Simple Assembly Program Problem

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Mahmoud899

Joined: 26 Sep 2012
Posts: 5

Mahmoud899 12 Sep 2013, 21:48

Hello, I am new to the x86 assembly language. I am trying to write a very simple assembly program that prints to the screen a string.

Code:

org 100h

jmp start       ; jump over data declaration

msg:    db      "Hello, World!", 0

start:  
        mov     al, [ds:si]
        cmp     al, 0
        je      done
        mov     ah, 0eh
        int     10h
        inc     si
        jmp     start

done:
        mov ax, 4C00h
        int 21h

the program prints the following:
δ♫Hello, World!

I am writing the program on the following assumptions, that any db i make is already put in the data segment, also that the SI, BX, DI are pointing at the data segment. I would like to know why doesn't it print Hello, World! only and it prints weird characters before it?

Thank You

12 Sep 2013, 21:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 12 Sep 2013, 22:09

You are printing the two bytes of the "jmp start" instruction. Your data follows after the jmp.

You might want to consider initialising SI to point to the data before using it.

Code:

;...
start:
        mov si,msg
loop:
        mov     al, [ds:si]
        cmp     al, 0
        je      done
        mov     ah, 0eh
        int     10h
        inc     si
        jmp     loop
;...

12 Sep 2013, 22:09

MHajduk

Joined: 30 Mar 2006
Posts: 6115
Location: Poland

MHajduk 12 Sep 2013, 22:17

Or just do something like that:

Code:

org 100h

jmp start       ; jump over data declaration

msg:    db      "Hello, World!", 0

start:  
        mov     al, [ds:si+2] ; <--- a small change here
        cmp     al, 0
        je      done
        mov     ah, 0eh
        int     10h
        inc     si
        jmp     start

done:
        mov ax, 4C00h
        int 21h

12 Sep 2013, 22:17

Mahmoud899

Joined: 26 Sep 2012
Posts: 5

Mahmoud899 13 Sep 2013, 11:42

Thank you guys, problem solved

13 Sep 2013, 11:42

Mahmoud899

Joined: 26 Sep 2012
Posts: 5

Mahmoud899 13 Sep 2013, 12:39

Hello, I am writing a very simple assembly program that adds 2 numbers and then moves their total to a defined variable that I have created

Code:

format MZ

entry .code:start
segment .code
start:
        mov ax, .data
        mov ds, ax
        mov al, data1
        mov bl, data2
        add al, bl
        mov total, al
        mov ah, 4CH
        int 21H 


segment .data
data1   db      52H
data2   db      29H
total   db      ?

when I assemble the program I get the following error:
F2_1.asm[11]:
mov total, al
error: invalid operand.

How is that possible, it should be possible that you could move data from general purpose registers to a user defined variable. How can I fix that error thank you

13 Sep 2013, 12:39

system error

Joined: 01 Sep 2013
Posts: 670

system error 13 Sep 2013, 12:49

Mahmoud899

mov al, data1 ;transfers the ADDRESS
mov al, [data1] ;transfers the VALUE

Every time you want to transfer data from or to memory (like your variables), use the brackets like [data] or [ds:data]. Without the brackets, the address is transferred instead.

13 Sep 2013, 12:49

system error

Joined: 01 Sep 2013
Posts: 670

system error 13 Sep 2013, 13:06

Download "emu8086" so that you can see how your instructions behave against the registers in step-by-step manner. It has direct support of FASM syntax. Start with a simpler flat memory model like org. It provides you with a natural transition to flat 32-bit programming.

13 Sep 2013, 13:06

Mahmoud899

Joined: 26 Sep 2012
Posts: 5

Mahmoud899 13 Sep 2013, 13:21

I tried your suggestion and edited the code and inserted square brackets around data1 and data2 and I am still getting the same problem in the same line. I have tried it on the EMU8086 and it also gave me the same error. I am also using 16-bit programming not 32-bit

13 Sep 2013, 13:21

system error

Joined: 01 Sep 2013
Posts: 670

system error 13 Sep 2013, 13:30

[total]

13 Sep 2013, 13:30

Mahmoud899

Joined: 26 Sep 2012
Posts: 5

Mahmoud899 13 Sep 2013, 13:51

excellent it worked. Thanks for the help

13 Sep 2013, 13:51

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 13 Sep 2013, 15:12

What about overflow / carry flag ?
You should interprete this.
Try to add 0FFh and 80h and see what happens ... Cool

13 Sep 2013, 15:12

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