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

2. Chapter Outline Numeric Constant . Named Constants .
Program Structure .

3. 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’
Octal : end with ‘O’ or ‘o’
Any number may have an optional sign.

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

5. 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.

6. Directives SEGMENT Directive Data Segment Stack segment Code Segment
END Directive
ex: ENDP directive ends a procedure
ex: END directive ends the entire program and appears as the last statement 9

7. Program Structure - Memory Models
The size of code and data a program can have is determined by
specifying a memory model using the .MODEL directive.
Syntax:
.MODEL memory_model
Model Description
SMALL code in 1 segment data in 1 segment
MEDIUM code > 1 segment data in 1 segment
COMPACT code in 1 segment data > 1 segment
LARGE code > 1 segment data > 1 segment
no array larger than 64k bytes
HUGE code > 1 segment data > 1 segment
arrays may be larger than 64k bytes

8. Program Structure - Memory Models
The appropriate model is SMALL, unless there is a lot of code or
data.
.MODEL directive should come before segment definitions.
A segment is 216 (64 k)

9. Program Structure - Stack Segment
The purpose of the stack segment declaration is to set aside a block
of memory (the stack area) to store the stack.
The stack area should be big enough to contain the stack at its
maximum size.
Syntax:
.STACK size ; where size is an optional number that specifies
; the stack area size in bytes.
Example:
.STACK 100H ; sets aside 100H bytes for the stack area.
; (reasonable size for most applications).
If size is omitted, 1KB is set aside for the stack area.

10. Program Structure - Data Segment
A program’s data segment contains all the variable definitions.
Constant definitions are often made here as well. (they may be
placed elsewhere in the program since no memory allocation is
involved).
To declare a data segment, we use the directive .DATA, followed by
variable and constant declarations.
Example:
.DATA
WORD1 DW 2
MSG DB ‘this is a message’

11. Program Structure - Code Segment
The code segment contains a program’s instructions.
Syntax:
.CODE name ; where name is an optional name of segment.
There is no need for a name in a SMALL program.
Inside a code segment, instructions are organized as procedures.

12. Program Structure - Code Segment
The simplest procedure definition is:
name PROC ; name: is the name of the procedure.
; body of the procedure ; PROC & ENDP: are pseudo-ops that
name ENDP ; delineate the procedure
Example of a code segment definition:
.CODE
MAIN PROC
; main procedure instructions
MAIN ENDP
; other procedures go here

13. A General Form of a .SMALL model program
Program Structure -
A General Form of a .SMALL model program
.MODEL SMALL
.STACK 100H
.DATA
; data definitions go here
.CODE
MAIN PROC
; instructions go here
MAIN ENDP
; other procedures go here
END MAIN