Chapter 2 Assemblers - Solomon.ipv6.club.tw

Chapter 2 Assemblers-- Basic Assembler Functions

OutlineBasic assembler functionsA simple SIC assemblerAssembler algorithm and data structure

Basic assembler functionsTranslating mnemonic operation codes totheir machine language equivalentsAssigning machine addresses to symboliclabels

Assembler directiveAssembler directives are pseudo instructionsThey provide instructions to the assembler itselfThey are not translated into machine operation codesSIC assembler directiveSTARTEND: specify name & starting address: end of source program, specify the firstexecution instructionBYTE, WORD, RESB, RESWEnd of recordEnd of file: a null char (00): a zero-length record

Example program (Figure 2.1 pp. 45)

Example program (Figure 2.1 pp. 45)!

Example program (Figure 2.1 pp. 45)"

Example program (Figure 2.1 pp. 45)Purpose of example programReads records from input device (code F1)Copies them to output device (code 05)At the end of the file, writes EOF on the output device, then RSUBto the operating systemData transfer (RD, WD)A buffer is used to store recordBuffering is necessary for different I/O ratesThe end of each record is marked with a null character (00)16The end of the file is indicated by a zero-length recordSubroutines (JSUB, RSUB)RDREC, WRRECSave link register first before nested jump#

A simple SIC assemblerAssembler’s functionsConvert mnemonic operation codes to their machinelanguage equivalentsConvert symbolic operands to their equivalent machineaddressesDecide the proper instruction formatConvert the data constants to internal machinerepresentationsWrite the object program and the assembly listing

DifficultConvert symbolic operands to their equivalentmachine addressesForward reference2 passesFirst pass: scan the source program forlabel definitions and assign addressesSecond pass: perform actual translation%

Example program with object code(Figure 2.2 pp. 47)

Example program with object code(Figure 2.2 pp. 47)

Example program with object code(Figure 2.2 pp. 47)

Format of object program(Figure 2.3 pp.49)Header recordCol. 1HCol. 2 7Program nameCol. 8 13Starting address of object program (hex)Col. 14-19 Length of object program in bytes (hex)Text recordCol. 1TCol. 2 7Starting address for object code in this record (hex)Col. 8 9Length of object code in this record in bytes (hex)Col. 10 69 Object code, represented in hex (2 col. per byte)End recordCol.1ECol.2 7Address of first executable instruction in object program (hex)“ ” is only for separation only

Format of object program(Figure 2.3 pp.49)% & % #' )* ,)' *./ 0' 12 *)' % !3 4%%%5!(

The two passes of an assemblerPass 1 (define symbols)Assign addresses to all statements in the programSave the addresses assigned to all labels for use in Pass 2Perform assembler directives, including those for addressassignment, such as BYTE and RESWPass 2 (assemble instructions and generate objectprogram)Assemble instructions (generate opcode and look up addresses)Generate data values defined by BYTE, WORDPerform processing of assembler directives not done during Pass 1Write the object program and the assembly listing!

Assembler algorithmand data structuresOPTAB: operation code tableSYMTAB: symbol tableLOCCTR: location counter- 66 ) 18 17 996:"

OPTABLEMnemonic operation codes Machine codeContain instruction format and lengthLOCCTR LOCCTR (instruction length)ImplementationIt is a static tableArray or hash tableUsually use a hash table (mnemonic opcodeas key)#

LOCCTRInitialize to be the beginning addressspecified in the “START” statementLOCCTR LOCCTR (instruction length)The current value of LOCCTR gives theaddress to the label encountered

SYMTABLabel name label address, type, length, flagTo indicate error conditions (Ex: multiple define)It is a dynamic tableInsert, delete and searchUsually use a hash tableThe hash function should perform non-randomkey (Ex: LOOP1, LOOP2, X, Y, Z)%

Algorithm Pass 1(Figure 2.4(a), pp.53)

Algorithm Pass 2(Figure 2.4(b), pp.54)

Chapter 2 Assemblers--Basic Assembler Functions. Outline Basic assembler functions A simple SIC assembler Assembler algorithm and data structure. Basic assembler functions Translating mnemonic operation codes to their machine language equivalents Assigning machine addresses to symbolic