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flat assembler > DOS > Code not working properly after int21 AH=4C [SOLVED]

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redsimplex



Joined: 13 May 2016
Posts: 4
Code not working properly after int21 AH=4C [SOLVED]
I was working on a code for some time and it went smooth until I added a variable called [origin_color] and after that the terminat interrupt started working incorectly.
Basiclly the code needs to return back to the dos terminal after you press ESC, but it just hangs on instead.

after deleting most of the code but leaving some parts in the code still not works, example:


Code:
format mz

entry code:start


segment varseg
        black_color db ?
        green_color db ?
        orgin_color db ?

        mouse_status db ?
        mouse_x dw ?
        mouse_y dw ?
        previous_x dw ?
        previous_y dw ?
        two dw ?


segment code
        start:

                mov [two], 02h
                mov [green_color], 02h
                mov [orgin_color], 09h

                mov ax13h
                int 10h

                main_loop:
                        keyboard_input:
                        in al60h
                        dec al
                        jz exit
                        jmp main_loop

        exit:

                mov ah4ch
                int 21h



if you erase


Code:
                mov [two], 02h
                mov [green_color], 02h
                mov [orgin_color], 09h



the code suddenly works.

Can someone explain why its happening, and how can I fix it? Crying or Very sad

edit1:

Following in a debug you find that that code runs until you press 'esc', then the IP jumps to 'exit' where it suppose to terminate the program and return, but after int 21h it just stacks Sad

and I searched for sometime for an answer in google and in the search in this board and could'nt find anything as the FAQ suggested.


Last edited by redsimplex on 13 May 2016, 06:24; edited 1 time in total
Post 13 May 2016, 03:29
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15233
Location: 1I/╩╗Oumuamua
You don't seem to be initialising the DS register with the value of varseg.
Post 13 May 2016, 04:50
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redsimplex



Joined: 13 May 2016
Posts: 4

revolution wrote:
You don't seem to be initialising the DS register with the value of varseg.


thanks! problem solved.
But I don't understand what happened there, will you mind to explain?
Post 13 May 2016, 06:24
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15233
Location: 1I/╩╗Oumuamua
When you don't specify a different segment register all the memory writes that don't use SP are controlled by DS.
Post 13 May 2016, 07:03
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Trinitek



Joined: 06 Nov 2011
Posts: 257
DOS does not set DS to a useful value when starting an EXE. You were effectively writing to a location in memory that wasn't where your variables were located.
Post 13 May 2016, 07:13
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