1. Defining Types of data
expression Dn
[name]
expression Dn
[name]
Name: a program that references a data item does so by means of name
Dn (Directive): define the data item – see next slide—
Expression: is an operand may specify an uninitialized value or constant value
an uninitialized value defined by item ?
EXAMPLE :
DATAX DB ?

2. Defining Types of data (Directive):
Pseudo-op Stands for
DB Define Byte
DW Define Word
DD Define Doubleword
DQ Define Quadword
DT Define Tenbytes

3. Defining Types of data -Examples
a memory byte is associated with the name ALPHA, and initialized to 4
ALPHA DB 4
A memory byte is associated with the name BYT, and uninitialized.
BYT DB ?
a memory word is associated with the name WRD, and initialized to -2.
WRD DW -2

4. High and Low Bytes of a Word
WORD1 DW 1234H
high byte
WORD1+1
low byte
WORD1

5. Defining Types of data The decimal range (fit a byte):
Unsigned representation: 0 to 255
Signed representation: -128 to 127
The decimal range (fit a word):
Unsigned representation: 0 to 65535
Signed representation: to 32767

6. Defining Types of data – Array byte
an array is a sequence of memory bytes or words.
Example:
B_ARRAY DB 10H,20H,30H
Symbol Address Contents
B_ARRAY H 10H
B_ARRAY H 20H
B_ARRAY H 30H

7. Defining Types of data – Array word
Example:
W_ARRAY DW ,40,29887,329
Symbol Address Contents
W_ARRAY H 1000D
W_ARRAY H 40D
W_ARRAY H D
W_ARRAY H D

8. Defining Types of data :The DUP Operator
It is possible to define arrays whose elements share a common initial value by using the DUP (duplicate) operator.
Syntax:
Example:
Repeat-count(exp) Dn
[name]
creates an array of 212 uninitialized bytes.
DELTA DB 212 DUP (?)
set up an array of 100 words, with each entry initialized to 0.
GAMMA DW 100 DUP (0)

9. Character String
ASCII codes can be initialized with a string of characters using single quotes like ‘PC’ or double quotes like “PC”.
Example:
LETTERS DB 'ABC' =
LETTERS DB 41H,42H,43H
Inside a string, the assembler differentiates between upper and lowercase.
It is possible to combine characters and numbers in one definition:
Example: MSG DB 'HELLO',0AH,0DH, '$'

11. Exercises: Q1. Which of the following names are legal ? a. ?1 b. T =
c. LET’S_GO
Q2. If it is legal, give data definition pseudo-op to define each of the following.
A word variable A initialized to 52.
A byte variable C initialized to 300.
A word variable WORD1,unintialized.
A byte variable B initialized to -129.

12. Numeric Constant
In an assembly language program we may express data as:
Binary: bit string followed by ‘B’ or ‘b’
Decimal: string of decimal digits followed by an optional ‘D’ or ‘d’
Hex: begins with a decimal digit and ends with ‘H’ or ‘h’
Real : end with ‘R’ and the assembler converts a given a decimal or hex constant to floating point number
Any number may have an optional sign.

13. Numeric Constant Number Type 11011 1101B 64223 decimal -21843D 1B4DH
FFFFH
0FFFFH
decimal
binary
decimal
decimal
hex
illegal
illegal
hex

14. Named Constants - EQU (Equates)
To assign a name to a constant, we can use the EQU pseudo-op.
Syntax:
name EQU constant
Examples:
LF EQU 0AH
MOV DL,0AH = MOV DL,LF
PROMPT EQU 'Any Thing'
MSG DB 'Any Thing' = MSG DB PROMPT
Note: no memory is allocated for EQU names.

15. Registers
Information inside the microprocessor is stored in registers.
The registers are classified according to the functions they perform
In general there are fourteen 16-bit registers:
Data registers:
There are four general data registers.
They hold data for an operation.
Address registers:
They are divided into segment, pointer, and index registers.
They hold the address of an instruction or data.
Status register:
It is called the FLAGS register.
It keeps the current status of the processor.

16. Pointer and Index Registers
Data Registers AX AH AL BH BL BX CX CH CL DX DH DL
Segment Registers CS DS SS ES
Pointer and Index Registers SI DI SP BP IP
FLAGS Register

17. Data Registers: AX, BX, CX, DX
These four registers, in addition to being general-purpose registers,
also perform special functions.
The high and low bytes of these registers can be accessed separately.
Ex. The high byte of AX is called AH, and the low byte is called AL.
This arrangement gives us more registers to use when dealing
with byte-size data.

18. Data Registers: AX, BX, CX, DX
AX (Accumulator Register) is the preferred register to use in arithmetic, logic, and data
transfer instructions
BX (Base Register) also serves as an address register.
CX (Count Register) Program loop constructions are facilitated by the use of CX, which
serves as a loop counter.
DX (Data Register) is used in multiplication and division.

19. Address Registers - Segment Registers CS, DS, SS, ES
To keep track of the various program segments, the 8086 is
equipped with four segments registers to hold segment numbers:
CS (Code Segment): contains the code segment number.
DS (Data segment): contains the data segment number.
SS (Stack Segment): contains the stack segment number.
ES (Extra Segment): is used if a program needs to access a
second data segment.

20. MOV Instruction The MOV (move) instruction is used to:
Transfer data between Registers.
Transfer data between registers and memory locations.
Move a number directly into a register or memory location.
Syntax:
MOV destination, source
Example:
MOV AX, WORD1
MOV AX, BX
MOV AX, 'A'
Note: any register can be used except CS & IP
Before After
AX AX
WORD WORD1

21. Legal Combinations of operands for MOV
Destination Operand
General Segment Memory
Source operand register register 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
Illegal: MOV WORD1, WORD2 • Legal: MOV AX, WORD2
MOV WORD1, AX
Illegal: MOV DS, CS • Legal: MOV AX, CS
MOV DS, AX

22. Type Agreement of Operands
The operands of any two-operand instruction must be of the same
type (i.e. Both bytes or words).
Illegal: MOV AX, BYTE1
However, the assembler will accept both of the following:
MOV AH, 'A' moves 41H into AH
MOV AX, 'A' moves 0041H into AX

23. LEA Instruction
LEA (Load Effective Address) puts a copy of the source offset
address into the destination.
Syntax:
LEA destination, source
Where destination is a general register and source is a memory
location
Example:
MSG DB 41H, 42H, 43H
LEA DX, MSG
puts the offset address of the variable MSG into DX.
Data Definition + Basic Instructions 17

24. ADD and SUB Instructions
The ADD (add) and SUB (subtract) instructions are used to:
Add/subtract the contents of:
Two registers.
A register and a memory location.
Add/subtract a number to/from a register or memory location.
Syntax:
ADD destination, source SUB destination, source
Examples:
ADD WORD1, AX SUB AX, DX
Before After
01BC BC
AX AX DF
WORD WORD1
Before After
FFFF
AX AX
DX DX

25. Legal Combinations of operands for ADD & SUB
Destination Operand
General Memory
Source Operand register location
General register yes yes
Memory location yes no
Constant yes yes
Illegal: ADD BYTE1, BYTE2
Legal: MOV AL, BYTE2
ADD BYTE1, AL

26. INT Instruction
To invoke a DOS or BIOS routine, the INT (interrupt) instruction is
used.
Format:
INT interrupt_number
where interrupt_number is a number that specifies a routine.

27. INT 21h INT 21h may be used to invoke a large number of DOS functions.
A particular function is requested by placing a function number in
the AH register and invoking INT 21h.
Some of the functions are:
INT21h functions expect input values to be in certain registers and
return output values in other registers.
Function number Routine
1 single-key input
2 single-character output
9 character string output

28. INT 21h Function 1: Single-Key Input Input: AH = 1
Output: AL = ASCII code if character key is pressed
= 0 if non-character key is pressed
To invoke the routine, the following instructions should be executed:
MOV AH,1 ; input key function
INT 21H ; ASCII code in AL

29. INT 21h Function 2: Display a character or execute a control function
Input: AH = 2
DL = ASCII code of the character
Output AL = ASCII code of the character
To invoke the routine, the following instructions should be executed:
MOV AH, 2 ; display character function
MOV DL, '?' ; character is '?' (or any other character)
INT 21H ; display character

30. INT 21h Function 2 may be used to perform control functions.
If DL contains the ASCII code of a control character, INT 21h
causes the control function to be performed.
The principal control characters are :
ASCII code (Hex) Symbol Function
07H BEL beep (sounds a tone)
08H BS backspace
09H HT tab
0AH LF line feed (new line)
0DH CR carriage return (start of current line)

31. INT 21h Function 9: Display a string Input: AH = 9
DX = offset address of string.
The string must end with a '$' character
To invoke the routine, the following instructions should be executed:
MOV
MOV DS, AX
MOV AH, 9 ; display string function
LEA DX, MSG ; get message (Load Effective Address)
INT 21H ; display string
A program containing a data segment should
begins with these two instructions