Download MICROPROCESSOR AND ASSEMBLY LANGUAGE

MICROPROCESSOR AND
ASSEMBLY LANGUAGE
LECTURE-5-PROGRAM STRUCTURE
LANGUAGE
MUHAMMAD HAFEEZ
DEPARTMENT OF COMPUTER SCIENCE
GC UNIVERSITY LAHORE
OF
ASSEMBLY
TODAY’S AGENDA
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Program Structure
Assembler Directives
MEMORY MODELS
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Memory Models defines how much memory
we need for a program
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TINY
SMALL
MEDIUM
COMPACT
LARGE
HUGE
MEMORY MODELS
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TINY
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SMALL
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data segment can exceed 64K
Code segment 64K
LARGE
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64K data segment
Code can exceed 64K
COMPACT
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64K for code segment
64K for data segment
MEDIUM
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64K for both code and data segment
Code can exceed 64K
Data can exceed 64K but Array declared in one data segment
can not exceed 64K
HUGE
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Code can exceed 64K
Data can exceed 64K, Array declared in one data segment can
exceed 64K
MEMORY MODELS
ASSEMBLY LANGUAGE STATEMENTS
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Assembly Language Statements are either,
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Instructions
Directive (also called Pseudo-Ops)
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Both Instruction and Directive can have up to 4
Fields
[Label] mnemonic
[operand]
[;comment]
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Brackets indicate the fields are optional
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ASSEMBLY LANGUAGE STATEMENTS
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Label:
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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
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As it tells the assembler to refer the code
ASSEMBLY LANGUAGE STATEMENTS
ASSEMBLY LANGUAGE STATEMENTS
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Mnemonic:
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Instructions and Operands for Processor
Real work of program
MOV AX,5
ADD AX,BX
Pseudo-Ops/ Directive for Assembler
MAIN
ENDP
DB
Comment:
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At end of each line
Optional BUT Highly important for readable/
understandable Assembly Language Programs
ASSEMBLY LANGUAGE PROGRAM
DATA MOVEMENT INSTRUCTIONS
simplified
Assigns
a Title
to your
Source
File
Label
Stack of
program
Data of
program
Assembler
Directive
Comment
DATA DEFINING DIRECTIVES
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Storage is defined using data definition
directives
Storage is created at assembly time.
Variables can be initialized to character
string
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Syntax
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Name
Data Directive Initial value, [values]
DATA DEFINING DIRECTIVES
DECLARATION OF DATA
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DATA1
DATA2
DATA3
DATA4
DB
DB
DB
DB
15H
15H, 1FH, 4AH
? ;SET ASIDE ONE BYTE
‘STRING OF CHARACTERS’,’$’
DB could be replaced with DW, DD, DQ or
DT depending upon requirement
NAMED CONSTANTS WITH
DIRECTIVE “EQU”
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To assign a name to constant, use EQU
pseudo-op
EQU Syntax
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EQU
Constant
EQU
EQU
0AH
0DH
Example:
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LF
CR
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PROMPT EQU
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Name
‘Enter Number’
No Memory is allocated for EQU pseudo-op
MORE DATA DEFINITION
DIRECTIVES
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To assign a name to constant, use EQU
pseudo-op
EQU Syntax
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EQU
Constant
EQU
EQU
0AH
0DH
Example:
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LF
CR
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PROMPT EQU
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Name
‘Enter Number’
No Memory is allocated for EQU pseudo-op
ASSEMBLE, LINK AND RUN A
PROGRAM
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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
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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.
ASSEMBLE, LINK AND RUN A
PROGRAM
FILES CREATED BY ASSEMBLER
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The Source Listing File (.LST)
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The Cross Reference File (.CRF)
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Display line number and corresponding machine code sideby-side, helpful in debugging
List of variables and labels in the programs
The Cross Reference File (.CRF)
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List of variables and labels in the programs
MORE DIRECTIVE OF MASM
FOR HANDLING DATA
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ORG [Number]
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Is used to mark the beginning of offset address
ORG 10H
OFFSET
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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]
FLAG REGISTER
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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
FLAG REGISTER
CONDITIONAL FLAGS
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The processor uses conditional flags to
make decision
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CARRY FLAG (CF):
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SUB AX, AX set Zero Flag = 1
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):
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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)
CONDITIONAL FLAGS
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Auxiliary Carry (AF):
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Zero Flag (ZF):
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ZF=1, If result is zero.
ZF=0, for non-zero results
Sign Flag (SF):
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AF=1, If carry out from D3 bit to D4 bit, otherwise
AF=0
SF=1, If after computations MSB=1, if you are giving
a signed interpretation to result
SF=0, if MSB = 0
Overflow Flag (OF):
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OF=1, If Singed overflow occurs
OF=0, otherwise
CONTROL FLAGS
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Trap Flag (TF):
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Interrupt Flag (IF):
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TF=1, Allow to program to execute in single step,
helpful in debugging
TF=0, otherwise
IF=1, Allow external maskable interrupts to the
processor
IF=0, otherwise
Direction Flag (DF):
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Used to control the direction of string operations
FLAG REGISTER IN DEBUG
UTILITY
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Command is --- R F
INSTRUCTIONS AFFECT FLAGS
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MOV/ XCHG
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ADD/ SUB
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All
INC/ DEC
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None
All except CF
NEG
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All, (By default set CF=1, only exception result is
zero)
OF=1, if Word operand is 8000H and byte
operand is 80H
EXAMPLES:
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AX=FFFFH, BX=FFFFH, ADD AX,BX
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MOV AL,80H, MOV BL,80H, ADD AL,BL
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SF=1, CF=1, AF=1
PF=1, ZF=1, CF=1, OF=1, SF=0
MOV CX, 5
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DEC CX, DEC CX, DEC CX, DEC CX, DEC CX
After execution of these instructions result will be
zero hence, ZF=1
EXAMPLES:
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Addition 0Fh + 08h?
Addition 0Fh + F8h?
Addition 4Fh + 40h?
Addition F8h + 81h?
 Represent in signed and unsigned
numbers
SOME BASIC I/O OPERATIONS
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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
SOME BASIC I/O OPERATIONS
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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
SOME BASIC I/O OPERATIONS
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Read a Character
Display a Character
Display a String
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
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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
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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
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QUESTIONS
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