INTRODUCTION TO IBM PC
ASSEMBLY LANGUAGE
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Assembly Language Syntax
• An assembly language program
consists of statements.
• The syntax of an assembly
language
program
statement
obeys the following rules:
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RULES
Only one statement is written per line
Each statement is either an instruction or
an assembler directive
instruction is translated into machine
code
assembler directive instructs the
assembler to perform some specific task
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Program Statement
• The general format for an assembly language
program statement is as follows:
name operation operand’(s) comment
Examples:
START:
MOV CX,5
MAIN
PROC
4
; initialize counter
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Name Field
• This field is used for:
instruction label: if present, a label must
be followed by a colon (:)
procedure names
variable names.
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Name Field
Assembler translates names into memory
addresses.
Names can be from 1 to 31 characters long:
(letters, digits, and special characters: ?, ., _, $,
@, %)
Embedded blanks are not allowed, names may
not begin with a digit, period (if used) must be
the first character
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Name Field
Examples:
Legal names
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Illegal names
COUNTER1
2ABC
@CHARACTER
$500
SUM_OF_DIGITS
.TEST
DONE?
TWO WORDS
A45.26
YOU&ME
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Operation Field
For an instruction
• This field consists of a symbolic
operation code, known as opcode
• The opcode describes the operation’s
function
• Symbolic opcodes are translated into
machine language opcode.
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Operation Field
For an assembler directive
• This field consists of a pseudooperation code (pseudo-op)
• pseudo-ops tell assembly to do
something
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Operand Field
For an instruction
• This field specifies data to be acted on. It may
have one, two or no operands at all.
• Examples of instructions with different operand
fields
NOP
; Instruction with no operand field
INC AX
; Instruction with one operand field
ADD AX, 2 ; Instruction with two operand field
If 2 operands: the first is destination, the second
is the source operand
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Operand Field
For an assembler directive
• This field contains more information
about the directive
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Comment Field
• A semicolon marks the beginning of
a comment
• A semicolon in the beginning of a
line makes it all a comment line
• Good programming practice
dictates the use of a comment on
almost every line.
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Key rules for the use of comments
• Do not say something that is
obvious
• Put instruction in context of
program
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Comment Field
Examples of good and bad Comments
MOV CX , 0 ; Move 0 to CX (This is not a good
comment.)
MOV CX , 0 ; CX counts terms, initially set to 0
(This is a good comment.)
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Numbers
• Binary number is written as a bit string followed
by the letter `b`.
• decimal number is written as a string of decimal
digits followed by the letter `d`.
• Hex number is written as a string of hex digits
followed by the letter `h`.
• Hex number must begin with a decimal digit
• Numbers may have an optional sign
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Numbers
Examples:
number
type
1010
1010B
-2134D
ABFFH
0ABFFH
1BHH
1BFFH
1,23
decimal
binary
decimal
illegal
hex
illegal
hex
illegal
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Characters
• Characters and character
segments must be enclosed in
single or double quotes; ‘A' ,
“hello“.
• Assembler translates
characters to their ASCII code
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Variables
Declaring Integer Variables:
• An integer is a whole number,
such as 4 or 4444. Integers have
no fractional part. Integer
variables can be initialized in
several ways with the data
allocation directives.
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Variables
Allocating Memory for Integer Variables:
• When an integer variable is declared,
the assembler allocates memory
space for the variable. The variable
name becomes a reference to the
memory space allocated to that
variable.
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Syntax
name directive initializer
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initial value
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Variables
Pseudo-op type
• DB unsigned
signed
• DW unsigned
signed
• DD unsigned
signed
size
1 byte
1 byte
2 bytes
2 bytes
4 bytes
range
0 to 255.
-128 to +127.
0 to 65,535 (64K).
-32,768 to +32,767.
0 to 4,294,967,295 (4 Mbytes).
4 bytes -2,147,483,648 to +2,147,483,647.
• DQ
8-byte integer
4 consecutive words
• DT
10-byte integer
10 consecutive bytes
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Byte variables
• Syntax:
Name
DB
initial value
Examples:
ALPHA
BYT
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DB
DB
4
?
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Word variables
• Syntax:
Name
DW
initial value
Example:
WRD
DW -2
• The assembler stores integers with the least
significant byte in the lowest address of the
memory area allocated to the integer
Example:
WD
DW 1234H
low byte WD contains 34h, high byte contains
12h
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Array Declaration
• An array is a sequential collection of variables,
all of the same size and type
• Array elements occupy contiguous memory
locations.
• The program references each element relative
to the start of the array.
• An array is declared by giving it a name, a type,
and a series of initializing values or
placeholders (?).
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Array Examples
B_ARRAY
DB
10, 25, 20
If array starts at offset address 0200h, it will look like this:
Symbol
B-ARRAY
B-ARRAY+1
B-ARRAY+2
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Address
0200H
0200H+1
0200H+2
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Contents
10
25
20
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Array Examples
W_ARRAY
DW 0FFFFh, 789Ah, 0BCDEh
If array starts at offset address 0100h, it will look
like this:
Symbol
W_ARRAY
W_ARRAY+2
W_ARRAY+4
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Address
0100H
0102H
0104H
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Contents
FFFFH
789AH
BCDEH
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Character strings
• An array of characters can be
initialized by a string of characters.
• Inside a string, the assembler
differentiates between upper and
lower cases (different ASCII codes).
• It is possible to combine characters
and numbers in one definition
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Character strings
Examples:
1)
LETTERS
Is equivalent to
LETTERS
2)
MSG
Is equivalent to
MSG
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DB
‘AaBCbc‘
DB
41H,61H,42H,43H,62H,63H
DB
‘ABC‘,0AH,0DH,‘$‘
DB
41H,42H,43H,0AH,0DH,24H
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Constant Declaration
• In an assembly language program, constants are
defined through the use of the EQU directive.
• Syntax:
Name
EQU
constant
 The EQU directive is used to assign a name to a
constant.
 Use of constant names makes an assembly language
easier to understand.
 No memory is allocated for a constant.
 The symbol on the right of EQU cab also be a string
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Constant Declaration
Examples:
1)
LF
EQU
0AH ; LF can be used in place of 0Ah
MOV
MOV
DL
DL
LF
0AH
PMT
EQU
‘TYPE YOUR NAME‘ ;
DB
‘TYPE YOUR NAME‘
DB
PMT
Have the same machine code
2)
instead of
MSG
We can use
MSG
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BASIC INSTRUCTIONS
MOV and XCHG
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MOV instruction
• Is used to transfer data :
– between registers,
– between a register & a memory location.
Or
– To move a number directly into a register or
memory location.
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Syntax
MOV
destination , source
Example:
MOV AX , WORD1
This reads “ Move WORD1 to AX “
The contents of register AX are replaced by the
contents of the memory location WORD1.
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Mov AX , WORD1
Before
After
0006
0008
AX
AX
0008
0008
WORD1
WORD1
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MOV AX , BX
• AX gets what was previously in
BX , BX is unchanged.
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MOV AH , ‘A’
• This is a move of the 041h ( the ASCII code of
“A” ) into register AH.
• The previous value of AH is overwritten
( replaced by new value )
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XCHG instruction
• (Exchange) operation is used to exchange
the contents of
– two registers, or
– a register and a memory location
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Syntax
XCHG destination , source
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Example
XCHG AH , BL
This instruction swaps the contents of AH and
BL.
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XCHG AH , BL
After
Before
1A
AH
00
BH
40
00
05
00
AL
AH
AL
00
1A
BH
BL
05
BL
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Example
XCHG AX , WORD1
• This swaps the contents of AX and memory
location WORD1.
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Restrictions on MOV & XCHG
MOV
Source
Operand
Destination Operand
General
Register
Segment
Register
Memory
Location
Constant
General
Register
yes
yes
yes
no
Segment
Register
yes
no
yes
no
Memory
Location
yes
yes
no
no
Constant
yes
no
yes
no
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Restrictions on MOV & XCHG
XCHG
Source
Operand
43
Destination Operand
General
Register
Memory
Location
General
Register
yes
yes
Memory
Location
yes
no
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Restrictions on MOV & XCHG
Example :
ILLEGAL : MOV WORD1 , WORD2
LEGAL:
MOV AX , WORD2
MOV WORD1 , AX
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ADD & SUB
• Are used to add & subtract the contents of
– two registers,
– a register & memory location , or
– add ( subtract ) a number to ( from ) a
register or a memory location.
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Syntax
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ADD
destination , source
SUB
destination , source
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Example
ADD WORD1 , AX
This instruction , “ Add AX to WORD1 “ , causes the
contents of AX & memory word WORD1 to be added,
and the sum is stored in WORD1. AX is unchanged.
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ADD WORD1 , AX
Before
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After
01BC
01BC
AX
AX
0523
06DF
WORD1
WORD1
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Example
SUB
AX , DX
This instruction , “ Subtract DX from AX “ , the value of
DX is subtracted from the value of AX , with the
difference being stored in AX. DX is unchanged.
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SUB AX , DX
Before
After
0000
50
FFFF
AX
AX
0001
0001
DX
DX
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Example
ADD BL , 5
This is an addition of the number 5 to the
contents of register BL.
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Legal combinations of operands for
ADD & SUB
Destination operand
Source Operand
General Register
Memory location
General Register
yes
yes
Memory location
yes
no
Constant
yes
yes
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ILLEGAL
ADD BYTE1 , BYTE2
Solution :
move BYTE2 to a register before adding
MOV
AL , BYTE2
; AL gets BYTE2
ADD
BYTE1 , AL
; add it to BYTE1
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INC ( increment )
Is used to add 1 to the contents
of a
• Register or
• Memory location
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DEC ( decrement )
Is used to subtract 1 from the
contents of a
• Register or
• Memory location
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Syntax
INC destination
DEC destination
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Example
INC WORD1
adds 1 to the contents of WORD1
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INC WORD1
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Before
After
0002
0003
WORD1
WORD1
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Example
DEC BYTE1
subtracts 1 to the variable BYTE1
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DEC BYTE1
Before
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After
FE
FD
BYTE1
BYTE1
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NEG
• Is used to negate the contents of the
destination.
• It does this by replacing
by its two’s complement.
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the
contents
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Syntax
NEG
destination
The destination may be a
register or
memory location.
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NEG BX
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Before
After
0002
FFFE
BX
BX
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Type agreement of operands
• For instruction with 2 operand, the two
operands must be of the same type; that is,
both words or bytes.
• Illegal …. MOV AX , BYTE1 …. Is not
allowed.
• Assembler will accept both the following
instructions :
MOV AH , ‘A’ ….. moves 41h into AH
MOV AX , ‘A’ ….. moves 0041h into AX
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Translation of HLL to
Assembly Language
Statement
B = A
Translation
MOV AX , A ; moves A into AX
MOV B , AX
; and then into B
WHY
Because direct memory – memory move is illegal we
must move the contents of A into a register
before moving it to B.
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Translation of HLL to
Assembly Language
StatementTranslation
A = 5–A
MOV AX , 5
SUB AX , A
MOV A , AX
; put 5 in AX
; AX…. 5 – A
; put it in A
There is another shorter way :
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NEG
A
; A = -A
ADD
A , 5
;A = 5 - A
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Translation of HLL to
Assembly Language
Statement
A= B–2*A
Translation
MOV AX , B
; AX has B
SUB AX , A ; AX has B – A
SUB AX , A
; AX has
B–2*A
MOV A , AX
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; move results to B
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Program Structure
• Machine language programs consist of :
–
–
–
Codes,
Data, and
Stack.
Each part occupies a memory segment. They
are structured as program segments. Each
program segment is translated into a memory
segment by the assembler.
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Memory Models
The size of the code & data a
program can have is determined
by specifying a memory model
using the . MODEL directive.
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Syntax
. MODEL
memory_mode1
LARGE
SMALL
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MEDUIM
COMPACT
Code in one
segment
Code in more
than one
segment
Code in
one
segment
Data in one
segment
Data in one
segment
Data in
more than
one
segment
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Code in more than
one segment
Data in more than
one segment
No array larger
than 64K bytes.
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• Unless there is a lot of code or data,
the appropriate model is SMALL.
• . MODEL directive should come
before any segment definition.
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Data Segment
• A program’s data segment contains all the
variable definitions. Constant definitions are
made here as well, but they may be placed
elsewhere in the program since no memory
allocation is involved.
• We use the . DATA directive followed by
variable & constant declarations.
• Variable addresses are computed as offsets
from the start of this segment
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Example
.DATA
WORD1
WORD2
MSG
MASK
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DW 2
DW 5
DB ‘ This is a message ‘
EQU 10010010B
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Stack Segment
• Used to set aside storage for the stack
• Stack addresses are computed as offsets into
this segment
• Use: .stack followed by a value that indicates
the size of the stack
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Declaration Syntax
.STACK
size
An optional number that
specifies the stack area size
in bytes.
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Example
.STACK
100 H
Sets aside 100h bytes for the stack area ( a reasonable
size for most applications ) .
If size is omitted , 1 KB is set aside for the stack area.
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Code Segment
• It contains a program’s
instructions.
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Syntax
.CODE
name
Optional name for the
segment
there is no need for a
name in a SMALL
program
Why??
The assembler will
generate an error
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Inside the code segment
• Instructions are organized as procedures.
• The simplest procedure definition is :
name
PROC
; body of the procedure
name
ENDP
name is the name of the procedure, PROC and ENDP
are pseudo-op that delineate the procedure
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Example
.CODE
MAIN
PROC
; main procedure body
MAIN
ENDP
; other procedures go here
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Program Structure
•
A program has always the following general structure:
.model
.stack
.data
small
100h
;Select a memory model
;Define the stack size
; Variable and array declarations
; Declare variables at this level
.code
main
proc
; Write the program main code at this level
main
endp
;Other Procedures
; Always organize your program into procedures
end
main
; To mark the end of the source file
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