introduction to assembly language(chapter 1)

8/11/2019 Introduction to Assembly Language(Chapter 1)

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Computer Organization &Assembly Languages

Introduction to Assembly Language

Chapter 1


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Assembly Language

Sometimes referred to

as assemblyor ASL, assembly languageis a

low-levelprogramming language used to

interface with computer hardware. Assemblylanguage uses structured commands as

substitutions for numbers allowing humans to

read the code easier than looking at binary.


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Basic Components in Assembly

Language

Instruction, Directive, Label, and Comment;NUMOFF.ASM: Turn NUM-LOCK indicator off.

.MODEL SMALL

.STACK

.CODE

.STARTUP

MOV AX,40H ;set AX to 0040H

D1: MOV DS,AX ;load data segment with 0040H

MOV SI,17H ;load SI with 0017H

AND BYTE PTR [SI],0DFH ;clear NUM-LOCK bit

.EXIT

END

Comments

Assembly directive

Instructions

Assembly directive

Label


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Assembler

A computer will not understand any program written in a language, otherthan its machine language. The programs written in other languages mustbe translated into the machine language. Such translation is performedwith the help of software. A program which translates an assemblylanguage program into a machine language program is called anassembler.If an assembler which runs on a computer and produces the

machine codes for the same computer then it is called self assembler orresident assembler. If an assembler that runs on a computer and producesthe machine codes for other computer then it is called Cross Assembler.

Assemblers are further divided into two types: One Pass Assembler andTwo Pass Assembler. One pass assembler is the assembler which assignsthe memory addresses to the variables and translates the source code intomachine code in the first pass simultaneously. A Two Pass Assembler is theassembler which reads the source code twice. In the first pass, it reads allthe variables and assigns them memory addresses. In the second pass, itreads the source code and translates the code into object code.


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Linker & Loader

Linker

A linkeror link editoris a computer program that

takes one or more object files generated by

a compiler and combines them into asingle executable program.

Loader

ALoader

is a program which accepts the objectprogram as input, makes them executable by the

computer and initiates execution.


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Assembler

Design and Implementation ofAssembler

Source

Program Assembler

Object

Code

Loader

Executable

Code

Linker


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Advantages & Disadvantages of Assembly Language

Advantages Assembly Language:

1.The symbolic programming of Assembly Language is easierto understand and saves a lot of time and effort of the

programmer.

2.It is easier to correct errors and modify program

instructions.

3.Assembly Language has the same efficiency of execution as

the machine level language. Because this is one-to-one

translator between assembly language program and its

corresponding machine language program.

Disadvantages Assembly Language:1.One of the major disadvantages is that assembly language is

machine dependent. A program written for one computer

might not run in other computers with different hardware

configuration.


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Functions of a Basic Assembler

Convert mnemonic operation codes tomachine language equivalents

Convert symbolic operands to machine

addresses (pass 1) Build machine instructions

Convert data constants to internal

representations Write the object program and assembly listing

files


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Basic Elements of Assembly Language

Integer Constants

If no radix is given, the integer is assumed to bedecimal.

Int 21h Int 21

A hexadecimal constant beginning with a letter

must have a leading 0

Mov ax, 0A3C9h


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Character and String Constants

A single or a string of characters

Enclosed in either single or double quotes

Embedded quotes are permittedAn embedded quote within quotation marks


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Reserved Words

Instruction Mnemonics

Directives

Attributes (BYTE or WORD)

Operators

Predefined Symbols

A complete list of MASM reserved wordsare in Appendix D (4thEdition)


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Names

(Identifiers)

Used to identify variables, constants, procedures, orcode labels. Max 247 characters

Not case-sensitive

First character can be a letter, _, $, @ Subsequent characters may also be digits.

Cannot use an assembler reserved word.

Should make identifier names descriptive and easy to

understand Avoid @ as first character.


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Examples

Mystring db this is a string,0

Mystring is the name

Length = $ - mystring

Length is a reserved word (can not use)

Loop1: Loop1 is the label for a loop instruction

Note difference between data names and code

labels in the data section (no colon after the name)

In the code section (colon after the name)


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Assembly Language

Statements

Generally fall into two classes:

Instructions- Executable StatementsDirectivesstatements that provide

information to the assembler

about how to generateexecutable code


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General Format of a Statement

[name] [mnemonic] [operands] [;comment]

Statements are free-formthey can be

written in any column with any number ofspaces between each operand

Blank lines are permitted

Must be written on a single line and notpass column 128 (use / for continuation of

line)


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Directives

A statement that is recognized and acted upon by

the assembler as the programs source code isbeing assembled.

Used for defining logical segments, choosing amemory module, defining variables, creatingprocedures,

.data (directive to identify the area of a programthat contains the data variables)

Count db 50 (db is a directive that tells theassembler to create storage for a byte namedcount and initialize it to 50.


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Standard Assembler Directives

Directive Description

title Title of the listing file

.model Specify the program's memory model

.stack Set the size of the stack segment

.data Mark the beginning of the data segment

.code Mark the beginning of the code segment

proc Begin procedure

endp End of procedure

end End of program assembly


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Instructions

A statement that is executed by the processorat runtime

Fall into five general categories

Data transfer (mov ax, 5) Arithmetic (add ax, 20)

Transfer of control (call MySub)

Logical (Jz next1) Input/output (In al, 20)


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General Format of an Instruction

Label (optional)

Instruction mnemonic (required)

Operand(s) (usually required)

Comment (optional)

A source code line may have only a label orcomment.

Label: Mnemonic ;CommentOperand(s)


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Labels

An identifier that acts as a place marker for either instructions ordata

Used to transfer program execution to a labeled instruction Act as place markers

marks the address (offset) of code and data

Data label must be unique example: count DWORD 100 (not followed by colon)

Code label target of jump and loop instructions

example: target: (followed by colon)Mov ax, bx.jmp target


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Instruction Mnemonics

A short word that identifies the operation

carried out by an instruction

Mov assign one value to another

Add add two values

Sub subtract one value from another

Call call a procedure


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Formatting of Instructions and

Number of Operands Needed

Mnemonicdestination operand, source operand

HLT ; zero operands

INC AX ; one operandMOV AX, 100 ; two operands

SHLD DX, AX, 4 ; three operands

Different instructions use different numbers of operands as shown above.


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No operandsstc ; set carry flag

One operandinc eax ; increment register eax

call Clrscr ; call procedure Clrscrjmp L1 ; jump to instruction with label L1

Two operandsadd ebx, ecx ; register ebx = ebx + ecx

sub var1, 25 ; memory variable var1 = var1 - 25 Three operands

imul eax,ebx,5 ; register eax = ebx * 5

Instruction Examples


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Operands

A memory operand is specified by either the name of

a variable or a register that holds the address of a

variable.

A variable name implies the address of the variable;96 constant operand

2 + 4 constant expression operand

Eax register operand

Count variable operand


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Comments

Comments are good!

explain the program's purpose

when it was written, and by whoms

revision information

tricky coding techniques

application-specific explanations Single line comments begin with a semicolon

; Words after the semicolon are comments.; Comments may begin anywhere on a line

Multi-line commentsBlock comments begin with the comment directive a user-specified

symbolCOMMENT &This line is a comment.So is this line. The ampersand symbol is personal choice.

&

introduction to assembly language(chapter 1)

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