Introduction to 8086 Assembly

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Introduction to 8086 Assembly Language

CS 272Sam Houston State UniversityDr. Tim McGuire

Structure of an assembly language program

Assembly language programs divide roughly into five sectionsheaderequatesdatabodyclosing

The Header

The header contains various directives which do not produce machine codeSample header:

%TITLE "Sample Header".8086.model small.stack 256

Named Constants

Symbolic names associated with storage locations represent addresses

Named constants are symbols created to represent specific valuesdetermined by an expressionNamed constants can be numeric or stringSome named constants can be redefinedNo storage is allocated for these values

Equates

Constant values are known as equatesSample equate section:

Count EQU 10

Element EQU 5Size = Count * ElementMyString EQU "Maze of twisty passages"Size = 0

= is used for numeric values onlyCannot change value of EQU symbolEQUated symbols are not variables

Introduction to 8086 Assembly Language http://www.shsu.edu/~csc_tjm/spring2005/cs272/intro_to

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EQU expressions are evaluated where used; = expressions are evaluatedwhere defined

The Data Segment

Begins with the.datadirectiveTwo kinds of variables, initializedand uninitialized.

Initialized variables take up space in the program's code fileDeclare uninitialized variables after initialized ones so they do not take upspace in the program's code file

Reserving space for variables

Sample DATA SEGMENT

.datanumRows DB 25numColumns DB ?videoBase DW 0800h

DB and DW are common directives (define byte) and (define word)The symbols associated with variables are called labelsStrings may be declared using the DB directive:

aTOm DB "ABCDEFGHIJKLM"

Program Data and Storage

Pseudo-ops to define data or reserve storageDB - byte(s)

DW - word(s)DD - doubleword(s)DQ - quadword(s)DT - tenbyte(s)

These directives require one or more operandsdefine memory contentsspecify amount of storage to reserve for run-time data

Defining Data

Numeric data values100 - decimal100b - binary100h - hexadecimal'100' - ASCII"100" - ASCII

Use the appropriate DEFINE directive (byte, word, etc.)A list of values may be used - the following creates 4 consecutive words


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DW 40Ch,10b,-13,0

A ? represents an uninitialized storage location

DB 255,?,-128,'X'

Naming Storage Locations

Names can be associated with storage locations

ANum DB -4 DW 17ONEUNO DW 1X DD ?

These names are called variablesANum refers to a byte storage location, initialized to FChThe next word has no associated name

ONE and UNO refer to the same wordX is an uninitialized doubleword

Arrays

Any consecutive storage locations of the same size can be called an array

X DW 040Ch,10b,-13,0Y DB 'This is an array'Z DD -109236, FFFFFFFFh, -1, 100b

Components of X are at X, X+2, X+4, X+6

Components of Y are at Y, Y+1, , Y+15Components of Z are at Z, Z+4, Z+8, Z+12

DUP

Allows a sequence of storage locations to be defined or reservedOnly used as an operand of a define directive

DB 40 DUP(?)DW 10h DUP(0)DB 3 DUP("ABC")DB 4 DUP(3 DUP (0,1), 2 DUP('$'))

Word Storage

Word, doubleword, and quadword data are stored in reverse byte order (inmemory)

Directive Bytes in StorageDW 256 00 01


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DD 1234567h 67 45 23 01DQ 10 0A 00 00 00 00 00 00 00X DW 35DAh DA 35

Low byte of X is at X, high byte of X is at X+1

The Program Body

Also known as the code segmentDivided into four columns: labels, mnemonics, operands, and commentsLabels refer to the positions of variables and instructions, represented bythe mnemonicsOperands are required by most assembly language instructionsComments aid in remembering the purpose of various instructions

An example

Label Mnemonic Operand Comment--------------------------------------------------------- .data

exCode DB 0 ;A byte variablemyWord DW ? ;Uninitialized word var. .codeMAIN PROC mov ax,@data ;Initialize DS to address mov ds,ax ; of data segment jmp Exit ;Jump to Exit label mov cx,10 ;This line skipped!Exit: mov ah,04Ch ;DOS function: Exit prog mov al, exCode ;Return exit code value int 21h ;Call DOS. Terminate progMAIN ENDP ;End Program END MAIN ; and specify entry point

The Label Field

Labels mark places in a program which other instructions and directivesreferenceLabels in the code segment always end with a colonLabels in the data segment never end with a colonLabels can be from 1 to 31 characters long and may consist of letters,digits, and the special characters ? . @ _ $ %If a period is used, it must be the first characterLabels must not begin with a digit

The assembler is case insensitive

Legal and Illegal Labels

Examples of legal namesCOUNTER1

@character


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SUM_OF_DIGITS

$1000

DONE?

.TEST

Examples of illegal namesTWO WORDS contains a blank

2abc begins with a digitA45.28 . not first characterYOU&ME contains an illegal character

The Mnemonic Field

For an instruction, the operation field contains a symbolic operation code(opcode)The assembler translates a symbolic opcode into a machine languageopcodeExamples are: ADD, MOV, SUB

In an assembler directive, the operation field contains a directive(pseudo-op)Pseudo-ops are not translated into machine code; they tell the assembler todo something

The Operand Field

For an instruction, the operand field specifies the data that are to be actedon by the instruction. May have zero, one, or two operands

NOP ;no operands -- does nothingINC AX ;one operand -- adds 1 to the contents of AXADD WORD1,2 ;two operands -- adds 2 to the contents ; of memory word WORD1

In a two-operand instruction, the first operand is the destination operand.The second operand is the source operand.For an assembler directive, the operand field usually contains moreinformation about the directive.

The Comment Field

A semicolon marks the beginning of a comment field

The assembler ignores anything typed after the semicolon on that lineIt is almost impossible to understand an assembly language programwithout good commentsGood programming practice dictates a comment on almost every line

Good and Bad Comments

Don't say something obvious, like


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MOV CX,0 ;move 0 to CX

Instead, put the instruction into the context of the program

MOV CX,0 ;CX counts terms, initially 0

An entire line can be a comment, or be used to create visual space in a

program

;; Initialize registers; MOV AX,0 MOV BX,0

The Closing

The last lines of an assembly language program are the closing

Indicates to assembler that it has reached the end of the program andwhere the entry point is

MAIN ENDP ;End of program END MAIN ; entry point for linker use

END is a pseudo-op; the single "operand" is the label specifying thebeginning of execution, usually the first instruction after the.codepseudo-op

Assembling a Program

The source file of an assembly language program is usually named with anextension of .asm

edit myprog.asm

The source file is processed (assembled) by the assembler (TASM) toproduce an object file (.obj)

tasm myprog produces myprog.obj

The object file must be linked by the linker (TLINK) to produce an

executable file (.exe)

tlink myprogproduces myprog.exe

Dealing with Errors

TASMwill report the line number and give an error message for each errorit finds


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Sometimes it is helpful to have a listing file (.lst), created by using TASMwith the -loptionThe .lstfile contains a complete listing of the program, along with linenumbers, object code bytes, and the symbol table

Using the Debugger

Useful for logic errors that the assembler missesSee the text for a complete tutorialYou do notneed to use the TDH386.SYS driver or the TD386.EXE debuggerwith the latest version of the assemblerTo use the debugger on myprog.asm

tasm /zi myprogtlink /v myprogtd myprog

.COM and .EXE files

The .COM code file format is a relic of the first version of MS-DOSNot recommended for general purposesAll code, data, and the stack occupy one 64K segment (Borland's "tiny"model).EXE code files are more efficient in use of RAMData and code occupy separate segmentsThe programmer is responsible for setting up the data and code segmentsproperly

Ending a Program

All programs, upon termination, must return control back to anotherprogram -- the operating systemUnder MS-DOS, this is COMMAND.COMThis is done by doing a DOS system call

Data Transfer Instructions

MOV destination,sourcereg, regmem, regreg, memmem, immedreg, immed

Sizes of both operands must be the samereg can be any non-segment register except IP cannot be the target registerMOV's between a segment register and memory or a 16-bit register arepossible


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Examples

mov ax, word1

"Move word1to ax"Contents of register axare replaced by the contents of the memorylocation word1

xchg ah, bl

Swaps the contents of ahand blIllegal: mov word1, word2

can't have both operands be memory locations

Sample MOV Instructions

b db 4Fhw dw 2048

mov bl,dhmov ax,wmov ch,b

mov al,255mov w,-100mov b,0

When a variable is created with a define directive, it is assigned a defaultsize attribute (byte, word, etc)You can assign a size attribute using LABEL

LoByte LABEL BYTEaWord DW 97F2h

Addresses with Displacements

b db 4Fh, 20h, 3Chw dw 2048, -100, 0

mov bx, w+2mov b+1, ahmov ah, b+5mov dx, w-3

Type checking is still in effectThe assembler computes an address based on the expression

NOTE: These are address computations done at assembly time

MOV ax,b-1

will not subtract 1 from the value stored at b

eXCHanGe

XCHG destination,source


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reg, regreg, memmem, reg

MOV and XCHG cannot perform memory to memory movesThis provides an efficient means to swap the operands

No temporary storage is neededSorting often requires this type of operationThis works only with the general registers

Arithmetic Instructions

ADD dest, sourceSUB dest, sourceINC destDEC destNEG dest

Operands must be of the same size

sourcecan be a general register, memory location, or constantdestcan be a register or memory location

except operands cannot both be memory

ADD and INC

ADD is used to add the contents oftwo registersa register and a memory locationa register and a constant

INC is used to add 1 to the contents of a register or memory location

Examples

add ax, word1

"Add word1to ax"Contents of register axand memory location word1 are added, andthe sum is stored inax

inc ah

Adds one to the contents of ahIllegal: add word1, word2


SUB, DEC, and NEG

SUB is used to subtract the contents ofone register from another registera register from a memory location, or vice versaa constant from a register

DEC is used to subtract 1 from the contents of a register or memory location


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NEG is used to negate the contents of a register or memory location

Examples

sub ax, word1

"Subtract word1from ax"Contents of memory location word1 is subtracted from the contents of

register ax, and the sum is stored inaxdec bx

Subtracts one from the contents of bxIllegal: sub byte1, byte2


Type Agreement of Operands

The operands of two-operand instructions must be of the same type (byteor word)

mov ax, bh ;illegal

mov ax, byte1 ;illegalmov ah,'A' ;legal -- moves 41h into ah

mov ax,'A' ;legal -- moves 0041h into ax

Translation of HLL Instructions

B = A

mov ax,Amov B,ax

memory-memory moves are illegal

A = B - 2*A

mov ax,Bsub ax,Asub ax,Amov A,ax

Program Segment Structure

Data SegmentsStorage for variablesVariable addresses are computed as offsets from start of this segment

Code Segmentcontains executable instructions

Stack Segmentused to set aside storage for the stackStack addresses are computed as offsets into this segment

Segment directives


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.DATA

.CODE

.STACK size

Memory Models

.Model memory_model

tiny: code+data


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The interrupt instruction is used to cause a software interrupt (system call)An interrupt interrupts the current program and executes a subroutine,eventually returning control to the original programInterrupts may be caused by hardware or software

int interrupt_number ;software interrupt

Output to Monitor

DOS Interrupts : interrupt 21hThis interrupt invokes one of many support routines provided by DOSThe DOS function is selected via AHOther registers may serve as arguments

AH = 2, DL = ASCII of character to outputCharacter is displayed at the current cursor position, the cursor isadvanced, AL = DL

Output a String

Interrupt 21h, function 09hDX = offset to the string (in data segment)The string is terminated with the '$' character

To place the address of a variable in DX, use one of the followinglea DX,theString ;load effective address

mov DX, offset theString ;immediate data

Print String Example

%TITLE "First Program -- HELLO.ASM" .8086 .MODEL small

.STACK 256 .DATAmsg DB "Hello, World!$" .CODEMAIN PROC mov ax,@data ;Initialize DS to address mov ds,ax ; of data segment lea dx,msg ;get message mov ah,09h ;display string function int 21h ;display messageExit: mov ah,4Ch ;DOS function: Exit program mov al,0 ;Return exit code value int 21h ;Call DOS. Terminate programMAIN ENDP ;End of program END MAIN ; entry point

Input a Character

Interrupt 21h, function 01hFiltered input with echo


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This function returns the next character in the keyboard buffer (waitingif necessary)The character is echoed to the screenAL will contain the ASCII code of the non-control character

AL=0 if a control character was entered

An Example Program

%TITLE "Case Conversion" .8086 .MODEL small .STACK 256 .DATAMSG1 DB 'Enter a lower case letter: $'MSG2 DB 0Dh,0Ah,'In upper case it is: 'CHAR DB ?,'$'exCode DB 0 .CODE

MAIN PROC;initialize ds mov ax,@data ; Initialize DS to address mov ds,ax ; of data segment;print user prompt mov ah,9 ; display string fcn lea dx,MSG1 ; get first message int 21h ; display it;input a character and convert to upper case mov ah,1 ; read char fcn int 21h ; input char into AL

sub al,20h ; convert to upper case mov CHAR,al ; and store it;display on the next line mov dx,offset MSG2 ; get second message mov ah,9 ; display string function int 21h ; display message and upper case;return to DOSExit: mov ah,4Ch ; DOS function: Exit program mov al,exCode ; Return exit code value int 21h ; Call DOS. Terminate programMAIN ENDP END MAIN ; End of program / entry point


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Introduction to 8086 Assembly

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