ss unit-iiassembler 2007

8/12/2019 Ss Unit-iiassembler 2007

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1MC9224 System SoftwareDepartment of Computer Applications

ADHIPARASAKTHI ENGINEERING COLLEGE

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Melmaruvathur-603 319


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2MC9224- System SoftwareDepartment of Computer Applications


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OutlineBasic Assembler FunctionsA simpler SIC AssemblerAssembler Algorithm and Data Structures

Machine-Dependent Assembler FeaturesInstruction Formats and Addressing Modes

Program LocationMachine-Independent Assembler Features

LiteralsSymbol-Defining StatementsExpressionsProgram Blocks

Control Section and Program LinkingAssembler Design Options

One-Pass AssemblerMulti-Pass Assembler

Implementation ExamplesMASM Assembler


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2.1. Basic Assembler FunctionsExample Program SIC AssemblerPurpose

Reads records from input device (code F1) and store in BUFFER

Copies them to output device (code 05) At the end of the file, writes an extra EOF on the output device,then RSUB to the operating system

Data transfer (RD, WD)End of each record is marked with a null characterEnd of the file is indicated by a zero-length record

Subroutines (JSUB, RSUB)RDREC, WRRECSave link register first before nested jump


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2.1. Basic Assembler Functions

5 COPY START 1000 LOAD PROG AT LOC 100010 FIRST STL RETADR SAVE RETURN ADDRESS15 CLOOP JSUB RDREC READ INPUT RECORD20 LDA LENGTH TEST FOR EOF (LENGTH = 0)25 COMP ZERO30 JEQ ENDFIL EXIT IF EOF FOUND35 JSUB WRREC WRITE OUTPUT RECORD40 J CLOOP LOOP45 ENDFIL LDA EOF INSERT END OF FILE MARKER50 STA BUFFER55 LDA THREE SET LENGTH = 360 STA LENGTH65 JSUB WRREC WRITE EOF70 LDL RETADR GET RETURN ADDRESS

75 RSUB RETURN TO CALLER80 EOF BYTE CEOF 85 THREE WORD 390 ZERO WORD 095 RETADR RESW 1100 LENGTH RESW 1105 BUFFER RESB 4096 4096-BYTE BUFFER AREA


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2.1. Basic Assembler Functions110 .115 . SUBROUTINE TO READ RECORD INTO BUFFER120 .125 RDREC LDX ZERO CLEAR LOOP COUNTER130 LDA ZERO CLEAR A TO ZERO

135 RLOOP TD INPUT TEST INPUT DEVICE140 JEQ RLOOP LOOP UNTIL READY145 RD INPUT READ CHARACTER INTO A150 COMP ZERO TEST FOR END OF RECORD155 JEQ EXIT EXIT LOOP IF EOR160 STCH BUFFER,X STORE CHAR IN BUFFER165 TIX MAXLEN LOOP UNLESS MAX LENGTH170 JLT RLOOP HAS BEEN REACHED175 EXIT STX LENGTH SAVE RECORD LENGTH180 RSUB RETURN TO CALLER185 INPUT BYTE XF1 CODE FOR INPUT DEVICE190 MAXLEN WORD 4096


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2.1. Basic Assembler Functions195 .200 . SUBROUTINE TO WRITE RECORD FROM BUFFER205 .210 WRREC LDX ZERO CLEAR LOOP COUNTER215 WLOOP TD OUTPUT TEST OUTPUT DEVICE220 JEQ WLOOP LOOP UNTIL READY225 LDCH BUFFER,X GET CHAR FROM BUFFER230 WD OUTPUT WRITE CHARACTER235 TIX LENGTH LOOP UNTIL ALL CHAR

240 JLT WLOOP HAVE BEEN WRITTEN245 RSUB RETURN TO CALLER250 OUTPUT BYTE X05 CODE FOR OUTPUT DEVICE255 END FIRST


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2.1. Basic Assembler Functions

Assembler DirectivesPseudo-Instructions / Basic assembler directives

Not translated into machine instructions

Providing information to the assemblerBasic assembler directives

START Specifies starting memory address of object programEND Marks the end of the program

BYTE Generate Character / Hexadecimal constant 1 byteWORD Generate 1 word constantRESB Reserve the indicated number of bytes for data areaRESW Reserve the indicated number of words for data area




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2.1.1. A Simple SIC Assembler

Mnemonic code (or instruction name) opcodeSymbolic operands (e.g., variable names) MachineaddressesChoose the proper instruction format and addressingmodeConstants Equivalent Internal Machine representationOutput to object files and listing files




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2.1.1. A Simple SIC Assembler Example Program &Object Code

Line Loc Source statement Object code5 1000 COPY START 100010 1000 FIRST STL RETADR 14103315 1003 CLOOP JSUB RDREC 482039

20 1006 LDA LENGTH 00103625 1009 COMP ZERO 28103030 100C JEQ ENDFIL 30101535 100F JSUB WRREC 48206140 1012 J CLOOP 3C100345 1015 ENDFIL LDA EOF 00102A50 1018 STA BUFFER 0C103955 101B LDA THREE 00102D60 101E STA LENGTH 0C103665 1021 JSUB WRREC 48206170 1024 LDL RETADR 08103375 1027 RSUB 4C0000




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Line Loc Source statement Object code

80 102A EOF BYTE CEOF 454F4685 102D THREE WORD 3 00000390 1030 ZERO WORD 0 00000095 1033 RETADR RESW 1100 1036 LENGTH RESW 1105 1039 BUFFER RESB 4096




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110 . 115 . SUB TO READ RECORD INTO BUFFER120 . 125 2039 RDREC LDX ZERO 041030

130 203C LDA ZERO 001030135 203F RLOOP TD INPUT E0205D140 2042 JEQ RLOOP 30203F145 2045 RD INPUT D8205D150 2048 COMP ZERO 281030155 204B JEQ EXIT 302057160 204E STCH BUFFER,X 549039165 2051 TIX MAXLEN 2C205E170 2054 JLT RLOOP 38203F175 2057 EXIT STX LENGTH 101036180 205A RSUB 4C0000185 205D INPUT BYTE XF1 F1190 205E MAXLEN WORD 4096 001000




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195 .200 . SUB TO WRITE RECORD FROM BUFFER205 .

210 2061 WRREC LDX ZERO 041030215 2064 WLOOP TD OUTPUT E02079220 2067 JEQ WLOOP 302064225 206A LDCH BUFFER,X 509039230 206D WD OUTPUT DC2079235 2070 TIX LENGTH 2C1036240 2073 JLT WLOOP 382064245 2076 RSUB 4C0000250 2079 OUTPUT BYTE X05 05255 END FIRST




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2.1.1. A Simple SIC Assembler Mnemonic code (or instruction name) opcodeExamples:STL 1033 14 10 33

LDA 1036 00 10 36

0001 0100 0 001 0000 0011 0011

0000 0000 0 001 0000 0011 0110




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2.1.1. A Simple SIC Assembler Converting Symbolto Numbers

Isn t it straightforward?Isn t it simply the sequential processing of the sourceprogram, one line at a time?

Not so, if we have forward references we don t know the value of the symbol, because it isdefined later in the code Need two pass Assembler

Loc Label Operator Operand1000 FIRST STL RETADR

1003 CLOOP JSUB RDREC

1012 J CLOOP 1033 RETADR RESW 1


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2.1.1. A Simple SIC Assembler Symbolic Operands

We are not likely to write memory addressesdirectly in our code

Instead, we will define variable names

Other examples of symbolic operands:Labels (for jump instructions)Subroutines

Constants


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2.1.1. A Simple SIC Assembler Two Pass AssemblerPass1

Scan the source programIdentify the label definitions and assign addresses

Pass2 All other assembler operations

Load and Execution


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2.1.1. A Simple SIC Assembler Object Program

H^ COPY ^ 001000 ^ 00107AT^ 001000 ^ 1E^ 141033 ^ 482039 ^ 001036 ^ 281030 ^ 301015 ^ 482061 ^ 3C1003 ^ 00102A ^ 0C1039 ^ 00102DT^ 00101E ^ 15 ^ 0C1036 ^ 482061 ^ 081044 ^ 4C0000 ^ 454F46 ^ 000003 ^ 000000T^ 002039 ^ 1E^ 041030 ^ 001030 ^ E0205D ^ 30203F ^ D8205D ^ 281030 ^ 302057 ^ 549039 ^ 2C205E ^ 38203FT^ 002057 ^ 1C^ 101036 ^ 4C0000 ^ F1^ 001000 ^ 041030 ^ E02079 ^ 302064 ^ 509039 ^ DC2079 ^ 2C1036T^ 002073 ^ 07 ^ 382064 ^ 4C0000 ^ 05E^ 001000 starting address


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2.1.1. A Simple SIC Assembler

Pass 1 Assign addresses to all statements in the program

Save the values assigned to all labels for use in Pass 2Perform some processing of assembler directives

Pass 2 Assemble instructions by translating opcode and symbolicoperandsGenerate data values defined by BYTE, WORDPerform processing of assembler directives not done in Pass 1Write the object program and the assembly listing

Functions of a Two Pass Assembler


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2.1.2. Assembler Algorithm and Data Structures

Internal Data Structures Two Major Data structures

Operation Code Table (OPTAB)

Symbol Table (SYMTAB)One variable - Location Counter (LOCCTR)

Pass 1 Pass 2Intermediatefile

OPTAB SYMTAB SYMTAB

Sourceprogram

Objectcode


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2.1.2. Assembler Algorithm and Data Structures OPTAB (Operation Code Table)

ContentMnemonic names, machine code (instruction format, length forSIC/XE) etc.

CharacteristicStatic table (assembler itself written)Implementation

Array or hash table(mnemonic operation code as key), easy forsearch

UsagePass 1 - look up and validate mnemonic operation codes in thesource programPass 2 translate opcodes into machine language


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2.1.2. Assembler Algorithm and Data Structures SYMTAB (Symbol Table)

ContentLabel name, value, flag, (type, length for dataarea ) etc.

CharacteristicDynamic table (insert, delete, search)

ImplementationHash table, non-random keys, hashingfunction

UsagePass 1 - enter symbol names into SYMTABalong with the assign address ( from LOCCTR)Pass 2 look up symbols, fetch address andinsert in object code

COPY 1000FIRST 1000CLOOP 1003

ENDFIL 1015EOF 1024THREE 102DZERO 1030RETADR 1033LENGTH 1036BUFFER 1039RDREC 2039


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2.1.2. Assembler Algorithm and Data Structures Intermediate File

Pass1 and pass2 use source program as input.Other information should be passed between twopasses.Pass 1 writes an intermediate file

Each source statement together with its assigned addressError indicatorsUsed as input for pass 2


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2.1.2. Assembler Algorithm and Data Structures Algorithm for Pass1 Assembler


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2.2. Machine-Dependent Assembler Features SIC/XE Assembler

We have learned the 2-pass assembler for SICWhats new for SIC/XE?

More addressing modesProgram relocation

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2.2.1. Instruction Formats and Addressing Modes

SIC/XE:PC-relative or base-relative addressing: op mIndirect addressing: op @mImmediate addressing: op #cExtended format: +op mIndex addressing: op m,xRegister-to-register instructionsLarger memory multi-programming (programallocation)

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2.2.1. Instruction Formats and Addressing ModesRegister translation during pass 2

Register name (A, X, L, B, S, T, F, PC, SW) translated totheir ids (0,1, 2, 3, 4, 5, 6, 8, 9)Use separate table or may be preloaded in SYMTAB

Address translationRegister-memory instructions: try PC-relative first, thenbase-relative addressing

Assembler makes its own decisionUser must specify extended format (format 4), otherwise errorwill be generated by the assembler

Format 3: 12-bit displacementBase-relative: 0~4095 PC-relative: -2048~2047

Format 4: 20-bit address field

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2.2.1. Instruction Formats and Addressing ModesLine Loc Source statement Object code 5 0000 COPY START 010 0000 FIRST STL RETADR 17202D12 0003 LDB #LENGTH 69202D13 BASE LENGTH15 0006 CLOOP +JSUB RDREC 4B10103620 000A LDA LENGTH 03202625 000D COMP #0 29000030 0010 JEQ ENDFIL 33200735 0013 +JSUB WRREC 4B10105D40 0017 J CLOOP 3F2FEC45 001A ENDFIL LDA EOF 03201050 001D STA BUFFER 0F201655 0020 LDA #3 010003

60 0023 STA LENGTH 0F200D65 0026 +JSUB WRREC 4B10105D70 002A J @RETADR 3E200380 002D EOF BYTE CEOF 454F4695 0030 RETADR RESW 1100 0033 LENGTH RESW 1105 0036 BUFFER RESB 4096

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2.2.1. Instruction Formats and Addressing Modes115 . READ RECORD INTO BUFFER120 .125 1036 RDREC CLEAR X B410130 1038 CLEAR A B400132 103A CLEAR S B440

133 103C +LDT #4096 75101000135 1040 RLOOP TD INPUT E32019140 1043 JEQ RLOOP 332FFA145 1046 RD INPUT DB2013150 1049 COMPR A, S A004155 104B JEQ EXIT 332008160 104E STCH BUFFER,X 57C003165 1051 TIXR T B850170 1053 JLT RLOOP 3B2FEA175 1056 EXIT STX LENGTH 134000180 1059 RSUB 4F0000185 105C INPUT BYTE XF1 F1

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2.2.1. Instruction Formats and Addressing Modes195 .200 . WRITE RECORD FROM BUFFER205 .210 105D WRREC CLEAR X B41012 105F LDT LENGTH 774000215 1062 WLOOP TD OUTPUT E32011220 1065 JEQ WLOOP 332FFA225 1068 LDCH BUFFER,X 53C003230 106B WD OUTPUT DF2008235 106E TIXR T B850240 1070 JLT WLOOP 3B2FEF245 1073 RSUB 4F0000250 1076 OUTPUT BYTE X05 05255 END FIRST

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2.2.1. Instruction Formats and Addressing Modes

10 0000 FIRST STL RETADR 17202D

Displacement= RETADR PC = 0030 0003 = 02D

40 0017 J CLOOP 3F2FEC

Displacement= CLOOP PC= 0006 001A= 14= FEC

OPCODE e Addressn i x b p

0001 01 0 (02D) 16 1 1 0 0 1


0011 11 0 (FEC) 16 1 1 0 0 1

PC Relative Addressing

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2.2.1. Instruction Formats and Addressing Modes Base Relative Addressing Mode

BASE register and directive:12 LDB #LENGTH13 BASE LENGTH

Base register is under the control of programmerBASE directive tells assembler that LENGHTH is base address;NOBASE releases the binding, until B value will be used as Baseaddress

160 104E STCH BUFFER, X 57C003

Displacement= BUFFER B = 0036 0033 = 3Compare lines 20 and 175 (PC vs Base addressing)


0101 01 0 (003) 16 1 1 1 1 0

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2.2.1. Instruction Formats and Addressing Modes Base Relative Addressing Mode

Why cannot we use PC-relative?

Assembler knows the PC value at execution time, it cannotbe changeableBase register is under the control of programmer, and setinitially and changeable

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55 0020 LDA #3 010003

133 103C +LDT #4096 75101000


0000 00 0 (003) 16 0 1 0 0 0


0111 01 1 (01000) 16 0 1 0 0 0

2.2.1. Instruction Formats and Addressing Modes Immediate Address Translation

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2.2.1. Instruction Formats and Addressing ModesImmediate Address Translation

12 0003 LDB #LENGTH 69202D

12 0003 LDB #LENGTH 690033

The immediate operand is the value of the symbol LENGTH, which isthe address assigned to LENGTHLENGTH=0033=PC+displacement=0006+02D


0110 10 0 (02D) 16 0 1 0 0 1


0110 10 0 (033) 16 0 1 0 0 0

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2.2.1. Instruction Formats and Addressing Modes Indirect Address Translation

Target addressing is computed as usual (PC-relativeor BASE-relative)Only the n bit is set to 1

70 002A J @RETADR 3E2003

TA=RETADR=0030TA=(PC)+displacement=002D+0003


0011 11 0 (003) 16 1 0 0 0 1

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2.2.2. Program Relocation

Need for program relocationExample :

If program starts at 1000

55 101B LDA THREE 00102DIf program starts at 2000

55 201B LDA THREE 00202DSome lines need not be modified (line 85, value 3)

Assembler does not know that where the program willbe loaded, but it can inform to loader via modificationrecords.

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Loaded at 0000 Loaded at 5000 Loaded at 7420


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Example Fig. 2.2 Absolute program , starting address 1000

5 2000 1000 COPY START 100010 2000 1000 FIRST STL RETADR 141033 14203315 2003 1003 CLOOP JSUB RDREC 482039 48303920 2006 1006 LDA LENGTH 001036 00203625 2009 1009 COMP ZERO 281030 28203030 200C 100C JEQ ENDFIL 301015 30201535 200F 100F JSUB WREC 482061 48306140 2012 1012 J CLOOP 3C1003 3C200345 2015 1015 ENDFIL LDA EOF 00102A 00202A50 2018 1018 STA BUFFER 0C1039 0C203955 201B 101B LDA THREE 00102D 00202D60 201E 101E STA LENGTH 0C1036 0C203665 2021 1021 JSUB WREC 482061 483061

70 2024 1024 LDL RETADR 081033 08203375 2027 1027 RSUB 4C0000 4C000080 202A 102A EOF BYTE C'EOF' 454E46 454E4685 202D 102D THREE WORD 3 000003 00000390 2030 1030 ZERO WORD 0 000000 00000095 2033 1033 RETADR RESW 1100 2036 1036 LENGTH RESW 1105 2039 1039 BUFFER RESB 4096

==== 2000==== 2000


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Example Fig. 2.6:Except for absolute address, rest of the instructions need not be modifiednot a memory address (immediate addressing)PC-relative, Base-relative

Parts requiring modification at load time are those with absolute addresses

== 10005 1000 0000 COPY START 010 1000 0000 FIRST STL RETADR 17202D 17202D12 1003 0003 LDB #LENGTH 69202D 69202D13 BASE LENGTH15 1006 0006 CLOOP +JSUB RDREC 4B101036 4B10 203620 100A 000A LDA LENGTH 032026 03202625 100D 000D COMP #0 290000 29000030 1010 0010 JEQ ENDFIL 332007 33200735 1013 0013 +JSUB WRREC 4B10105D 4B10 205D40 1017 0017 J CLOOP 3F2FEC 3F2FEC45 101A 001A ENDFIL LDA EOF 032010 032010

50 101D 001D STA BUFFER 0F2016 0F201655 1020 0020 LDA #3 010003 01000360 1023 0023 STA LENGTH 0F200D 0F200D65 1026 0026 +JSUB WRREC 4B10105D 4B10 205D70 102A 002A J @RETADR 3E2003 3E200380 102D 002D EOF BYTE C'EOF' 454F46 454F4695 1030 0030 RETADR RESW 1100 1036 0036 BUFFER RESB 4096


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Use relative addressesDid you notice that we didn t modify the addresses for JEQ, JLTand J instructions?

We didn t modify the addresses for RETADR, LENGTH, andBUFFER in Figure 2.6 either.

The sample SIC/EX program is easierMostly PC or base relative

Only extended format instructions have direct addresses andrequire modification

Making Program Relocation Easier



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Modification records are added to the object files.(See pp.67 and Figure 2.8.)Example:

HCOPY 001000 001077T000000 1D 17202D4B101036 T00001D

M000007 05 Modification Record E000000

Object File with M-Records


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Modification Record

0009 3 61 0

1 04 B2 D

0008

000700060005

+JSUB RDREC

0004 2 06 92 D2 01 7

0003000200010000

LDB #LENGTH

STL RETADR

000C 2 62 00 3

000B000A LDA LENGTH

M000007 05

Address 0007

5 half- bytes


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Object Code


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50MC9224 - System SoftwareDepartment of Computer Applications


Design ideaLet programmers write the value of a constantoperand as a part of the instruction that uses it

Avoids having to define the constant elsewhere inthe program and make up a label for it

Example (Fig. 2.10)045 001A ENDFIL LDA =CEOF 215 1062 WLOOP TD =X 05

2.3. Machine-Independent Assembler Features - Literals

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Example Program5 0000 COPY START 010 0000 FIRST STL RETADR 17202D13 0003 LDB #LENGTH 69202D14 BASE LENGTH15 0006 CLOOP +JSUB RDREC 4B10103620 000A LDA LENGTH 03202625 000D COMP #0 29000030 0010 JEQ ENDFIL 33200735 0013 +JSUB WRREC 4B10105D40 0017 J CLOOP 3F2FEC45 001A ENDFIL LDA =CEOF 03201050 001D STA BUFFER 0F201655 0020 LDA #3 01000360 0023 STA LENGTH 0F200D65 0026 +JSUB WRREC 4B10105D

70 002A J @RETADR 3E200393 LTORG95 0030 RETADR RESW 1100 0033 LENGTH RESW 1105 0036 BUFFER RESB 4096106 1036 BUFEND EQU *107 1000 MAXLEN EQU BUFEND - BUFFER

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Example Program115 . READ RECORD INTO BUFFER120 .125 1036 RDREC CLEAR X B410130 1038 CLEAR A B400132 103A CLEAR S B440

133 103C +LDT #MAXLEN 75101000135 1040 RLOOP TD INPUT E32019140 1043 JEQ RLOOP 332FFA45 1046 RD INPUT DB2013150 1049 COMPR A,S A004155 104B JEQ EXIT 332008160 104E STCH BUFFER,X 57C003165 1051 TIXR T B850170 1053 JLT RLOOP 3B2FEA175 1056 EXIT STX LENGTH 134000180 1059 RSUB 4F0000185 105C INPUT BYTE XF1 F1

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Example Program

195 .200 . WRITE RECORD FROM BUFFER205 .

210 105D WRREC CLEAR X B410212 105F LDT LENGTH 774000215 1062 WLOOP TD =X05 E32011220 1065 JEQ WLOOP 332FFA225 1068 LDCH BUFFER,X 53C003230 106B WD =X05 DF2008

235 106E TIXR T B850240 1070 JLT WLOOP 3B2FEF245 1073 RSUB 4F0000255 END FIRST

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Literal vs. Immediate Operands

Immediate operandsOperand value is assembled as part of the machineinstruction

55 0020 LDA #3 010003Literals

The assembler generates the specified value as a constantat some other memory location

45 001A ENDFIL LDA =CEOF 032010Compare (Fig. 2.6)45 001A ENDFIL LDA EOF 03201080 002D EOF BYTE CEOF 454F46

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Literal Implementation (1/4)

Literal pools All the literals used in the program grouped together in toone or more literal pools

Normally literals are placed into a pool at the end of theprogram

See Fig. 2.10 (after END statement)

In some cases, it is desirable to place literals into a pool atsome other location in object program

Use assembler directive LTORG: create a literal pool thatcontains all of the literal operands used since the previousLTROG, and place it where LTORG was encounteredReason: keep literal operand close to the instruction

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Example Program with Literal Implementation5 0000 COPY START 010 0000 FIRST STL RETADR 17202D13 0003 LDB #LENGTH 69202D14 BASE LENGTH15 0006 CLOOP +JSUB RDREC 4B10103620 000A LDA LENGTH 03202625 000D COMP #0 290000

30 0010 JEQ ENDFIL 33200735 0013 +JSUB WRREC 4B10105D40 0017 J CLOOP 3F2FEC45 001A ENDFIL LDA =CEOF 03201050 001D STA BUFFER 0F201655 0020 LDA #3 01000360 0023 STA LENGTH 0F200D65 0026 +JSUB WRREC 4B10105D70 002A J @RETADR 3E200393 LTORG002D * =CEOF 454F4695 0030 RETADR RESW 1100 0033 LENGTH RESW 1105 0036 BUFFER RESB 4096106 1036 BUFEND EQU *107 1000 MAXLEN EQU BUFEND - BUFFER

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115 . READ RECORD INTO BUFFER120 .125 1036 RDREC CLEAR X B410130 1038 CLEAR A B400

132 103A CLEAR S B440133 103C +LDT #MAXLEN 75101000135 1040 RLOOP TD INPUT E32019140 1043 JEQ RLOOP 332FFA145 1046 RD INPUT DB2013150 1049 COMPR A,S A004155 104B JEQ EXIT 332008160 104E STCH BUFFER,X 57C003165 1051 TIXR T B850170 1053 JLT RLOOP 3B2FEA175 1056 EXIT STX LENGTH 134000180 1059 RSUB 4F0000185 105C INPUT BYTE XF1 F1

Example Program with Literal Implementation

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Example Program with Literal Implementation

195 .200 . WRITE RECORD FROM BUFFER205 .210 105D WRREC CLEAR X B410212 105F LDT LENGTH 774000215 1062 WLOOP TD =X05 E32011220 1065 JEQ WLOOP 332FFA225 1068 LDCH BUFFER,X 53C003230 106B WD =X05 DF2008235 106E TIXR T B850240 1070 JLT WLOOP 3B2FEF245 1073 RSUB 4F0000255 END FIRST

1076 * =X05 05

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Duplicate literals215 1062 WLOOP TD =X05

230 106B WD =X05 Assembler should recognize duplicate literals andstore only one copy of specified data value

By comparing defining character string, e.g. =X05 By comparing the generated data value, e.g. =CEOF and=X454F46, but benefits are usually not great enough to

justify the additional complexity in the assembler


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Literal Implementation (3/4) Problem of duplicate literalrecognition

'*' denotes a literal refer to the current value oflocation counter.Some have same name, but different values.

BASE *LDB =*

If the literal value represents an 'address' in theprogram, the assembler must also generate theappropriate 'Modification records'

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Use LITTAB:Literal name, operand value & length, operand address

Pass 1

Build LITTAB with literal name, operand value/lengthWhen LTORG is encountered, assign address to each literal not yetassigned an address, update LOCCTR

Pass 2Search LITTAB for each literal encountered and generate the

operand addressData values of literals in literal pool are generated similarly as usingBYTE or WORD statementsGenerate modification record for literals that represent an addressin the program

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2.3.2. Machine-Independent Assembler Features Symbol Defining

statement

Users can define labels on instructions or data areasThe value of a label is the address assigned to the statement

Users can also define symbols with valuessymbol EQU value

value can be constants, other symbols, expressionsMaking source program easier to understandNo forward reference

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statementExample 1: replacing symbolic names133 LDT #4096replaced as

MAXLEN EQU 4096133 +LDT #MAXLEN

Example 2: defining mnemonic names for registers A EQU 0

X EQU 1

RMO A,Xcan be replaced as

RMO 0,1

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statementExample 3: defining registers with symbolic namesv(SIC)

BASE EQU R1COUNT EQU R2INDEX EQU R3

To represent registers 1,2,3


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statement Assembler directive ORG

Allow the assembler to reset the PC to values. ORG values

Use to indirectly assign values to symbols When ORG is encountered, the assembler resets its

LOCCTR to the specified value ORG will affect the values of all labels defined until the

next ORG. If the previous value of LOCCTR can be automaticallyremembered, we can return to the normal use ofLOCCTR by simply write ORG


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statement

In the data structureSymbol :6 bytes

Value :3 bytesFlags :2 bytes

If EQU statements are used

STAB RESB 1100SYMBOL EQU STAB VALUE EQU STAB+6FLAG EQU STAB+9

Symbol table structure to support ORG

Symbol Value Flags

STAB

(100 entries)

: : :


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statement

If ORG statements are usedSTAB RESB 1100

ORG STAB Set LOCCTR to STAB

SYMBOL RESB 6 VALUE RESW 1 Size of each fieldFLAGS RESB 2

ORG STAB +1100 Restore LOCCTR

VALUE field fetched byLDA VALUE,X


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statement

All symbols used in the right hand side must be definedpreviously in the program

ALPHA RESW 1BETA EQU ALPHA

BETA EQU ALPHA ALPHA RESW 1

ALPHA EQU BETABETA EQU DELTADELTA RESW 1

Restriction with EQU

Valid

Invalid

Invalid


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statement

All symbols used to specify the new location countervalue must have been previously defined

ORG ALPHABYTE1 RESB 1BYTE2 RESB 2

BYTE3 RESB 3ORG ALPHA RESB 1

Restriction with ORG

Invalid


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2.3.3. Machine-Independent Assembler Features Expressions

Assemblers allow the use of expressions as operand The assembler calculates the expressions and produce a single

operand address or value. Operator +,-,*,/

Division is usually defined to produce an integer result Expressions may be constants, user-defined symbols or specialterms.

106 1036 BUFFEND EQU * Gives BUFFEND a value that is the address of the next byte after

the buffer area Examples MAXLEN EQU BUFEND-BUFFER STAB RESB (6+3+2)*MAXENTRIES


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Values of terms can be Absolute (independent of program location)

Constants

Relative (to the beginning of the program) Address labels * (value of LOCCTR)


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Expressions can be Absolute

Only absolute termsRelative terms in pairs with opposite signs for each pair.

Relative All the relative terms except one can be paired as described in absolute .The remaining unpaired relative term must have a positivesign.No relative terms may enter into a multiplication or divisionoperation

Expressions that do not meet the conditions of either absolute or relative should be flagged as errors.


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Assembler needs to track type of anexpression, to generate Modificationrecords if neededNeed a flag in the SYMTAB for

indication Absolute value

BUFFEND BUFFER

Illegal

BUFFEND + BUFFER100 BUFFER3* BUFFER

Handling relative terms in SYMTAB

Symbol Type Flag

RETADR R 0030

BUFFER R 0036BUFEND R 1036

MAXLEN A 1000


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2.3.4. Machine-Independent Assembler Features Program Blocks

Allow the generated machine instructions and data toappear in the object program in a different order

Gather all code segments, data segments and stack segments

Program blocks vs. Control sectionsProgram blocks

Segments of code that are rearranged within a single objectprogram unit

Control sectionSegments of code that are translated into independent objectprogram units


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Assembler directive : USEUSE [blockname]

At the beginning, statements are assumed to be part of the unnamed(default) block If no USE statements are included, the entire program belongs to this

single block Each program block may actually contain several separate segments ofthe source program

Assembler rearrange these segments to gather together the pieces ofeach block and assign address

Separate the program into blocks in a particular orderLarge buffer area is moved to the end of the object programProgram readability is better if data areas are placed in the sourceprogram close to the statements that reference them


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Three blocks are useddefault: executable instructionsCDATA: all data areas that are less in length

CBLKS: all data areas that consists of largerblocks of memory


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Line Source statement Object Code 5 0000 0 COPY START 010 0000 0 FIRST STL RETADR 17202D15 0003 0 CLOOP JSUB RDREC 4B202120 0006 0 LDA LENGTH 03202625 0009 0 COMP #0 29000030 000C 0 JEQ ENDFIL 33200635 000F 0 JSUB WRREC 4B203B40 0012 0 J CLOOP 3F2FEE45 0015 0 ENDFIL LDA =CEOF 03205550 0018 0 STA BUFFER 0F205655 001B 0 LDA #3 01000360 001E 0 STA LENGTH 0F204865 0021 0 JSUB WRREC 4B202970 0024 0 J @RETADR 3E203F

92 0000 1 USE CDATA95 0000 1 RETADR RESW 1100 0003 1 LENGTH RESW 1103 0000 2 USE CBLKS105 0000 2 BUFFER RESB 4096106 1000 2 BUFEND EQU *107 1000 MAXLEN EQU BUFEND - BUFFER

(Figure 2.11)


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115 . READ RECORD INTO BUFFER120 .123 0027 0 USE125 0027 0 RDREC CLEAR X B41030 0029 0 CLEAR A B400132 002B 0 CLEAR S B440133 002D 0 +LDT #MAXLEN 75101000135 0031 0 RLOOP TD INPUT E32038140 0034 0 JEQ RLOOP 332FFA145 0037 0 RD INPUT DB2032150 003A 0 COMPR A,S A004155 003C 0 JEQ EXIT 332008160 003F 0 STCH BUFFER,X 57A02F165 0042 0 TIXR T B850170 0044 0 JLT RLOOP 3B2FEA175 0047 0 EXIT STX LENGTH 13201F180 004A 0 RSUB 4F0000183 0006 1 USE CDATA185 0006 1 INPUT BYTE XF1 F1


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195 .200 . WRITE RECORD FROM BUFFER205 .208 004D 0 USE210 004D 0 WRREC CLEAR X B410212 004F 0 LDT LENGTH 772017

215 0052 0 WLOOP TD = X05 E3201B220 0055 0 JEQ WLOOP 332FFA225 0058 0 LDCH BUFFER,X 53A016230 005B 0 WD = X05 DF2012235 005E 0 TIXR T B850240 0060 0 JLT WLOOP 3B2FEF245 0063 0 RSUB 4F0000252 0007 1 USE CDATA253 LTORG

0007 1 * = CEOF 454F46000A 1 * = X05 05

255 END FIRST


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Pass 1 A separate location counter for each program block

Save and restore LOCCTR when switch between blocks At the beginning of a block, LOCCTR is set to 0.

Assign each label an address relative to the start of the block

Store the block name or number in the SYMTAB along with the assignedrelative address of the labelIndicate the block length as the latest value of LOCCTR for each blockat the end of Passssss1

Assign to each block a starting address in the object program byconcatenating the program blocks in a particular order

Pass 2The address of each symbol can be computed by adding the assignedblock starting address and the relative address of the symbol to thatblock


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Line Loc/Block Source statement Object Code 5 0000 0 COPY START 010 0000 0 FIRST STL RETADR 17202D15 0003 0 CLOOP JSUB RDREC 4B202120 0006 0 LDA LENGTH 03202625 0009 0 COMP #0 29000030 000C 0 JEQ ENDFIL 33200635 000F 0 JSUB WRREC 4B203B40 0012 0 J CLOOP 3F2FEE45 0015 0 ENDFIL LDA =CEOF 03205550 0018 0 STA BUFFER 0F205655 001B 0 LDA #3 01000360 001E 0 STA LENGTH 0F204865 0021 0 JSUB WRREC 4B202970 0024 0 J @RETADR 3E203F92 0000 1 USE CDATA95 0000 1 RETADR RESW 1100 0003 1 LENGTH RESW 1103 0000 2 USE CBLKS105 0000 2 BUFFER RESB 4096106 1000 2 BUFEND EQU *107 1000 MAXLEN EQU BUFEND - BUFFER

(Figure 2.12)

3 blocks:Default (0)CDATA (1)CBLKS (2)


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115 . READ RECORD INTO BUFFER120 .123 0027 0 USE125 0027 0 RDREC CLEAR X B41030 0029 0 CLEAR A B400132 002B 0 CLEAR S B440133 002D 0 +LDT #MAXLEN 75101000135 0031 0 RLOOP TD INPUT E32038140 0034 0 JEQ RLOOP 332FFA145 0037 0 RD INPUT DB2032150 003A 0 COMPR A,S A004155 003C 0 JEQ EXIT 332008160 003F 0 STCH BUFFER,X 57A02F165 0042 0 TIXR T B850170 0044 0 JLT RLOOP 3B2FEA175 0047 0 EXIT STX LENGTH 13201F180 004A 0 RSUB 4F0000183 0006 1 USE CDATA185 0006 1 INPUT BYTE XF1 F1


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195 .200 . WRITE RECORD FROM BUFFER205 .208 004D 0 USE210 004D 0 WRREC CLEAR X B410212 004F 0 LDT LENGTH 772017

215 0052 0 WLOOP TD = X05 E3201B220 0055 0 JEQ WLOOP 332FFA225 0058 0 LDCH BUFFER,X 53A016230 005B 0 WD = X05 DF2012235 005E 0 TIXR T B850240 0060 0 JLT WLOOP 3B2FEF245 0063 0 RSUB 4F0000252 0007 1 USE CDATA253 LTORG

0007 1 * = CEOF 454F46000A 1 * = X05 05

255 END FIRST


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At the end of pass1, assembler constructs a tabEach source line is given a relative address and a blocknumber

Absolute symbol has no block number (line 107)20 0006 0 LDA LENGTH 032060

LENGTH = (Block 1) + 0003 = 0066 + 0003 = 0069LOCCTR = (Block 0) + 0009 = 0009Displacement = 0069 0009 = 060

Block Name Block Number Address Length

(default) 0 0000 066CDATA 1 0066 000B

CBLKS 2 0071 1000


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Program readabilityNo extended format instructions (lines 15, 35, 65)No need for base relative addressing (line 13, 14)LTORG is used to make sure the literals are placedahead of any large data areas (line 253)

Object codeIt is not necessary to physically rearrange the

generated code in the object program; loader canhandle itSee Fig. 2.13, Fig. 2.14


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CDATA


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2.3.5. Machine-Independent Assembler Features Control section

and Program LinkingControl sectionsMost often used for subroutines or other logicalsubdivisions of a program

Programmer can assemble, manipulate, and loadeach of these control sections separatelyInstructions in one control section may need to referto instructions or data in another section

Thus, there should be some means for linking controlsections togetherFig. 2.15, 2.16: three control sections (COPY, RDREC,WRREC)


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and Program Linking

(Figure 2.16)


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and Program Linking


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and Program Linking


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and Program LinkingExternal definition

EXTDEF name [, name]Declare symbols that are defined in this control section and used byother sections

External referenceEXTREF name [,name]

Declare symbols that are used in this control section and are definedelsewhere

For EXTREF labels, assembler has no idea where the correspondingcontrol section will be loaded use 015 0003 CLOOP +JSUB RDREC 4B100000

160 0017 +STCH BUFFER,X 57900000190 0028 MAXLEN WORD BUFEND-BUFFER 000000

External Definition and References


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and Program Linking

Assembler must include information in object program that willcause loader to insert proper values where requiredDefine record

Col. 1 DCol. 2-7 Name of external symbol defined in this control

sectionCol. 8-13 Relative address within this control section (hex)Col.14-73 Repeat info in Col. 2-13 for other external symbols

Refer recordCol. 1 RCol. 2-7 Name of external symbol referred to in this sectionCol. 8-73 Name of other external reference symbols

Implementation


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and Program Linking

Modification recordCol. 1 MCol. 2-7 Starting address of the field to be modified (hex)Col. 8-9 Length of the field to be modified, in half-bytes (hex)Col.11-16 External symbol whose value is to be added to or subtractedfrom the indicated fieldNote: control section name is automatically an external symbol, i.e. it isavailable for use in Modification records.

ExampleFigure 2.17M00000405+RDRECM00000705+COPY

Modification Record


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and Program Linking

H COPY 000000 001033D BUFFER 00033 BUFEND 001033 LENGTH 00002DR RDREC WRRECT 000000 1D 172027 4B100000 032023 290000 332007 4B100000 ...T 00001D 0D 010003 0F200A 4B10000 3E2000T 000030 03 454F46M 000004 05+RDRECM 000011 05+WRRECM 000024 05+WRRECE 000000

Figure 2.17 (1/2)


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and Program LinkingH RDREC 000000 00002BR BUFFER LENGTH BUFENDT 000000 1D B410 B400 B440 77201F E3201B 332FFA DB2015 ...T 00001D 0E 3B2FE9 13100000 4F0000 F1 000000

M 000018 05+BUFFERM 000021 05+LENGTHM 000028 06+BUFENDM 000028 06-BUFFERE

H WRREC 000000 00001CR LENGTH BUFFERT 000000 1C B410 77100000 E32012 332FFA 53900000 DF2008 ...M 000003 05+LENGTHM 00000D 05+BUFFERE

Figure 2.17 (2/2)

190 MAXLEN WORD BUFEND - BUFFER


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and Program Linking

Earlier definitionsRequired all of the relative terms be paired in an expression (anabsolute expression), or that all except one be paired (a relative

expression)New restriction

Both terms in each pair must be relative within the same controlsectionEx: BUFEND-BUFFER

Ex: RDREC-COPYIn general, assembler cannot determine whether or not theexpression is legal at assembly time. This work will be handledby a linking loader.

External Reference in Expressions


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2.4. 1 Assembler Design Options One Pass Assembler

Main problemForward references

Data items

Labels on instructionsSolution

Data items: require all such areas be defined

before they are referencedLabels on instructions: no good solution


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Two types of one-pass assemblerType 1: Load-and-go

Produces object code directly in memory for

immediate executionType 2:

Produces usual kind of object code for laterexecution


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CharacteristicsUseful for program development and testing

Avoids the overhead of writing the object program

out and reading it backBoth one-pass and two-pass assemblers can bedesigned as load-and-goHowever one-pass also avoids the overhead of an

additional pass over the source programFor a load-and-go assembler, the actual addressmust be known at assembly time, we can use anabsolute program

Load-and-Go Assembler


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Load-and-Go Assembler Assembler operations:

1. Omit address translation for any undefined symbol2. Insert the symbol into SYMTAB, mark it undefined3. The address that refers to the undefined symbol is

added to a list of forward references associated withthe symbol table entry

4. When the definition for a symbol is encountered, theproper address for the symbol is then inserted intoany instructions previously generated according tothe forward reference list


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Load-and-Go Assembler

At the end of the program Any SYMTAB entries that are still marked with *indicate undefined symbolsSearch SYMTAB for the symbol named in the ENDstatement and jump to this location to beginexecution

The actual starting address must be specified atassembly time


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Program for one pass assembler

Fig 2.18Line Loc Source statement Object code0 1000 COPY START 10001 1000 EOF BYTE CEOF 454F462 1003 THREE WORD 3 0000033 1006 ZERO WORD 0 0000004 1009 RETADR RESW 15 100C LENGTH RESW 1

6 100F BUFFER RESB 409610 200F FIRST STL RETADR 14100915 2012 CLOOP JSUB RDREC 48203D20 2015 LDA LENGTH 00100C25 2018 COMP ZERO 28100630 201B JEQ ENDFIL 30202435 201E JSUB WRREC 48206240 2021 J CLOOP 3C201245 2024 ENDFIL LDA EOF 00100050 2027 STA BUFFER 0C100F55 202A LDA THREE 00100360 202D STA LENGTH 0C100C65 2030 JSUB WRREC 48206270 2033 LDL RETADR 08100975 2036 RSUB 4C0000


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Fig 2.19(a)

Before Line 40Memory Address Contents

1000 454F4600 00030000 00xxxxxx xxxxxxxx1010 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx

.

.2000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx14

2010 100948-- --00100C 28100630 ----48--2020 --3C2012

.

. Address unknown yet

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Fig 2.19(a) - SYMTAB

LENGTH 100C RDREC * THREE 1003

ZERO 1006 WRREC * EOF 1000 ENDFIL * RETADR 1009

BUFFER 100F CLOOP 2012 FIRST 200F ... ...

Symbol Value

2013

201F

201C




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Fig 2.19(a)

After Line 45 Memory Address Contents

1000 454F4600 00030000 00xxxxxx xxxxxxxx

1010 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx..

2000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx14

2010 100948-- --00100C 28100630 2024 48--2020 --3C2012 001000

.

.




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Fig 2.19(a) - SYMTAB

LENGTH 100C RDREC * THREE 1003

ZERO 1006 WRREC * EOF 1000 ENDFIL 2024 RETADR 1009

BUFFER 100F CLOOP 2012 FIRST 200F ... ...

Symbol Value

2013

201F




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Fig 2.18110 .115 . SUB TO READ RECORD INTO BUFFER120 .121 2039 INPUT BYTE XF1 F1122 203A MAXLEN WORD 4096 001000124 .

125 203D RDREC LDX ZERO 041006130 2040 LDA ZERO 001009135 2043 RLOOP TD INPUT E02039140 2046 JEQ RLOOP 302043145 2049 RD INPUT D82039150 204C COMP ZERO 281006155 204F JEQ EXIT 30205B

160 2052 STCH BUFFER,X 54900F165 2055 TIX MAXLEN 2C203A170 2058 JLT RLOOP 382043175 205B EXIT STX LENGTH 10100C180 205E RSUB 4C0000




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Fig 2.19 (b)

After Line 160 Memory Address Contents

1000 454F4600 00030000 00xxxxxx xxxxxxxx1010 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx

.2000 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx142010 10094820 3D00100C 28100630 202448--2020 --3C2012 0010000C 100F0010 030C100C2030 48----08 10094C00 00F10010 000410062040 001006E0 20393020 43DB2039 281006302050 ----5490 0F

.




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LENGTH 100C RDREC 203D

THREE 1003

ZERO 1006

WRREC *

EOF 1000

ENDFIL 2024

RETADR 1009

BUFFER 100F

CLOOP 2012

FIRST 200F

MAXLEN 203A

INPUT 2039

EXIT *

RLOOP 2043

... ...

201F

2031

2050


Fig 2.19 (b)

After Line 160




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Fig 2.18195 .200 . SUB TO WRITE RECORD FROM BUFFER205 .206 2061 OUTPUT BYTE X05 05207 .210 2062 WRREC LDX ZERO 041006215 2065 WLOOP TD OUTPUT E02061220 2068 JEQ WLOOP 302065225 206B LDCH BUFFER,X 50900F230 206E WD OUTPUT DC2061235 2071 TIX LENGTH 2C100C240 2074 JLT WLOOP 382065245 2077 RSUB 4C0000255 END FIRST




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Type 2 AssemblerWill produce object code Assembler operations:

Forward references are entered into list as beforeInstruction referencing are written into object file as a Textrecord, even with incorrect addressesWhen definition of a symbol is encountered, assembler mustgenerate another Text record with the correct operandaddressLoader is used to insert correct addresses into instructionswith forward references that could not be handled by theassemblerObject program records must be kept in original order whenthey are presented to the loader


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Type 2 Assembler

Fig. 2.20




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2.4.2 Assembler Design Options Multi Pass Assembler

Restriction on EQU and ORGNo forward reference, since symbols value cant bedefined during the first pass

Example: ALPHA EQU BETABETA EQU DELTADELTA RESW 1

Assemblers with 2 passes cannot resolve




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Resolve forward references with as manypasses as needed

Portions that involve forward references in symboldefinition are saved during Pass 1

Additional passes through stored definitionsFinally a normal Pass 2

Example implementation:

Use link lists to keep track of whose value dependon an undefined symbol




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1 HALFSZ EQU MAXLEN/2

2 MAXLEN EQU BUFEND-BUFFER

3 PREVBT EQU BUFFER-1

...4 BUFFER RESB 4096

5 BUFEND EQU *

HALFSZ &1 MAXLEN/2

MAXLEN *

1 symbol undefined

HALFSZ

Fig. 2.21 (a)

After Pass 1




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3 PREVBT EQU BUFFER-1...

4 BUFFER RESB 4096

5 BUFEND EQU *

BUFEND *

HALFSZ &1 MAXLEN/2

MAXLEN &2 BUFEND-

BUFFER

BUFFER * MAXLEN

MAXLEN

HALFSZ

Fig. 2.21 (c)

MAXLEN defined


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Fig. 2.21 (d)

PERVBT defined1 HALFSZ EQU MAXLEN/2



...

4 BUFFER RESB 4096

5 BUFEND EQU *

BUFEND *

HLFSZ &1 MAXLEN/2

PREVBT &1 BUFFER-1

MAXLEN &2 BUFEND-BUFFER

BUFFER *

MAXLEN

PREVBT

HALFSZ

MAXLEN




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Fig. 2.21 (e)

After Line 41 HALFSZ EQU MAXLEN/2



...

4 BUFFER RESB 4096

5 BUFEND EQU *

BUFEND *

HLFSZ &1 MAXLEN/2

PREVBT 1033

MAXLEN &1 BUFEND-BUFFER

BUFFER 1034

MAXLEN

Assumeloc=1034

HALFSZ




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Fig. 2.21 (f)

After Line 5




...

4 BUFFER RESB 4096

5 BUFEND EQU *

BUFEND 2034

HLFSZ 800

PREVBT 1033

MAXLEN 1000

BUFFER 1034

ss unit-iiassembler 2007

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