Machine Independent Assember Features

7/30/2019 Machine Independent Assember Features

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LITERALS

It is often convenient for the programmer to

be able to write the value of a constant

operand as a part of the instruction that uses

it

This avoids having to define the constant

elsewhere in the program and makeup alabel for it


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A literal is identified with the prefix = which isfollowed by a specification of the literal value

Example:

LDA =CEOF

Statement specifies a 3-byte operand whosevalue is the character string EOF


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There is a difference between a literal and animmediate operand

With immediate addressing, the operand value is

assembled as part of the machine instruction

With a literal, the assembler generates the specifiedvalue as a constant at some other memory location

The address of the generated constant is used asthe target address for the machine instruction


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ExampleLOC SOURCE STATEMENT OBJECT CODE

:

:001A ENDFIL LDA =CEOF 032010

.

.

002A J @RETADR 3E2003

LTORG

002D * =CEOF 454F46:

:

106B WD =X05 DF2008

:

1073 RSUB 4F0000END FIRST

1076 * =X05 05


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All of the literal operands used in a program aregathered together into one or more Literal Pools

Normally literals are placed into a pool at the end of

the program

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

To allow this, the assembler directive LTORG isused


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When the assembler encounters a LTORGstatement, it creates a literal pool that

contains all of the literal operands used since

the previous LTORG

Of course, literals placed in a pool by LTORG

will not be repeated in the pool at the end ofthe program


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The basic data structure needed is a LiteralTableLITTAB

For each Literal used, this table contains theLiteral Name, the operand value and lengthand the address assigned to the operandwhen it is placed in a literal pool

LITTAB is often organized as a hash table,using the literal name or value as the key


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During Pass 1, as each literal operand isrecognized, the assembler searches LITTAB for thespecified literal name

If the literal is already present in the table, no actionis needed; if it is not present, the literal is added toLITTAB (leaving the address unassigned)

When pass 1 encounters a LTORG statement or theend of the program, the assembler makes a scan ofthe literal table


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At this time each literal currently in the table

is assigned an address

During Pass 2, the operand address for use

in generating object code is obtained by

searching LITTAB for each literal operand

encountered


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Symbol-Defining Statements

EQU (EQUATE)

This assembler directive allows the programmer to

define symbols and specify their values

Symbol EQU Value

This statement defines the symbol and assigns to it

the value specified


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Consider the statement

+LDT #4096 to load value 4096 into T

If we include the statement

MAXLEN EQU 4096

We can write above line as

+LDT #MAXLEN


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When the assembler encounters the EQU

statement, it enters MAXLEN into SYMTAB

with value 4096

During the assembly of the LDT instruction,

the assembler searches SYMTAB for the

symbol MAXLEN, using its value as theoperand in the instruction


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Contd..

ORG

ORG stands forOrigin

Its form is : ORG Value

where Value is a constant or an expressioninvolving constants and previously defined

symbols


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Contd..

When ORG statement is encountered during

assembly of a program, the assembler resets

its location counter (LOCCTR) to the

specified value

Since the values of symbols used as labels

are taken from LOCCTR, the ORG statementwill affect the values of all labels defined until

the next ORG


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Example..

SYMBOL VALUE FLAGSSTAB

(100 entries)

. . .

. . .

. . .

Let :

SYMBOL: 6bytes

VALUE: 1wordFLAGS: 2bytes


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Contd..

We could reserve space for this table with thestatement

STAB RESB 1100

We want to be able to refer to the fields SYMBOL,VALUE and FLAGS individually, so we must alsodefine these labels

We can make the structure of the table as clear as itmight be using ORG statement


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Contd..

STAB RESB 1100

ORG STAB

SYMBOL RESB 6

VALUE RESW 1

FLAGS RESB 2

ORG STAB+1100


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Expressions

Most assemblers allow the use of expressions

Each such expression must be evaluated by theassembler to produce a single operand address or

value

Individual terms in the expression may be constants,user defined symbols or special terms

The most common special term is the current valueof the location counter (often designated by * )


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Example..

:

0036 BUFFER RESB 4096

1036 BUFEND EQU *

1000 MAXLEN EQU BUFEND - BUFFER

:

:


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Contd..

Expressions are classified as either AbsoluteExpressions or Relative Expressions depending

upon the type of value they produce

An expression that contains only absolute terms is

anAbsolute Expression

Absolute expression may contain relative termsprovided the relative terms occur in pairs and the

terms in each such pair have opposite signs


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Contd..

In the statement

MAXLEN EQU BUFEND - BUFFER

Both BUFEND and BUFFER are relative terms,

each representing an address within the program

However, the expression represents an absolutevalue: the difference between the two addresses,

which is the length of the buffer area in bytes


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Program Blocks

So far all the programs we come across were

handled by the assembler as one entity resulting in

a single block of object code

Within this object program the generated machine

instructions and data appeared in the same order as

they were written in the source program

Program Blocks refer to segments of code that are

rearranged within a single object program unit

Line Loc / Block Source statement Object code


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5 0000 0 COPY START 0

10 0000 0 FIRST STL RETADR 172063

15 0003 0 CLOOP JSUB RDREC 4B2021

20 0006 0 LDA LENGTH 032060

25 0009 0 COMP #0 290000

30 000C 0 JEQ ENDFIL 332006

35 000F 0 JSUB WRREC 4B203B

40 0012 0 J CLOOP 3F2FEE

45 0015 0 ENDFIL LDA =CEOF 032055

50 0018 0 STA BUFFER 0F2056

55 001B 0 LDA #3 010003

60 001E 0 STA LENGTH 0F2048

65 0021 0 JSUB WRREC 4B2029

70 0024 0 J @RETADR 3E203F

80 0000 1 USE CDATA

95 0000 1 RETADR RESW 1

100 0003 1 LENGTH RESW 1

105 0000 2 USE CBLKS

106 0000 2 BUFFER RESB 4096

107 1000 2 BUFEND EQU *

108 1000 2 MAXLEN EQU BUFEND -BUFFER


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115 . SUBROUTINE TO READ RECORD INTO BUFFER

120 .

0027 0 USE

125 0027 0 RDREC CLEAR X B410

127 0029 0 CLEAR A B400

130 002B 0 CLEAR S B440

002D 0 +LDT #MAXLEN 75101000

135 0031 0 RLOOP TD INPUT E32038

140 0034 0 JEQ RLOOP 332FFA

145 0037 0 RD INPUT DB2032

150 003A 0 COMPR A, S A004

155 003C 0 JEQ EXIT 332008

160 003F 0 STCH BUFFER, X 57A02F

165 0042 0 TIXR T B850

170 0044 0 JLT RLOOP 3B2FEA

175 0047 0 EXIT STX LENGTH 13201F

180 004A 0 RSUB 4F0000

181 0006 1 USE CDATA

185 0006 1 INPUT BYTE XF1 F1


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Line Loc / Block Source statement Object code200 . SUBROUTINE TO WRITE RECORD FROM BUFFER

.

004D 0 USE

205 004D 0 WRREC CLEAR X B410

210 004F 0 LDT LENGTH 772017

215 0052 0 WLOOP TD =X05 E3201B

220 0055 0 JEQ WLOOP 332FFA

225 0058 0 LDCH BUFFER,X 53A016230 005B 0 WD =X05 DF2012

235 005E 0 TIXR T B850

240 0060 0 JLT WLOOP 3B2FEF

245 0063 0 RSUB 4F0000

0007 1 USE CDATA

LTORG

250 0007 1 * =CEOF 454F46

000A 1 * =X05 05

255 END FIRST


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The assembler directive USE indicates which portions ofthe source program belong to the various blocks

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 of the source program


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During Pass 1, a separate location counter for each programblock is assigned

The location counter for a block is initialized to 0 when the blockis first begun

The current value of this location counter is saved whenswitching to another block, and the saved value is restored whenresuming a previous block

Each label in the program is assigned an address that is relativeto the start of the block that contains it


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When labels are entered into the symbol table, the

Block Name or Number is stored along with the

assigned relative address

At the end of the Pass 1 the latest value of the

location counter for each block indicates the length

of that block

The assembler can then assign to each block a

starting address in the object program


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For code generation during Pass 2, the assembler

needs the address for each symbol relative to the

start of the object program

This is easily found from the information in SYMTAB

The assembler simply adds the location of the

symbol, relative to the start of its block, to theassigned block starting address


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Contd..

At the end of Pass 1 the assembler constructs a tablethat contains the starting addresses and lengths for allblocks

For our sample program, this table looks like:

Block name Block number Address Length

(default) 0 0000 0066

CDATA 1 0066 000B

CBLKS 2 0071 1000


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Default (1)

CDATA (1)

CBLCKS (1)

Default (2)

CDATA (2)

Default (3)

CDATA (3)

Default (1)

Default (2)

CDATA (2)

Default (3)

CDATA (3)

Default (1)

Default (2)

Default (3)

CDATA (1)

CDATA (2)

CDATA (3)

CBLCKS (1)

Source Program Object Program Program Loaded in Memory

0000

0027

004D

0066

006C

006D

0071

1070

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Machine Independent Assember Features