Lecture 06

Programming The 8086 In

AAiT

Programming The 8086 In

Assembly Language…Continued

Daniel D.Daniel D.Daniel D.Daniel D. DECEECEg ECEg ECEg ECEg ---- 4501450145014501

� Assembler directives & operators

IssuesAAiT

� program examples & exercises

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Assembler directives (Preprocessors)

ASSUME tell the assembler the name of the logical segment it

should use for a specified segment.

e.g. ASUME CS: code

DB (define byte) Used to declare byte type variable(s) and set aside

storage for it

e.g. High_scores DB 82H, 78H, 75H

AAiT

e.g. High_scores DB 82H, 78H, 75H

Name DB “ZELALEM”

Also, DW, DD, DQ, DT

END end of program. Assembler ignores any instruction after END

EQU (equate) Used to give a name to some value or symbol.

e.g. Ratio EQU 05h

Henceforth, Ratio can be used instead of 05h in the program

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Processor directives

EVEN tell the assembler to align the data or instruction to the even

bank.

EXTRN Used to tell the assembler that the names or labels following

the directive are in some other assembly module.

GLOBAL can be used in place of PUBLIC or EXTRN directives. It is used

to make the symbol available to other modules.

INCLUDE Include Source Code from File

AAiT

INCLUDE Include Source Code from File

e.g. INCLUDE “program_2.asm”

LENGTH Is an operator, which tells the assembler to determine the

number of elements in some named data item, such as a

string or an array.

e.g. MOV CX, LENGTH string1 CX = no of elements in string1

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Processor directives

NAME Used to give a specific name to each assembly module

when programs consisting of several modules are written.

e.g. NAME “Main”

OFFSET Is an operator which tells the assembler to determine the

offset or the displacement of a named data item (variable) or

procedure from start of the segment which contains it.

e.g. MOV BX, OFFSET TABLE

AAiT

e.g. MOV BX, OFFSET TABLE

ORG allows to set the location counter to a desired value at any

point in the program.

e.g. ORG 100H set the location counter to 0100H

ORG $ +100 Reserve the next 100 bytes

NB: Refer to the handout for other directives

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Program Examples

E.g. 1

Determine the contents of the GP registers and flags after these

programs.

MOV AX,1234H V DB 5, 8, 12, 1

MOV BX,5678H MOV AL, 0

PUSH AX MOV BX, 0

PUSH BX MOV DL, 24

ORG 40H

JMP START

NAME DB “Abebe”

START:

MOV SI, OFFSET NAME

AAiT

PUSH BX MOV DL, 24

POP AX DO: ADD AL, V[BX]

POP BX INC BX

CMP AL, DL

JLE DO

AX ? AL ? DL ? BX ?

BX ? ZF ? CF ? PF ? IP? AL? CX? [SI]? ZF?

MOV SI, OFFSET NAME

INC SI

LOADS [SI]

MOV CX, 05H

REPEAT: INC SI

CMP AL, [SI]

LOOPE REPEAT

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Program Examples

E.g. 2

1. Write a program that takes temperature values in Celsius

from the keyboard; changes to Fahrenheit; and displays

the result on the screen, like:

20 C = 68 F

or 05 C = 41 F

AAiT

or 05 C = 41 F

�take input range 00-99 c

�Use INT 21h/ah = 1……………for input function

� Use INT 21h/ah = 2……………for character display

�TF = 9/5 Tc + 32

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E.g. 2 solution

The input sub-program

name “input”

org 100h ;locate code starting here

jmp start

input db 0,0 ;reserve memory for i/p

AAiT

input db 0,0 ;reserve memory for i/p

output db 0,0,0 ;and for o/p

;take a two digit no and save @input

start:

lea bx, input ;points to 1st input variable

mov si, bx ;keep the input address

mov cx, 2 ;no of chars.

mov ah, 1 ;for input function

INP:

int 21h ;input function

mov [bx], al ;save the i/p

inc bx

loop INP

Next slide

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E.g. 2 solution….cnt’d

The conversion sub-program

name “conversion”

;form a single no from the inputs

mov dl, 10

mov al, [si] ;fist digit

and al, 0Fh ;change to decimal

;convert to Fahrenheit

mov dl, 5

div dl ;divide by 5

AAiT

and al, 0Fh ;change to decimal

mul dl ;10’s decimal place

inc si

mov dl, [si] ;second digit

and dl, 0Fh ;change to decimal

add al, dl ;one’s decimal place

div dl ;divide by 5

mov dl, 9

mul dl ;multiply by 9

add al, 32 ;add 32

Next slide

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E.g. 2 solution….cnt’d

The formatting sub-program (to ASCII)

name “format”

; change result to ASCII & store @output

mov cx, 3

lea bx, output ;point in memory @output

add bx, 2 ;start at one’s decimal place

mov di, bx ;keep the o/p address

ascii:

mov ah, 0

cmp al, 10

jb done

div dl ;ax/10: q=al, r=ah

or ah, 30h ;convert to ASCII

AAiT

mov di, bx ;keep the o/p address

mov dl, 10

or ah, 30h ;convert to ASCII

mov [bx], ah

dec bx

loop ascii

done: or al, 30h ;one’s place

mov [bx], al

NB: Character input and output is implemented in ASCII format,

that’s why we need to convert ASCII to decimal at input and

decimal to ASCII at output.

Next slide

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E.g. 2 solution….cnt’d

The display sub-program

; display the result in the

; required format

name “display”

mov cx, 2

mov ah, 2

lea si, input

mov cx,3 ;display output

op: cmp bx, di

ja finish

mov dl,[bx]

AAiT

lea si, input

ip: mov dl, [si] ;display input

int 21h

inc si

loop ip

mov dl, 'C' ;display C

int 21h

mov dl, ' = ' ;display =

int 21h

mov dl,[bx]

int 21h

inc bx

loop op

finish: mov dl,'F‘ ;display F

int 21h

ret ;return to operating system

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Exercise:

2. Consider a login system. Suppose you have a 5-character

password stored in DS in memory. Write a program that

reads a 5-char password string from the keyboard port;

compare the string with the password in memory; prints

“access granted” if correct or “access denied” otherwise.

AAiT

“access granted” if correct or “access denied” otherwise.

�Use INT 21h/ah = 1…………for input function

� Use INT 21h/ah = 9…………to display a string

�Use CMPSB to compare strings

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