Unit II Assemblers

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UNIT II ASSEMBLERS

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1. BASIC ASSEMBLER FUNCTIONS

2. A SIMPLE SIC ASSEMBLER

3. ASSEMBLER ALGORITHM AND DATA STRUCTURES

4. MACHINE DEPENDENT ASSEMBLER FEATURES

5. INSTRUCTION FORMATS AND ADDRESSING MODES

6. PROGRAM RELOCATION

7. MACHINE INDEPENDENT ASSEMBLER FEATURES

8. LITERALS

9. SYMBOL-DEFINING STATEMENTS


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10. EXPRESSIONS

11. ONE PASS ASSEMBLERS

12. MULTI PASS ASSEMBLERS

13. IMPLEMENTATION EXAMPLE

14. MASM ASSEMBLER

Assemblers Basic Assembler F unctions

M achine-dependent Assembler F eatur es M achine-independent Assembler Featur es Assembl er D esign Opti ons

Role of Assembler


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Introduction to Assemblers F undamental fu nctions

translating mnemonic operation codes to their machine language equivalents assigning machine addresses to symbolic labels

M achine dependency different machine instruction formats and codes

SourceProgram

Assembler ObjectCode

Loader

ExecutableCode

Linker


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Example Program

Object Code


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Purpose reads 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 RSUB to the operatingsystem program

Data transfer (RD , WD) a buffer is used to store record buffering is necessary for different I/O rates the end of each record is marked with a null character (00 16) the end of the file is indicated by a zero-length record

Subroutin es (JSUB , RSUB ) RDREC, WRREC save link register first before nested jump

Assembler Directives Pseudo-I nstructions

Not translated into machine instructions
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Providing information to the assembler Basic assembl er directives

START END BYTE WORD

RESB RESW

Functions of a Basic Assembler Mnemonic code (or instruction name) opcode. Symbolic operands (e.g., variable names) addresses. Choose the proper instruction format and addressing mode. Constants Numbers. Output to object files and listing files.

SIC Instruction Set Load/Store: LDA/STA, LDX/STXetc. Arithmetic: ADD, SUB, MUL, DIV Compare: COMP Jump: J Conditional Jump: JLT, JEQ, JGT See Appendix A for the complete list.

SIC Instruction Format Opcode : 8 b i t s Ad dress : one bi t f lag (x) and 15 bi ts of address

Examples Mnemon ic cod e (o r ins t ruc t ion name) Opco de. Variable names, Labels , Subro ut ines , Constants A d d r e s s

Examples :

OPCODE X Address

8 1 15


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STL RETADR 14 10 33

STCH BUFFER,X 54 90 39

Converting Symbols to Numbers Is n t i t s t r a igh t fo rward?

Isn t it simply the sequential processing of the source program, one line ata time?

Not so, if we have forw ard references.

Two Pass Assembler Pass 1

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

Pass 2 Assemble instructions Generate data values defined by BYTE, WORD Perform processing of assembler directives not done in Pass 1 Write the object program and the assembly listing

Read f rom inpu t l ine LABEL, OPCODE, OPERAND

COPY START 1000

LDA LEN

LEN RESW 1

0001 0100 0 001 0000 0011 0011

0101 0100 1 001 0000 0011 1001


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Assign addresses to all statements in the program Save the values (addresses) assigned to all labels for use in Pass 2 Perform some processing of assembler directives. (This includes processing that affects address

assignment, such as determining the length of data areas defined by BYTE, RESW, etc.

PASS-I Algorithm

beginread first input lineif OPCODE = 'START' then begin

save #[OPERAND] as starting addressinitialized LOCCTR to starting addresswrite line to intermediate fileread next input line

end {if START}else

initialized LOCCTR to 0while OPCODE != 'END' do

beginif this is not a comment line then

beginif there is a symbol in the LABEL field then begin

search SYMTAB for LABELif found then

set error flag (duplicate symbol)else

insert (LABEL, LOCCTR) into SYMTAB

Pass 1 Pass 2Intermediat

eObjectcodes

Source progra

OPTAB SYMTAB SYMTAB


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end {if symbol}search OPTAB for OPCODEif found then

add 3 {instruction lengh} to LOCCTRelse if OPCODE = 'WORD' then

add 3 to LOCCTRelse if OPCODE = 'RESW' then

add 3 * #[OPERAND] to LOCCTRelse if OPCODE = 'RESB' then

add #[OPERAND] to LOCCTRelse if OPCODE = 'BYTE' then begin

find length of constant in bytesadd length to LOCCTR

end {if BYTE}

elseset error flag (invalid operation code)end {if not a comment}

write line to intermediate fileread next input line

end {while not END}write last line to intermediate filesave (LOCCTR - starting address) as program length

end


Assemble instructions (translating operation codes and looking up addresses). Generate data values defined by BYTE, WORD, etc. Perform processing of assembler directives not done during Pass 1 Write the object program and the assembly listing

PASS-2 Algorithm

beginread first input file {from intermediate file}if OPCODE = 'START' then begin

write listing lineread next input line

end {if START}write header record to object programinitialized first Text record

while OPCODE != 'END' do begin

if this is not a comment line then


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beginsearch OPTAB for OPCODEif found then begin

if there is a symbol in OPERAND field then begin

search SYMTAB for OPERAND

if found thenstore symbol value as operand address

else begin

store 0 as operand addressset error flag (undefined symbol)

endend {if symbol}

elsestore 0 as operand address

assemble the object code instruction

end {if opcode found}else if OPCODE = 'BYTE' or 'WORD' then

convert constant to object codeif object code not fit into the current Text record then begin

write Text record to object programinitialized new Text record

endadd object code to Text record

end {if not comment}write listing line

read next input lineend {while not END}

write last Text record to object programwrite End record to object programwrite last listing line

end

Data Structures Operatio n Cod e Table (OPTAB ) Sym bol Table (SYMTAB ) Lo cat ion Coun ter(LOCCTR)

OPTAB (operation code table)

Operation Code Table (OPTAB) Used to look up mnemonic operation codes and translate them into machine language

equivalents Contains the mnemonic operation code and its machine language equivalent In more complex assemblers, contains information like instruction format and length


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SYMTAB (symbol table)Used to store values (addresses) assigned to labelsIncludes the name and value for each labelFlags to indicate error conditions, e.g. duplicate definition of labelsMay contain other info like type or length about the data area or

instruction labeled

LOCCTR(Location counter) Used to help in the assignment of addresses Initialized to the beginning address specified in the START

statement After each source statement is processed, the length of the

assembled instruction or data area to be generated is added Gives the address of a label

COPY 1000FIRST 1000CLOOP 1003ENDFIL 1015EOF 1024THREE 102DZERO 1030

RETADR 1033LENGTH 1036BUFFER 10 39RDREC 2039


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Machine-dependent Assembler Features

I I nn s st t r r uucct t iioonn f f oor r mmaa t t s s aa nnd d aa d d d d r r ee s s s siinn g g mmood d ee s s P P r r oo g g r r aa mm r r eel l ooccaa t t iioonn

For Eg STL RETADR 17 20 2D The mode bit p=1, meaning PC relative addressing mode.

In struction Format and Addressing Mode SIC/XE PC-relative or Base-relative addressing: op m Indirect addressing: op @m Immediate addressing: op #c Extended format: +op m Index addressing: op m,x register-to-register instructions larger memory -> multi-programming (program allocation)

Translation Register tr anslation

register name (A, X, L, B, S, T, F, PC, SW) and their values (0,1, 2, 3, 4, 5, 6, 8, 9) preloaded in SYMTAB

Addr ess tr anslation Most register-memory instructions use program counter relative or base relative addressing Format 3: 12-bit address field

base-relative: 0~4095 pc-relative: -2048~2047

Format 4: 20-bit address field

OPCODE e Address

612 bits

n i x b p

0001 01 0 0000 0010 11011 1 0 0 1

1

2D2


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PC-Relative Addressing Modes PC-relative

10 0000 FIRST STL RETADR 17202D

displacement= RETADR - PC = 30-3 = 2D

40 0017 J CLOOP 3F2FEC

displacement= CLOOP-PC= 6 - 1A= -14= FEC

Base-Relative Addressing Modes Base-relative

base register is under the control of the programmer 12 LDB #LENGTH 13 BASE LENGTH 160 104E STCH BUFFER, X 57C003

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


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the immediate operand is the symbol LENGTH the address of this symbol LENGTH is loaded into register B LENGTH=0033=PC+displacement=0006+02D if immediate mode is specified, the target address becomes the operand

Indirect Address Translation I ndi rect addressing

target addressing is computed as usual (PC-relative or BASE-relative) only the n bit is set to 1 70 002A J @RETADR 3E2003

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

Program Relocation


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


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


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Example Absolu te program , starting address 1000

e.g. 55 101B LDA THREE 00102D

Relocate the program to 2000

e.g. 55 101B LDA THREE 00202D

Each Absolute address should be modified Example

Except for absolute address, the rest of the instructions need not be modified not a memory address (immediate addressing) PC-relative, Base-relative

The only parts of the program that require modification at load time are those that specify directaddresses

Relocatable Program M odif ication record

Col 1 M Col 2-7 Starting location of the address field to be


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modified, relative to the beginning of the program

Col 8-9 length of the address field to be modified, in half-

bytes

Object Code with M-Records

Modification records are added to the object files. (See pp.64-65 and Figure 2.8.) Example:

HCOPY 001000 001077T000000 1D 17202D4B101036 T00 001D M000007 05 Modification Record E000000

Object Code

Machine-Independent Assembler Features L Liit t eer r aa l l s s S S y ymmbbool l D D ee f f iinniinn g g S S t t aa t t eemmeennt t

E E x x p p r r ee s s s s iioonn s s P P r r oo g g r r aa mm B Bl l oocck k s s C C oonnt t r r ool l S S eecct t iioonn s s aa nnd d P P r r oo g g r r aa mm L Liinnk k iinn g g

Literals Design idea

Let programmers to be able to write the value of a constant operand as a part of theinstruction that uses it. This avoids having to define the constant elsewhere in the program and make up alabel for it.

Example e.g. 45 001A ENDFIL LDA =CEOF 032010 93 LTORG 002D * =CEOF 454F46 e.g. 215 1062 WLOOP TD =X05 E32011
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Literals vs. Immediate Operands I mmediate Operands

The operand value is assembled as part of the machine instruction e.g. 55 0020 LDA #3 010003

Literals

The assembler generates the specified value as a constant at some other memorylocation e.g. 45 001A ENDFIL LDA =CEOF 032010

Compare (Fi g. 2.6) e.g. 45 001A ENDFIL LDA EOF 032010 80 002D EOF BYTE CEOF 454F46

Literal - Implementation L iteral pools

Normally literals are placed into a pool at the end of the program see (END statement)

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

assembler directive LTORG reason: keep the literal operand close to the instruction

Du plicate li terals e.g. 215 1062 WLOOP TD =X05 e.g. 230 106B WD =X05 The assemblers should recognize duplicate literals and store only one copy of thespecified data value

Comparison of the defining expression Same l iteral name with dif ferent value, e.g. L OCCTR=*

Comparison of the generated data value The benefi ts of using generate data value are usual ly not great enough to justif y

the addi tional complexi ty in the assembler

LITTAB literal name, the operand value and length, the address assigned to the operand

Pass 1 build LITTAB with literal name, operand value and length, leaving the address unassigned when LTORG statement is encountered, assign an address to each literal not yet assigned anaddress

Pass 2

C'EOF' 454F46 3 002D

X'05' 05 1 1076


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search LITTAB for each literal operand encountered generate data values using BYTE or WORD statements generate modification record for literals that represent an address in the program

Symbol-Defining Statements L abels on i nstructions or data areas

the value of such a label is the address assigned to the statement Defini ng symbols

symbol EQU value value can be: constant, other symbol, expression making the source program easier to understand no forward reference

Example 1 MAXLEN EQU 4096 +LDT #MAXLEN

Example 2 (M any general pur pose registers) BASE EQU R1 COUNT EQU R2 INDEX EQU R3

Example 3 MAXLEN EQU BUFEND- BUFFER

Counter Example BETA EQU ALPHA ALPHA RESW 1

BETA cannot be assigned a value when it is encountered during Pass 1 of the assembly(because ALPHA does not yet have a value).

ORG (origin) I ndi rectly assign valu es to symbols Reset the location counter to the specif ied value

ORG value

Val ue can be: constant, other symbol, expr ession No forward reference Example

SYMBOL: 6bytes VALUE: 1word FLAGS: 2bytes LDA VALUE, X

+LDT #4096


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ORG Example Using EQU statements

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

Using ORG statements STAB RESB 1100 ORG STAB SYMBOL RESB 6 VALUE RESW 1 FLAGS RESB 2 ORG STAB+1100

Expressions Expressions can be classif ied as absolu te expr ession s or relative expressions

MAXLEN EQU BUFEND-BUFFER BUFEND and BUFFER both are relative terms, represent ing addresses within the program However the expression BUFEND-BUFFER represents an absolute value

When r elative terms are pair ed with opposite signs, the dependency on the progr am star tingaddress is canceled out; the resul t i s an absolu te valu e

SYMTAB None of the relative terms may enter in to a mul tipl ication or divi sion operati on Errors:

BUFEND+BUFFER 100-BUFFER 3*BUFFER

The type of an expression keep track of the types of all symbols defined in the program

Program Blocks Program blocks

refer to segments of code that are rearranged within a single object program unit USE [blockname] Default block Each program block may actually contain several separate segments of the sourceprogram

SYMBOL VALUE FLAGSSTAB

(100 entries)

. . .. . .

. . .

Symbol Type ValueRETADR R 30BUFFER R 36BUFEND R 1036MAXLEN A 1000


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Implementation Pass 1

each program block has a separate location counter each label is assigned an address that is relative to the start of the block that contains it at the end of Pass 1, the latest value of the location counter for each block indicates the length ofthat block

the assembler can then assign to each block a starting address in the object program Pass 2

The address of each symbol can be computed by adding the assigned block starting address and therelative address of the symbol to that block

Each sour ce l ine is given a relative address assigned and a block number

F or absolu te symbol, there is no block number line 107

Example 20 0006 0 LDA LENGTH 032060 LENGTH=(Block 1)+0003= 0066+0003= 0069 LOCCTR=(Block 0)+0009= 0009

Block name Block number Address Length(default) 0 0000 0066

CDATA 1 0066 000BCBLKS 2 0071 1000


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Program Readability Program readabil ity

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

Obj ect code It is not necessary to physically rearrange the generated code in the object program

Control Sections and Program Linking Control Sections

are most often used for subroutines or other logical subdivisions of a program the programmer can assemble, load, and manipulate each of these control sectionsseparately instruction in one control section may need to refer to instructions or data located inanother section because of this, there should be some means for linking control sections together


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External Definition and References External defi ni tion

EXTDEF name [, name] EXTDEF names symbols that are defined in this control section and may be used by

other sections External reference

EXTREF name [,name] EXTREF names symbols that are used in this control section and are definedelsewhere

Example 15 0003 CLOOP +JSUB RDREC 4B100000 160 0017 +STCH BUFFER,X 57900000 190 0028 MAXLEN WORD BUFEND-BUFFER 000000

Implementation The assembler must inclu de in formation in th e object program that wi ll cause the loader to in sert

proper values where they are requi red Define record

Col. 1 D Col. 2-7 Name of external symbol defined in this control section Col. 8-13 Relative address within this control section (hexadeccimal) Col.14-73 Repeat information in Col. 2-13 for other external symbols

Refer r ecord Col. 1 D Col. 2-7 Name of external symbol referred to in this control section


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Col. 8-73 Name of other external reference symbols

Modification Record M odif ication record

Col. 1 M Col. 2-7 Starting address of the field to be modified (hexiadecimal) Col. 8-9 Length of the field to be modified, in half-bytes (hexadeccimal) Col.11-16 External symbol whose value is to be added to or subtracted from the indicated field Note: control section name is automatically an external symbol, i.e. it is available for use inModification records.

Example Figure 2.17 M00000405+RDREC M00000705+COPY

External References in Expression

Earl ier defi ni tions required all of the relative terms be paired in an ex pression (an absolute expression),or that all except one be paired (a relative expression)

New restri ction Both terms in each pair must be relative within the same control section Ex: BUFEND-BUFFER Ex: RDREC-COPY

I n general, the assembler cannot determi ne whether or not the expr ession i s legal at assemblytime. This work wi ll be handled by a li nki ng loader .

Assembler Design OptionsOOnnee-- p paa s s s s aa s s s seemmbbl l eer r s s

M M uul l t t ii-- p paa s s s s aa s s s seemmbbl l eer r s s T T wwoo-- p paa s s s s aa s s s seemmbbl l eer r wwiit t hh oovveer r l l aa y y s s t t r r uucct t uur r ee

Two-Pass Assembler with Overlay Structure F or small memory

pass 1 and pass 2 are never required at the same time three segments

root: driver program and shared tables and subroutines pass 1 pass 2

tree structure overlay program

One-Pass Assemblers M ain problem

forward references


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data items labels on instructions

Solution data items: require all such areas be defined before they are referenced labels on instructions: no good solution

M ain Problem forward reference

data items labels on instructions

Two types of one-pass assembler load-and-go

produces object code directly in memory for immediate execution the other

produces usual kind of object code for later execution

Load-and-go Assembler Characteristics

Useful for program development and testing Avoids the overhead of writing the object p rogram out and reading it back Both one-pass and two-pass assemblers can be designed as load-and-go. However one-pass also avoids the over head of an additional pass over the sourceprogram For a load-and-go assembler, the actual address must be known at assembly time,we can use an absolute program

Forward Reference in One-pass Assembler F or any symbol that h as not yet been defined

1. omit the address translation

2. insert the symbol into SYMTAB, and mark this symbol undefined

3. the address that refers to the undefined symbol is added to a list of forward referencesassociated with the symbol table entry

4. when the definition for a symbol is encountered, the proper address for the symbol isthen inserted into any instructions previous generated according to the forward referencelist

At the end of the program any SYMTAB entries that are still marked with * indicate undefined symbols search SYMTAB for the symbol named in the END statement and jump to thislocation to begin execution


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Th e actual star ting addr ess must be specif ied at assembl y time Example

Producing Object Code When extern al wor king-storage devices are not avail able or too slow (f or the in termediate fil e

between th e two passes Solution:

When definition of a symbol is encountered, the assembler must generate another Tex record withthe correct operand address


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The loader is used to complete forward references that could not be handled by the assembler The object program records must be kept in their original order when they are presented to theloader

Example

Multi-Pass Assemblers Restr iction on EQU and ORG

no forward reference, since symbols value cant be defined during the first pass Example

Use link list to keep track of whose value depend on an undefined symbol


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ASSEMBLERS

1. Introduction

There are two main classes of programming languages: high level (e.g., C, Pascal) and lowlevel . Assembly Language is a low level programming language. Programmers code symbolicinstructions, each of which generates machine instructions.

An assembler is a program that accepts as input an assembly language program (source) and produces its machine language equivalent (object code) along with the information for the loader.

Assembly language programAssembler Linker EXE

Figure 1 . Executable program generation from an assembly source code

Advantages of coding in assembly language are:Provides more control over handling particular hardware componentsMay generate smaller, more compact executable modulesOften results in faster execution

Disadvantages: Not portableMore complexRequires understanding of hardware details (interfaces)

Assembler:

An assembler does the following:

1. Generate machine instructions- evaluate the mnemonics to produce their machine code


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- evaluate the symbols, literals, addresses to produce their equivalent machine addresses- convert the data constants into their machine representations

2. Process pseudo operations

2. Two Pass Assembler

A two-pass assembler performs two sequential scans over the source code:

Pass 1 : symbols and literals are defined Pass 2 : object program is generated

Parsing : moving in program lines to pull out op-codes and operands

Data Structures:

- Location counter (LC): points to the next location where the code will be placed

- Op-code translation table : contains symbolic instructions, their lengths and their op-codes (orsubroutine to use for translation)

- Symbol table (ST): contains labels and their values

- String storage buffer (SSB): contains ASCII characters for the strings

- Configuration table : contains pointer to the string in SSB and offset where its value will be insertedin the object code

assembly machinelanguage Pass1 Pass 2 language

program Symbol table programConfiguration table

String storage bufferPartially configured object file

Figure 2 . A simple two pass assembler .


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Example 1: Decrement number 5 by 1 until it is equal to zero.

assembly language memory object code program address in memory

-----------------------START 0100H

LDA #5 0100 010101 000102 05

LOOP: SUB #1 0103 1D0104 000105 01

COMP #0 0106 290107 000108 00

JGT LOOP 0109 34010A 01 placed in Pass 1

010B 03RSUB 010C 4C

010D 00010E 00

END

Op-code Table

M nemoni c Addressin gmode

Opcode

LDA immediate 01SUB immediate 1DCOMP immediate 29LDX immediate 05ADD indexed 18TIX direct 2CJLT direct 38JGT direct 34RSUB implied 4C

Symbol Table

Symbo l

Value

LOOP 0103


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Example 2: Sum 6 elements of a list which starts at location 200.


-----------------------START 0100H

LDA #0 0100 010101 000102 00

LDX #0 0103 050104 000105 00

LOOP: ADD LIST, X 0106 180107 01 placed in Pass 2 0108 12

TIX COUNT 0109 2C010A 01 placed in Pass 2

010B 15JLT LOOP 010C 38

010D 01 placed in Pass 1 010E 06

RSUB 010F 4C0110 000111 00

LIST: WORD 200 0112 000113 020114 00

COUNT: WORD 6 0115 00

0116 000117 06END

Symbol Table

Symbo l

Addres s

LOOP 0106


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LIST 0112COUNT

0115

Configuration Table

Off set SSB pointerfor thesymbol

0007 DC00000A DC05

SSBDC00 4C

HDC01 49H ASCII for L,I,S,TDC02 53HDC03 54HDC04 5E

HASCII for separationcharacter

DC05

Pass1All symbols are identified and put in STAll op-codes are translatedMissing symbol values are marked

LC = origin

Read next statement

Parse the statement

YComment

N

END Y Pass 2


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N

N pseudo-op Y whatkind?

N

Label EQU WORD/ RESW/RESBBYTE

Y N Label N Label

Enter label in ST Enter label in STY Y

Enter label in ST Enter label in STCall translator

Place constant in

machine codeAdvance LC by thenumber of bytes specified

Advance LC in the pseudo-op

Figure 3 . First pass of a two-pass assembler.


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Translator Routine

Find opcode and the number of bytesin Op-code Table

Write opcode in machine code

Write the data or address that is knownat this time in machine code

moreinformation Ywill be needed Set up an entry in

in Pass 2 ? Configuration Table

N

Advance LC by the number of bytesin op-code table

return

Figure 4 . Flowchart of a translator routine

Pass 2- Fills addresses and data that was unknown during Pass 1.

Pass 1

More lines in NConfiguration Done

Table

Y

Get the next line

Retrieve the name of the symbol from SSB

Get the value of the symbol from ST


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Compute the location in memorywhere this value will be placed(starting address + offset)

Place the symbol value at this location

Figure 5 . Second pass of a two-pass assembler.

Relocatable CodeProducing an object code, which can be placed to any specific area in memory.

Direct Address Table (DAT): contains offset locations of all direct addresses in the program (e.g., 8080instructions that specify direct addresses are LDA, STA, all conditional jumps...). To relocate the

program, the loader adds the loading point to all these locations.

assembly language program Assembler machine language programand DAT

Figure 6. Assembler output for a relocatable code.

Example 3: Following relocatable object code and DAT are generated for Example 1.


-----------------------

STARTLDA #0 0000 01

0001 000002 00

LDX #0 0003 050004 000005 00

LOOP: ADD LIST, X 0006 180007 000008 12

TIX COUNT 0009 2C

000A 00000B 15JLT LOOP 000C 38

000D 00000E 06

RSUB 000F 4C0010 000011 00

LIST: WORD 200 0012 000013 02


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0014 00COUNT: WORD 6 0015 00

0016 000017 06

END

DAT0007000A000D

Forward and backward references in the machine code are generated relative to address 0000. Torelocate the code, the loader adds the new load-point to the references in the machine code which are

pointed by the DAT.

One-Pass Assemblers

Two methods can be used:

- Eliminating forward references Either all labels used in forward references are defined in the source program before they arereferenced, or forward references to data items are prohibited.

- Generating the object code in memory No object program is written out and no loader is needed. The program needs to be re-assembledevery time.

Multi-Pass Assemblers

Make as many passes as needed to process the definitions of symbols.

Example 3:

A EQU BB EQU CC DS 1

3 passes are required to find the address for A.


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Such references can also be solved in two passes: entering symbol definitions thatinvolve forward references in the symbol table. Symbol table also indicates whichsymbols are dependent on the values of others.

Example 4:

A EQU BB EQU DC EQU DD DS 1

At the end of Pass1:

Symbol TableA &1 B 0

B &1 D A 0C &1 D 0D 200 B C 0

After evaluating dependencies:

Symbol TableA 200 0B 200 0C 200 0D 200 0

Documents

Unit II Assemblers