Chapter 2 Assemblers - Course.ipv6.club.tw

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Chapter 2 Assemblers-- 2.4 Assembler Design Options

OutlineOne-pass assemblersMulti-pass assemblersTwo-pass assembler with overlay structure

Load-and-Go AssemblerLoad-and-go assembler generates their object codein memory for immediate execution.No object program is written out, no loader is needed.It is useful in a system oriented toward programdevelopment and testing such that the efficiency ofthe assembly process is an important consideration.Programs are re-assembled nearly every time theyare run, efficiency of the assembly process is animportant consideration.

One-Pass AssemblersScenario for one-pass assemblersGenerate their object code in memory for immediate execution –load-and-go assemblerExternal storage for the intermediate file between two passes isslow or is inconvenient to useMain problem - Forward referencesData itemsLabels on instructionsSolutionRequire that all areas be defined before they are referenced.It is possible, although inconvenient, to do so for data items.Forward jump to instruction items cannot be easily eliminated.Insert (label, address to be modified) toSYMTABUsually, address to be modified is stored in alinked-list

Sample program for a one-pass assemblerFigure 2.18, pp. 94

Forward Referencein One-pass AssemblerOmits the operand address if the symbol has not yet beendefinedEnters this undefined symbol into SYMTAB and indicates that itis undefinedAdds the address of this operand address to a list of forwardreferences associated with the SYMTAB entryWhen the definition for the symbol is encountered, scans thereference list and inserts the address.At the end of the program, reports the error if there are stillSYMTAB entries indicated undefined symbols.For Load-and-Go assemblerSearch SYMTAB for the symbol named in the END statement and jumpsto this location to begin execution if there is no error

Object Code in Memory and SYMTABFigure 2.19(a), pp.95

Object Code in Memory and SYMTABFigure 2.19(b), pp.96

If One-Pass Assemblers Need to ProduceObject CodesIf the operand contains an undefined symbol, use 0as the address and write the Text record to the objectprogram.Forward references are entered into lists as in theload-and-go assembler.When the definition of a symbol is encountered, theassembler generates another Text record with thecorrect operand address of each entry in thereference list.When loaded, the incorrect address 0 will be updatedby the latter Text record containing the symboldefinition.

Object code generated by one-pass assemblerFigure 2.18, pp.97

Multi-Pass AssemblersFor a two pass assembler, forward references insymbol definition are not allowed:ALPHABETADELTAEQUEQURESWBETADELTA1Symbol definition must be completed in pass 1.Prohibiting forward references in symboldefinition is not a serious inconvenience.Forward references tend to create difficulty for a personreading the program.

ImplementationFor a forward reference in symbol definition, we storein the SYMTAB:The symbol nameThe defining expressionThe number of undefined symbols in the defining expressionThe undefined symbol (marked with a flag *)associated with a list of symbols depend on thisundefined symbol.When a symbol is defined, we can recursivelyevaluate the symbol expressions depending on thenewly defined symbol.

Multi-pass assembler exampleFigure 2.21, pp. 99-101# of undefined symbols in thedefining expressionThe defining expressionDepending listUndefined symbol

Multi-pass assembler exampleFigure 2.21, pp. 99-1012 MAXLEN EQU BUFEND-BUFFER3PREVBTEQUBUFFER-1

Multi-pass assembler exampleFigure 2.21, pp. 99-1014BUFFERRESB40965BUFENDEQU*

Two-pass assemblerwith overlay structureWhen memory is not enoughPass 1 and pass 2 are never required at the same timeThree segmentsOverlay program!" # !" # !" #%&!" # !" # !" #

Chapter 2 Assemblers --2.4 Assembler Design Options. Outline One-pass assemblers Multi-pass assemblers Two-pass assembler with overlay structure. Load-and-Go Assembler Load-and-go assembler generates their object code in memory for immediate execution. No object program is written out, no loader is needed.

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