits |
	lowBits _ byteAddress bitAnd: 2.
	longWord _ self longAt: byteAddress - lowBits.
	shift _ (2 - lowBits) * 8.
	longWord _ longWord - (longWord bitAnd: (16rFFFF bitShift: shift)) +
(halfWord bitShift: shift).
	self longAt: byteAddress put: longWord
! !

!InterpreterSimulatorLSB methodsFor: 'memory access' stamp: 'GVG 1/27/1999
20:43'!
halfWordAt: byteAddress
	| lowBits |
	lowBits _ byteAddress bitAnd: 2.
	^((self longAt: byteAddress - lowBits)
		bitShift: (0 - lowBits) * 8)
		bitAnd: 16rFFFF! !

!InterpreterSimulatorLSB methodsFor: 'memory access' stamp: 'GVG 1/27/1999
21:30'!
halfWordAt: byteAddress put: halfWord
	| longWord shift lowBits |
	lowBits _ byteAddress bitAnd: 2.
	longWord _ self longAt: byteAddress - lowBits.
	shift _ lowBits * 8.
	longWord _ longWord - (longWord bitAnd: (16rFFFF bitShift: shift)) +
(halfWord bitShift: shift).
	self longAt: byteAddress - lowBits put: longWord
! !