1. Microprocessor and Assembly Language
Lecture-5-Program Structure of Assembly Language
Muhammad Hafeez
Department of Computer Science
GC University Lahore

2. Today’s Agenda
Program Structure
Assembler Directives

3. Memory Models
Memory Models defines how much memory we need for a program
TINY
SMALL
MEDIUM
COMPACT
LARGE
HUGE

4. Memory Models TINY SMALL MEDIUM COMPACT LARGE HUGE
64K for both code and data segment
SMALL
64K for code segment
64K for data segment
MEDIUM
64K data segment
Code can exceed 64K
COMPACT
data segment can exceed 64K
Code segment 64K
LARGE
Data can exceed 64K but Array declared in one data segment can not exceed 64K
HUGE
Data can exceed 64K, Array declared in one data segment can exceed 64K

5. Memory Models

6. Assembly Language Statements
Assembly Language Statements are either,
Instructions
Directive (also called Pseudo-Ops)
Both Instruction and Directive can have up to 4 Fields
[Label] mnemonic [operand] [;comment]
Brackets indicate the fields are optional

7. Assembly Language Statements
Label:
Label composed of A-Z, a-z, 0-9 and special character ?,
Must begin with an alphabetic and special character
Cannot exceed 31 characters
. Can only be used as first character
No Case Sensitivity
Label for directive do not need to end with Colon (:)
Label for Instruction needs to end with (:) colon
As it tells the assembler to refer the code

8. Assembly Language Statements

9. Assembly Language Statements
Mnemonic:
Instructions and Operands for Processor
Real work of program
MOV AX,5
ADD AX,BX
Pseudo-Ops/ Directive for Assembler
MAIN
ENDP DB
Comment:
At end of each line
Optional BUT Highly important for readable/ understandable Assembly Language Programs

10. Assembly Language Program

11. Data Movement Instructions
simplified
Assigns a Title to your Source File
Stack of program
Data of program
Label
Assembler Directive
Comment

12. Data Defining Directives
Storage is defined using data definition directives
Storage is created at assembly time. Variables can be initialized to character string
Syntax
Name Data Directive Initial value, [values]

13. Data Defining Directives

14. Declaration of Data DATA1 DB 15H DATA2 DB 15H, 1FH, 4AH
DATA3 DB ? ;SET ASIDE ONE BYTE
DATA4 DB ‘STRING OF CHARACTERS’,’$’
DB could be replaced with DW, DD, DQ or DT depending upon requirement

15. Named Constants with Directive “EQU”
To assign a name to constant, use EQU pseudo-op
EQU Syntax
Name EQU Constant
Example:
LF EQU 0AH
CR EQU 0DH
PROMPT EQU ‘Enter Number’
No Memory is allocated for EQU pseudo-op

16. More Data Definition Directives
To assign a name to constant, use EQU pseudo-op
EQU Syntax
Name EQU Constant
Example:
LF EQU 0AH
CR EQU 0DH
PROMPT EQU ‘Enter Number’
No Memory is allocated for EQU pseudo-op

17. Assemble, Link and Run a Program
Software you need can be downloaded from blog, along with instructions to set up your assembly language programming environment.
Three Steps are required to create, assemble and link an assembly language program
Step 1: A simple Text Editor to create assembly language program file and save it with Extension (.ASM)
An Assembler MASM (Microsoft Macro Assembler) that takes (.ASM) file and create an Object File with Extension (.OBJ)
A Linker Program (LINK) that takes an (.OBJ) file and create Executable file with Extension (.EXE)
Run Executable File
Execute under Debug, DEBUG PRG1.EXE (Enter) .. U CS:0 1 to find data segment address, D Datasegment: 0 F , G, Then again D DataSegment: 0 F
Note: Other assembler and linker program also exist, like Borland’s TASM and TLINK etc.

18. Assemble, Link and Run a Program

19. Files Created by Assembler
The Source Listing File (.LST)
Display line number and corresponding machine code side-by-side, helpful in debugging
The Cross Reference File (.CRF)
List of variables and labels in the programs

20. More Directive of MASM For handling Data
ORG [Number]
Is used to mark the beginning of offset address
ORG 10H
OFFSET
Returns the offset of a variable from the start of its segment address
The destination must be 16-bit
MOV BX, OFFSET DATA2
[More on Data Definitions, PTR, SEG later]

21. Flag Register
Flag Register defines processor status and help it to make decisions
Decision making is done by looking at current state of processor
Nine individual bits called FLAGS in flag register represents a particular state of processor
Flags are classified as control flags and status flags
Status flags are affected by result of computations, also called conditional flags

22. Flag Register

23. Conditional Flags
The processor uses conditional flags to make decision
SUB AX, AX set Zero Flag = 1
CARRY FLAG (CF):
CF=1, If carry out from MSB (D7) for Byte, (D15) for Word in case of addition, or Borrow in MSB in case of subtraction, also affected by rotate instructions
Parity Flag (PF):
PF=1, If low byte of a result has even number of 1 bits (even parity).
PF=0, if low byte of a result has odd number of 1 bits (odd parity)

24. Conditional Flags Auxiliary Carry (AF): Zero Flag (ZF):
AF=1, If carry out from D3 bit to D4 bit, otherwise AF=0
Zero Flag (ZF):
ZF=1, If result is zero.
ZF=0, for non-zero results
Sign Flag (SF):
SF=1, If after computations MSB=1, if you are giving a signed interpretation to result
SF=0, if MSB = 0
Overflow Flag (OF):
OF=1, If Singed overflow occurs
OF=0, otherwise

25. Control Flags Trap Flag (TF): Interrupt Flag (IF):
TF=1, Allow to program to execute in single step, helpful in debugging
TF=0, otherwise
Interrupt Flag (IF):
IF=1, Allow external maskable interrupts to the processor
IF=0, otherwise
Direction Flag (DF):
Used to control the direction of string operations

26. Flag Register in Debug Utility
Command is --- R F

27. Instructions Affect Flags
MOV/ XCHG
None
ADD/ SUB
All
INC/ DEC
All except CF
NEG
All, (By default set CF=1, only exception result is zero)
OF=1, if Word operand is 8000H and byte operand is 80H

28. Examples: AX=FFFFH, BX=FFFFH, ADD AX,BX
SF=1, CF=1, AF=1
MOV AL,80H, MOV BL,80H, ADD AL,BL
PF=1, ZF=1, CF=1, OF=1, SF=0
MOV CX, 5
DEC CX, DEC CX, DEC CX, DEC CX, DEC CX
After execution of these instructions result will be zero hence, ZF=1

29. Examples: Addition 0Fh + 08h? Addition 0Fh + F8h? Addition 4Fh + 40h?
Represent in signed and unsigned numbers

30. Some Basic I/O Operations
INT 21H , an interrupt also called DOS function call
87 different interrupts are supported by this DOS function call
Each interrupt is identified by a function number placed in AH register

31. Some Basic I/O Operations
INT 21H , an interrupt also called DOS function call
87 different interrupts are supported by this DOS function call
Each interrupt is identified by a function number placed in AH register

32. Some Basic I/O Operations
Read a Character
Display a Character
Display a String

33. Some Basic I/O Operations
Read a Character
INT 21h / AH=1 - read character from standard input, with echo, result is stored in AL.
If there is no character in the keyboard buffer, the function waits until any key is pressed.
Example:
mov ah, 1
int 21h

34. Some Basic I/O Operations
Display a Character
INT 21h / AH=2 - write character to standard output.
DL = character to write, after execution AL = DL.
Example:
mov ah, 2
mov dl, ’a’
int 21h

35. Some Basic I/O Operations
Display a String
INT 21h / AH=9 - display a string character string on the console.
The offset of string must be in DX
string must be end with $ (24h)
mov ah,9 ;string output function
mov dx,offset msg ;offset address of the string
int 21 ;call DOS
obvious disadvantage, a dollar sign can not be displayed as part of a string

36. Questions
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