Blue Book Chapter 27

[TOC] [26] 27 [28] [29] [30]

27
Specification of the Virtual Machine

Form of the Specification

Object Memory Interface

Objects Used by the Interpreter

Compiled Methods

Contexts

Classes

Chapter 26 described the function of the Smalltalk virtual machine, which consists of an interpreter and an object memory. This chapter and the next three present a more formal specification of these two parts of the virtual machine. Most implementations of the virtual machine will be written in machine language or microcode. However, for specification purposes, these chapters will present an implementation of the virtual machine in Smalltalk itself. While this is a somewhat circular proposition, every attempt has been made to ensure that no details are hidden as a result.
This chapter consists of three sections. The first describes the conventions and terminology used in the formal specification. It also provides some warnings of possible confusion resulting from the form of this specification. The second section describes the object memory routines used by the interpreter. The implementation of these routines will be described in Chapter 30. The third section describes the three main types of object that the interpreter manipulates, methods, contexts, and classes. Chapter 28 describes the bytecode set and how it is interpreted; Chapter 29 describes the primitive routines.

Form of the Specification

Two class descriptions named Interpreter and ObjectMemory make up the formal specification of the Smalltalk-80 virtual machine. The implementation of Interpreter will be presented in detail in this chapter and the following two; the implementation of ObjectMemory in Chapter 30.
    A potential source of confusion in these chapters comes from the two Smalltalk systems involved in the descriptions, the system containing Interpreter and ObjectMemory and the system being interpreted. Interpreter and ObjectMemory have methods and instance variables and they also manipulate methods and instance variables in the system they interpret. To minimize the confusion, we will use a different set of terminology for each system. The methods of Interpreter and ObjectMemory will be called routines; the word method will be reserved for the methods being interpreted. Similarly, the instance variables of Interpreter and ObjectMemory will be called registers; the word instance variable will be reserved for the instance variables of objects in the system being interpreted.
    The arguments of the routines and the contents of the registers of Interpreter and ObjectMemory will almost always be instances of Integer (SmallIntegers and LargePositiveIntegers). This can also be a source of confusion since there are Integers in the interpreted system. The Integers that are arguments to routines and contents of registers represent object pointers and numerical values of the interpreted system. Some of these will represent the object pointers or values of Integers in the interpreted system.
    The interpreter routines in this specification will all be in the form of Smalltalk method definitions. For example

routineName: argumentName

| temporaryVariable |

temporaryVariable := self anotherRoutine: argumentName.

^temporaryVariable - 1

The routines in the specification will contain five types of expression.

1.	Calls on other routines of the interpreter. Since both the invocation and definition of the routine are in `Interpreter`, they will appear as messages to `self`.
	self headerOf: newMethod self storeInstructionPointerValue: value inContext: contextPointer
2.	Calls on routines of the object memory. An `Interpreter` uses the name `memory` to refer to its object memory, so these calls will appear as messages to `memory`.
	memory fetchClassOf: newMethod memory storePointer: senderindex ofObject: contextPointer withValue: activeContext
3.	Arithmetic operations on object pointers and numerical values. Arithmetic operations will be represented by standard Smalltalk arithmetic expressions, so they will appear as messages to the numbers themselves.
	receiverValue + argumentValue selectorPointer bitShift: -1
4.	Array accesses. Certain tables maintained by the interpreter are represented in the formal specification by `Arrays`. Access to these will appear as `at:` and `at:put:` messages to the `Arrays`.
	methodCache at: hash semaphoreList at: semaphoreIndex put: semaphorePointer
5.	Conditional control structures. The control structures of the virtual machine will be represented by standard Smalltalk conditional control structures. Conditional selections will appear as messages to Booleans. Conditional repetitions will appear as messages to blocks.
	index < length ifTrue: [ ... ] sizeFlag = 1 ifTrue: [ ... ] ifFalse: [ ... ] [currentClass ~= NilPointer] whileTrue: [ ... ]

The definition of Interpreter describes the function of the Smallta1k-80 bytecode interpreter; however, the form of a machine language implementation of the interpreter may be very different, particularly in the control structures it uses. The dispatch to the appropriate routine to execute a bytecode is an example of something a machine language interpreter might do differently. To find the right routine to execute, a machine language interpreter would probably do some kind of address arithmetic to calculate where to jump; whereas, as we will see, Interpreter does a series of conditionals and routine calls. In a machine language implementation, the routines that execute each bytecode would simply jump back to the beginning of the bytecode fetch routine when they were finished, instead of returning through the routine call structure.
Another difference between Interpreter and a machine language implementation is the degree of optimization of the code. For the sake of clarity, the routines specified in this chapter have not been optimized. For example, to perform a task, Interpreter may fetch a pointer from the object memory several times in different routines, when a more optimized interpreter might save the value in a register for later use. Many of the routines in the formal specification will not be subroutines in a machine language implementation, but will be written in-line instead.

Object Memory Interface

Chapter 26 gave an informal description of the object memory. Since the routines of Interpreter need to interact with the object memory, we need its formal functional specification. This will be presented as the protocol specification of class ObjectMemory. Chapter 30 will describe one way to implement this protocol specification.
The object memory associates a 16-bit object pointer with

the object pointer of a class-describing object and
a set of 8- or 16-bit fields that contain object pointers or numerical values.

The interface to the object memory uses zero-relative integer indices to indicate an object's fields. Instances of Integer are used for both object pointers and field indices in the interface between the interpreter and object memory.
The protocol of ObjectMemory contains pairs of messages for fetching and storing object pointers or numerical values in an object's fields.

object pointer access

fetchPointer: fieldIndex ofObject: objectPointer

Return the object pointer found in the field numbered fieldIndex of the object associated with objectPointer.

storePointer: fieldIndex ofObject: objectPointer withValue: valuePointer

Store the object pointer valuePointer in the field numbered fieldIndex of the object associated with objectPointer.

word access

fetchWord: fieldIndex ofObject: objectPointer

Return the 16-bit numerical value found in the field numbered fieldIndex of the object associated with objectPointer.

storeWord: fieldIndex ofObject: objectPointer withValue: valueWord

Store the 16-bit numerical value valueWord in the field numbered fieldIndex of the object associated with objectPointer.

byte access

fetchByte: byteIndex ofObject: objectPointer

Return the 8-bit numerical value found in the byte numbered byteIndex of the object associated with objectPointer.

storeByte: byteIndex ofObject: objectPointer withValue: valueByte

Store the 8-bit numerical value valueByte in the byte numbered byteIndex of the object associated with objectPointer.

Note that fetchPointer:ofObject: and fetchWord:ofObject: will probably be implemented in an identical fashion, since they both load a 16-bit quantity. However, the implementation of storePointer:ofObject: will be different from the implementation of storeWord:ofObject: since it will have to perform reference counting (see Chapter 30) if the object memory keeps dynamic reference counts. We have maintained a separate interface for fetchPointer:ofObject: and fetchWord:ofObject: for the sake of symmetry.
Even though most of the maintenance of reference counts can be done automatically in the storePointer:ofObject:withValue: routine, there are some points at which the interpreter routines must directly manipulate the reference counts. Therefore, the following two routines are included in the object memory interface. If an object memory uses only garbage collection to reclaim unreferenced objects, these routines are no-ops.

reference counting

increaseReferencesTo: objectPointer

Add one to the reference count of the object whose object pointer is objectPointer.

decreaseReferencesTo: objectPointer

Subtract one from the reference count of the object whose object pointer is objectPointer.

Since every object contains the object pointer of its class description, that pointer could be considered the contents of one of the object's fields. Unlike other fields, however, an object's class may be fetched, but its value may not be changed. Given the special nature of this pointer, it was decided not to access it in the same way. Therefore, there is a special protocol for fetching an object's class.

class pointer access

fetchClassOf: objectPointer

Return the object pointer of the class-describing object for the object associated with objectPointer.

The length of an object might also be thought of as the contents of one of its fields. However, it is like the class field in that it may not be changed. There are two messages in the object memory protocol that ask for the number of words in an object and the number of bytes in an object. Note that we have not made a distinction between words and pointers in this case since we assume that they both fit in exactly one field.

length access

fetchWordLengthOf: objectPointer

Return the number of fields in the object associated with objectPointer.

fetchByteLengthOf: objectPointer

Return the number of byte fields in the object associated with objectPointer.

Another important service of the object memory is to create new objects. The object memory must be supplied with a class and a length and will respond with a new object pointer. Again, there are three versions for creating objects with pointers, words, or bytes.

object creation

instantiateClass: classPointer withPointers: instanceSize

Create a new instance of the class whose object pointer is classPointer with instanceSize fields that will contain pointers. Return the object pointer of the new object.

instantiateClass: classPointer withWords: instanceSize

Create a new instance of the class whose object pointer is classPointer with instanceSize fields that will contain 16-bit numerical values. Return the object pointer of the new object.

instantiateClass: classPointer withBytes: instanceByteSize

Create a new instance of the class whose object pointer is classPointer with room for instanceByteSize 8-bit numerical values. Return the object pointer of the new object.

Two routines of the object memory allow the instances of a class to be enumerated. These follow an arbitrary ordering of object pointers. Using the numerical order of the pointers themselves is reasonable.

instance enumeration

initialInstanceOf: classPointer

Return the object pointer of the first instance of the class whose object pointer is classPointer in the defined ordering (e.g., the one with the smallest object pointer).

instanceAfter: objectPointer

Return the object pointer of the next instance of the same class as the object whose object pointer is objectPointer in the defined ordering (e.g., the one with the next larger object pointer).

Another routine of the object memory allows the object pointers of two objects to be interchanged.

pointer Swapping

swapPointersOf: firstPointer and: secondPointer

Make firstPointer refer to the object whose object pointer was secondPointer and make secondPointer refer to the object whose object pointer was firstPointer.

As described in Chapter 26, integers between -16384 and 16383 are encoded directly as object pointers with a 1 in the low-order bit position and the appropriate 2's complement value stored in the high-order 15 bits. These objects are instances of class SmallInteger. A SmallInteger's value, which would ordinarily be stored in a field, is actually determined from its object pointer. So instead of storing a value into a SmallInteger's field, the interpreter must request the object pointer of a SmallInteger with the desired value (using the integerObjectOf: routine). And instead of fetching the value from a field, it must request the value associated with the object pointer (using the integerValueOf: routine). There are also two routines that determine whether an object pointer refers to a SmallInteger (isIntegerObject:) and whether a value is in the right range to be represented as a SmallInteger (isIntegerValue:). The function of the isIntegerObject: routine can also be performed by requesting the class of the object and seeing if it is SmallInteger.

integer access

integerValueOf: objectPointer

Return the value of the instance of SmallInteger whose pointer is objectPointer.

integerObjectOf: value

Return the object pointer for an instance of SmallInteger whose value is value.

isIntegerObject: objectPointer

Return true if objectPointer is an instance of SmallInteger, false if not.

isIntegerValue: value

Return true if value can be represented as an instance of SmallInteger, false if not.

The interpreter provides two special routines to access fields that contain SmallIntegers. The fetchInteger:ofObject: routine returns the value of a SmallInteger whose pointer is stored in the specified field. The check to make sure that the pointer is for a SmallInteger is made for uses of this routine when non-SmallIntegers can be tolerated. The primitiveFail routine will be described in the section on primitive routines.

fetchInteger: fieldIndex ofObject: objectPointer

| integerPointer |

integerPointer := memory fetchPointer: fieldIndex

ofObject: objectPointer.

(memory isIntegerObject: integerPointer)

ifTrue: [^memory integerValueOf: integerPointer]

ifFalse: [^self primitiveFail]

The storeInteger:ofObject:withValue: routine stores the pointer of the SmallInteger with specified value in the specified field.

storeInteger: fieldIndex ofObject: objectPointer withValue: integerValue

| integerPointer |

(memory isIntegerValue: integerValue)

ifTrue: [integerPointer := memory integerObjectOf: integerValue.

memory storePointer: fieldIndex

ofObject: objectPointer

withValue: integerPointer]

ifFalse: [^self primitiveFail]

The interpreter also provides a routine to perform a transfer of several pointers from one object to another. It takes the number of pointers to transfer, and the initial field index and object pointer of the source and destination objects as arguments.

transfer: count fromIndex: firstFrom ofObject: fromOop toIndex: firstTo ofObject: toOop

| fromIndex toIndex lastFrom oop |

fromIndex := firstFrom.

lastFrom := firstFrom + count.

toIndex := firstTo.

[fromIndex < lastFrom] whileTrue:

[oop := memory fetchPointer: fromIndex

ofObject: fromOop.

memory storePointer: toIndex

ofObject: toOop

withValue: oop.

memory storePointer: fromIndex

ofObject: fromOop

withValue: NilPointer.

fromIndex := fromIndex + 1.

toIndex := toIndex + 1]

The interpreter also provides routines to extract bit fields from numerical values. These routines refer to the high-order bit with index 0 and the low-order bit with index 15.

extractBits: firstBitIndex to: lastBitIndex of: anInteger

^(anInteger bitShift: lastBitIndex - 15)

bitAnd: (2 raisedTo: lastBitIndex - firstBitIndex + 1) - 1

highByteOf: anInteger

^self extractBits: 0 to: 7 of: anInteger

lowByteOf: anInteger

^self extractBits 8 to: 15 of: anInteger

Objects Used by the Interpreter

This section describes what might be called the data structures of the by the interpreter. Although they are objects, and therefore more than data interpreter structures, the interpreter treats these objects as data structures. The first two types of object correspond to data structures found in the interpreters for most languages. Methods correspond to programs, subroutines, or procedures. Contexts correspond to stack frames or activation records. The final structure described in this section, that of classes, is not used by the interpreter for most languages but only by the compiler. Classes correspond to aspects of the type declarations of some other languages. Because of the nature of Smalltalk messages, the classes must be used by the interpreter at runtime.
There are many constants included in the formal specification. They mostly represent object pointers of known objects or field indices for certain kinds of objects. Most of the constants will be named and a routine that initializes them will be included as a specification of their value. As an example, the following routines initialize the object pointers known to the interpreter.

initializeSmallIntegers

"SmallIntegers"

MinusOnePointer := 65535.

ZeroPointer := 1.

OnePointer := 3.

TwoPointer := 5

initializeGuaranteedPointers

"UndefinedObject and Booleans"

NilPointer := 2.

FalsePointer := 4.

TruePointer := 6.

"Root"

SchedulerAssociationPointer := 8.

"Classes"

ClassStringPointer := 14.

ClassArrayPointer := 16.

ClassMethodContextPointer := 22.

ClassBlockContextPointer := 24.

ClassPointPointer := 26.

ClassLargePositiveintegerPointer := 28.

ClassMessagePointer := 32.

ClassCharacterPointer := 40.

"Selectors"

DoesNotUnderstandSelector := 42.

CannotReturnSelector := 44.

MustBeBooleanSelector := 52.

"Tables"

SpecialSelectorsPointer := 48.

CharacterTablePointer := 50

Compiled Methods

The bytecodes executed by the interpreter are found in instances of CompiledMethod. The bytecodes are stored as 8-bit values, two to a word. In addition to the bytecodes, a CompiledMethod contains some object pointers. The first of these object pointers is called the method header and the rest of the object pointers make up the method's literal frame. Figure 27.1 shows the structure of a CompiledMethod and the following routine initializes the indices used to access fields of CompiledMethods.

Figure 27.1

initializeMethodIndices

"Class CompiledMethod"

HeaderIndex := 0.

LiteralStart := 1

The header is a SmallInteger that encodes certain information about the CompiledMethod.

headerOf: methodPointer

^memory fetchPointer: HeaderIndex

ofObject: methodPointer

The literal frame contains pointers to objects referred to by the bytecodes. These include the selectors of messages that the method sends, and shared variables and constants to which the method refers.

literal: offset ofMethod: methodPointer

^memory fetchPointer: offset + LiteralStart

ofObject: methodPointer

Following the header and literals of a method are the bytecodes. Methods are the only objects in the Smalltalk system that store both object pointers (in the header and literal frame) and numerical values (in the bytecodes). The form of the bytecodes will be discussed in the next chapter.

Method Headers Since the method header is a SmallInteger, its value will be encoded in its pointer. The high-order 15 bits of the pointer are available to encode information; the low-order bit must be a one to indicate that the pointer is for a SmallInteger. The header includes four bit fields that encode information about the CompiledMethod. Figure 27.2 shows the bit fields of a header.

Figure 27.2

The temporary count indicates the number of temporary variables used by the CompiledMethod. This includes the number of arguments.

temporaryCountof: methodPointer

^self extractBits: 3 to: 7 of: (self headerOf: methodPointer)

The large context flag indicates which of two sizes of MethodContext are needed. The flag indicates whether the sum of the maximum stack depth and the number of temporary variables needed is greater than twelve. The smaller MethodContexts have room for 12 and the larger have room for 32.

largeContextFlagOf: methodPointer

^self extractBits: 8 to: 8 of: (self headerOf: methodPointer)

The literal count indicates the size of the CompiledMethod's literal frame. This, in turn, indicates where the CompiledMethod's bytecodes start.

literalCountOf: methodPointer

^self literalCountOfHeader: (self headerOf: methodPointer)

literalCountOfHeader: headerPointer

^self extractBits: 9 to: 14 of: headerPointer

The object pointer count indicates the total number of object pointers in a CompiledMethod, including the header and literal frame.

objectPointerCountOf: methodPointer

^(self literalCountOf: methodPointer) + LiteralStart

The following routine returns the byte index of the first bytecode of a CompiledMethod.

initialInstructionPointerOfMethod: methodPointer

^((self literalCountOf: methodPointer) + LiteralStart) * 2 + 1

The flag value is used to encode the number of arguments a CompiledMethod takes and whether or not it has an associated primitive routine.

flagValueOf: methodPointer

^self extractBits: 0 to: 2 of: (self headerOf: methodPointer)

The eight possible flag values have the following meanings:

flag value	meaning
0-4	no primitive and 0 to 4 arguments
5	primitive return of `self` (0 arguments)
6	primitive return of an instance variable (0 arguments)
7	a header extension contains the number of arguments and a primitive index

Since the majority of CompiledMethods have four or fewer arguments and do not have an associated primitive routine, the flag value is usually simply the number of arguments.

Special Primitive Methods Smalltalk methods that only return the receiver of the message (self) produce CompiledMethods that have no literals or bytecodes, only a header with a flag value of 5. In similar fashion, Smalltalk methods that only return the value of one of the receiver's instance variables produce CompiledMethods that contain only headers with a flag value of 6. All other methods produce CompiledMethods with bytecodes. When the flag value is 6, the index of the instance variable to return is found in the header in the bit field ordinarily used to indicate the number of temporary variables used by the CompiledMethod. Figure 27.3 shows a CompiledMethod for a Smalltalk method that only returns a receiver instance variable.

Figure 27.3

The following routine returns the index of the field representing the instance variable to be returned in the case that the flag value is 6.

fieldIndexOf: methodPointer

^self extractBits: 3 to: 7 of: (self headerOf: methodPointer)

Method Header Extensions If the flag value is 7, the next to last literal is a header extension, which is another SmallInteger. The header extension includes two bit fields that encode the argument count and primitive index of the CompiledMethod. Figure 27.4 shows the bit fields of a header extension.

Figure 27.4

The following routines are used to access a header extension and its bit fields.

headerExtensionOf: methodPointer

| literalCount |

literalCount := self literalCountOf: methodPointer.

^self literal: literalCount - 2

ofMethod: methodPointer

argumentCountOf: methodPointer

| flagValue |

flagValue := self flagValueOf: methodPointer.

flagValue < 5 ifTrue: [^flagValue].

flagValue < 7

ifTrue: [^0]

ifFalse: [^self extractBits: 2 to: 6

of: (self headerExtensionOf: methodPointer)]

primitiveIndexOf: methodPointer

| flagValue |

flagValue := self flagValueOf: methodPointer.

flagValue = 7

ifTrue: [^self extractBits: 7 to: 14

of: (self headerExtensionOf: methodPointer)]

ifFalse: [^0]

Any CompiledMethod that sends a superclass message (i.e., a message to super) or contains a header extension, will have as its last literal an Association whose value is the class in whose message dictionary the CompiledMethod is found. This is called the method class and is accessed by the following routine.

methodClassOf: methodPointer

| literalCount association |

literalCount := self literalCountOf: methodPointer.

association := self literal: literalCount - 1

ofMethod: methodPointer.

^memory fetchPointer: ValueIndex

ofObject: association

An example of a CompiledMethod whose literal frame contained a method class was given in the last chapter. The CompiledMethod for the intersect: message to ShadedRectangle was shown in the section of the last chapter called Messages.

Contexts

The interpreter uses contexts to represent the state of its execution of CompiledMethods and blocks. A context can be a MethodContext or a BlockContext. A MethodContext represents the execution of a CompiledMethod that was invoked by a message. Figure 27.5 shows a MethodContext and its CompiledMethod.

Figure 27.5

A BlockContext represents a block encountered in a CompiledMethod. A BlockContext refers to the MethodContext whose CompiledMethod contained the block it represents. This is called the BlockContext'shome. Figure 27.6 shows a BlockContext and its home.
The indices used to access the fields of contexts are initialized by the following routine.

initializeContextIndices

"Class MethodContext"

SenderIndex := 0.

InstructionPointerIndex := 1.

StackPointerIndex := 2.

MethodIndex := 3.

ReceiverIndex := 5.

TempFrameStart := 6.

"Class BlockContext"

CallerIndex := 0.

BlockArgumentCountIndex := 3.

InitialIPIndex := 4.

HomeIndex := 5

Both kinds of context have six fixed fields corresponding to six named instance variables. These fixed fields are followed by some indexable fields. The indexable fields are used to store the temporary frame (arguments and temporary variables) followed by the contents of the evaluation stack. The following routines are used to fetch and store the instruction pointer and stack pointer stored in a context.

Figure 27.6

instructionPointerOfContext: contextPointer

^self fetchInteger: InstructionPointerIndex

ofObject: contextPointer

storeInstructionPointerValue: value inContext: contextPointer

self storeInteger: InstructionPointerIndex

ofObject: contextPointer

withValue: value

stackPointerOfContext: contextPointer

^self fetchInteger: StackPointerIndex

ofObject: contextPointer

storeStackPointerValue: value inContext: contextPointer

self storeInteger: StackPointerIndex

ofObject: contextPointer

withValue: value

A BlockContext stores the number of block arguments it expects in one of its fields.

argumentCountOfBlock: blockPointer

^self fetchInteger: BlockArgumentCountIndex

ofObject: blockPointer

The context that represents the CompiledMethod or block currently being executed is called the active context. The interpreter caches in its registers the contents of the parts of the active context it uses most often. These registers are:

**Context-related Registers of the Interpreter**
`activeContext`	This is the active context itself. It is either a `MethodContext` or a `BlockContext`.
`homeContext`	If the active context is a `MethodContext`, the home context is the same context. If the active context is a `BlockContext`, the home context is the contents of the home field of the active context. This will always be a `MethodContext`.
`method`	This is the `CompiledMethod` that contains the bytecodes the interpreter is executing.
`receiver`	This is the object that received the message that invoked the home context's method.
`instructionPointer`	This is the byte index of the next bytecode of the method to be executed.
`stackPointer`	This is the index of the field of the active context containing the top of the stack.

Whenever the active context changes (when a new CompiledMethod is invoked, when a CompiledMethod returns or when a process switch occurs), all of these registers must be updated using the following routine.

fetchContextRegisters

(self isBlockContext: activeContext)

ifTrue: [homeContext := memory fetchPointer: HomeIndex

ofObject: activeContext]

ifFalse: [homeContext := activeContext].

receiver := memory fetchPointer: ReceiverIndex

ofObject: homeContext.

method := memory fetchPointer: MethodIndex

ofObject: homeContext.

instruction Pointer := (self instructionPointerOfContext: activeContext) - 1.

stackPointer := (self stackPointerOfContext: activeContext) + TempFrameStart - 1

Note that the receiver and method are fetched from the homeContext and the instructionPointer and stackPointer are fetched from the activeContext. The interpreter tells the difference between MethodContexts and BlockContexts based on the fact that MethodContexts store the method pointer (an object pointer) and BlockContexts store the number of block arguments (an integer pointer) in the same field. If this location contains an integer pointer, the context is a BlockContext; otherwise, it is a MethodContext. The distinction could be made on the basis of the class of the context, but special provision would have to be made for subclasses of MethodContext and BlockContext.

isBlockContext: contextPointer

| methodOrArguments |

methodOrArguments := memory fetchPointer: MethodIndex

ofObject: contextPointer.

^memory isIntegerObject: methodOrArguments

Before a new context becomes the active context, the values of the instruction pointer and stack pointer must be stored into the active context with the following routine.

storeContextRegisters

self storeInstructionPointerValue: instructionPointer + 1

inContext: activeContext.

self storeStackPointerValue: stackPointer - TempFrameStart + 1

inContext: activeContext

The values of the other cached registers do not change so they do not need to be stored back into the context. The instruction pointer stored in a context is a one-relative index to the method's fields because subscripting in Smalltalk (i.e., the at: message) takes one-relative indices. The memory, however, uses zero-relative indices; so the fetchContextRegisters routine subtracts one to convert it to a memory index and the storeContextRegisters routine adds the one back in. The stack pointer stored in a context tells how far the top of the evaluation stack is beyond the fixed fields of the context (i.e.. how far after the start of the temporary frame) because subscripting in Smalltalk takes fixed fields into account and fetches from the indexable fields after them. The memory, however, wants an index relative to the start of the object; so the fetchContextRegisters routine adds in the offset of the start of the temporary frame (a constant) and the storeContextRegisters routine subtracts the offset.
The following routines perform various operations on the stack of the active context.

push: object

stackPointer := stackPointer + 1.

memory storePointer: stackPointer

ofObject: activeContext

withValue: object

popStack

| stackTop |

stackTop := memory fetchPointer: stackPointer

ofObject: activeContext.

stackPointer := stackPointer - 1.

^stackTop

stackTop

^memory fetchPointer: stackPointer

ofObject: activeContext

stackValue: offset

^memory fetchPointer: stackPointer - offset

ofObject: activeContext

pop: number

stackPointer := stackPointer - number

unPop: number

stackPointer := stackPointer + number

The active context register must count as a reference to the part of the object memory that deallocates unreferenced objects. If the object memory maintains dynamic reference counts, the routine to change active contexts must perform the appropriate reference counting.

newActiveContext: aContext

self storeContextRegisters.

memory decreaseReferencesTo: activeContext.

activeContext := aContext.

memory increaseReferencesTo: activeContext.

self fetchContextRegisters

The following routines fetch fields of contexts needed by the interpreter infrequently enough that they are not cached in registers. The sender is the context to be returned to when a CompiledMethod returns a value (either because of a "^" or at the end of the method). Since an explicit return from within a block should return from the CompiledMethod enclosing the block, the sender is fetched from the home context.

sender

^memory fetchPointer: SenderIndex

ofObject: homeContext

The caller is the context to be returned to when a BlockContext returns a value (at the end of the block).

caller

^memory fetchPointer: SenderIndex

ofObject: activeContext

Since temporaries referenced in a block are the same as those referenced in the CompiledMethod enclosing the block, the temporaries are fetched from the home context.

temporary: offset

^memory fetchPointer: offset + TempFrameStart

ofObject: homeContext

The following routine provides convenient access to the literals of the currently executing CompiledMethod.

literal: offset

^self literal: offset

ofMethod: method

Classes

The interpreter finds the appropriate CompiledMethod to execute in response to a message by searching a message dictionary. The message dictionary is found in the class of the message receiver or one of the superclasses of that class. The structure of a class and its associated message dictionary is shown in Figure 27.7. In addition to the message dictionary and superclass the interpreter uses the class's instance specification to determine its instances' memory requirements. The other fields of a class are used only by Smalltalk methods and ignored by the interpreter. The following routine initializes the indices used to access fields of classes and their message dictionaries.

Figure 27.7

initializeClassIndices

"Class Class"

SuperclassIndex := 0.

MessageDictionaryIndex := 1.

InstanceSpecificationIndex := 2.

"Fields of a message dictionary"

MethodArrayIndex := 1.

SelectorStart := 2

The interpreter uses several registers to cache the state of the message lookup process.

**Class-related Registers of the Interpreter**
`messageSelector`	This is the selector of the message being sent. It is always a `Symbol`.
`argumentCount`	This is the number of arguments in the message currently being sent. It indicates where the message receiver can be found on the stack since it is below the arguments.
`newMethod`	This is the method associated with the `messageSelector`.
`primitiveIndex`	This is the index of a primitive routine associated with `newMethod` if one exists.

A message dictionary is an IdentityDictionary. IdentityDictionary is a subclass of Set with an additional Array containing values associated with the contents of the Set. The message selectors are stored in the indexed instance variables inherited from Set. The CompiledMethods are stored in an Array added by IdentityDictionary. A CompiledMethod has the same index in that Array that its selector has in the indexable variables of the dictionary object itself. The index at which to store the selector and CompiledMethod are computed by a hash function.
The selectors are instances of Symbol, so they may be tested for equality by testing their object pointers for equality. Since the object pointers of Symbols determine equality, the hash function may be a function of the object pointer. Since object pointers are allocated quasirandomly, the object pointer itself is a reasonable hash function. The pointer shifted right one bit will produce a better hash function, since all object pointers other than SmallIntegers are even.

hash: objectPointer

^objectPointer bitShift: -1

The message selector lookup assumes that methods have been put into the dictionary using the same hashing function. The hashing algorithm reduces the original hash function modulo the number of indexable locations in the dictionary. This gives an index in the dictionary. To make the computation of the modulo reduction simple, message dictionaries have an exact power of two fields. Therefore the modulo calculation can be performed by masking off an appropriate number of bits. If the selector is not found at the initial hash location, successive fields are examined until the selector is found or a nil is encountered. If a nil is encountered in the search, the selector is not in the dictionary. If the end of the dictionary is encountered while searching, the search wraps around and continues with the first field.
The following routine looks in a dictionary for a CompiledMethod associated with the Symbol in the messageSelector register. If it finds the Symbol, it stores the associated CompiledMethod's pointer into the newMethod register, its primitive index into the primitiveIndex register and returns true. If the Symbol is not found in the dictionary, the routine returns false. Since finding a nil or an appropriate Symbol are the only exit conditions of the loop, the routine must check for a full dictionary (i.e., no nils). It does this by keeping track of whether it has wrapped around. If the search wraps around twice, the selector is not in the dictionary.

lookupMethodInDictionary: dictionary

| length index mask wrapAround nextSelector methodArray |

length := memory fetchWordLengthOf: dictionary.

mask := length - SelectorStart - 1.

index := (mask bitAnd: (self hash: messageSelector)) + SelectorStart.

wrapAround := false.

[true] whileTrue:

[nextSelector := memory fetchPointer: index

ofObject: dictionary.

nextSelector = NilPointer ifTrue: [^false].

nextSelector = messageSelector

ifTrue: [methodArray := memory fetchPointer: MethodArrayIndex

ofObject: dictionary.

newMethod := memory fetchPointer: index - SelectorStart

ofObject: methodArray.

primitiveIndex := self primitiveIndexOf: newMethod.

^true].

index := index + 1.

index = length

ifTrue: [wrapAround ifTrue: [^false].

wrapAround := true.

index := SelectorStart]]

This routine is used in the following routine to find the method a class associates with a selector. If the selector is not found in the initial class's dictionary, it is looked up in the next class on the superclass chain. The search continues up the superclass chain until a method is found or the superclass chain is exhausted.

lookupMethodInClass: class

| currentClass dictionary |

currentClass := class.

[currentClass ~= NilPointer] whileTrue:

[dictionary := memory fetchPointer: MessageDictionaryIndex

ofObject: currentClass.

(self lookupMethodInDictionary: dictionary)

ifTrue: [^true].

currentClass := self superclassOf: currentClass].

messageSelector = DoesNotUnderstandSelector

ifTrue: [self error: 'Recursive not understood error encountered'].

self createActualMessage.

messageSelector := DoesNotUnderstandSelector.

^self lookupMethodInClass: class

superclassOf: classPointer

^memory fetchPointer: SuperclassIndex

ofObject: classPointer

The interpreter needs to do something out of the ordinary when a message is sent to an object whose class and superclasses do not contain a CompiledMethod associated with the message selector. In keeping with the philosophy of Smalltalk, the interpreter sends a message. A CompiledMethod for this message is guaranteed to be found. The interpreter packages up the original message in an instance of class Message and then looks for a CompiledMethod associated with the selector doesNotUnderstand:. The Message becomes the single argument for the doesNotUnderstand: message. The doesNotUnderstand: message is defined in Object with a CompiledMethod that notifies the user. This CompiledMethod can be overridden in a user-defined class to do something else. Because of this, the lookupMethodInClass: routine will always complete by storing a pointer to a CompiledMethod in the newMethod register.

createActualMessage

| argumentArray message |

argumentArray := memory instantiateClass: ClassArrayPointer

withPointers: argumentCount.

message := memory instantiateClass: ClassMessagePointer

withPointers: self messageSize.

memory storePointer: MessageSelectorIndex

ofObject: message

withValue: messageSelector.

memory storePointer: MessageArgumentsIndex

ofObject: message

withValue: argumentArray.

self transfer: argumentCount

fromField: stackPointer - (argumentCount - 1)

ofObject: activeContext

toField: 0

ofObject: argumentArray.

self pop: argumentCount.

self push: message.

argumentCount := 1

The following routine initializes the indices used to access fields of a Message.

initializeMessageIndices

MessageSelectorIndex := 0.

MessageArgumentsIndex := 1.

MessageSize := 2

The instance specification field of a class contains a SmallInteger pointer that encodes the following four pieces of information:

Whether the instances' fields contain object pointers or numerical values
Whether the instances' fields are addressed in word or byte quantities
Whether the instances have indexable fields beyond their fixed fields
The number of fixed fields the instances have

Figure 27.8 shows how this information is encoded in the instance specification.

Figure 27.8

The four pieces of information are not independent. If the instances' fields contain object pointers, they will be addressed in word quantities. If the instances' fields contain numerical values, they will have indexable fields and no fixed fields.

instanceSpecificationOf: classPointer

^memory fetchPointer: InstanceSpecificationIndex

ofObject: classPointer

isPointers: classPointer

| pointersFlag |

pointersFlag := self extractBits: 0 to: 0

of: (self instanceSpecificationOf: classPointer).

^pointersFlag = 1

isWords: classPointer

| wordsFlag |

wordsFlag := self extractBits: 1 to: 1

of: (self instanceSpecificationOf: classPointer)

^wordsFlag = 1

isIndexable: classPointer

| indexableFlag |

indexableFlag := self extractBits: 2 to: 2

of: (self instanceSpecificationOf: classPointer).

^indexableFlag = 1

fixedFieldsOf: classPointer

^self extractBits: 4 to: 14

of: (self instanceSpecificationOf: classPointer)

Note: the instance specification of CompiledMethod does not accurately reflect the structure of its instances since CompiledMethods are not homogeneous. The instance specification says that the instances do not contain pointers and are addressed by bytes. This is true of the bytecode section of a CompiledMethod only. The storage manager needs to know that CompiledMethods are special and actually contain some pointers. For all other classes, the instance specification is accurate.

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