1. Chapter 4 –Requirements for coding in Assembly Language
Lecture 2
Chapter 4 –Requirements for coding in Assembly Language

2. Chapter Outline Assembly Language Features
Simplified segment Directive
Defining Types of data
Equate Directive

3. 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 ?

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

5. Defining Types of data -Examples
Syntax:
name DB initial_value
Example:
ALPHA DB a memory byte is associated with the name
ALPHA, and initialized to 4.
BYT DB ? a memory byte is associated with the name
BYT, and uninitialized.
WRD DW a memory word is associated with the
name WRD, and initialized to -2.
The decimal range is:
Unsigned representation: 0 to 255
Signed representation: -128 to 127

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:
DELTA DB DUP (?) creates an array of 212
uninitialized bytes.
GAMMA DW DUP (0) set up an array of 100 words,
with each entry initialized to 0.
Repeat-count(exp) Dn
[name]

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

10. 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 lower
case.
It is possible to combine characters and numbers in one definition:
Example: MSG DB 'HELLO',0AH,0DH, '$'

11. 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.

12. Numeric Constant Number Type 11011 1101B 64223 -21843D 1,234 1B4DH
FFFFH
0FFFFH
decimal
binary
decimal
decimal
illegal
hex
illegal
illegal
hex

13. 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.