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teencoder

May 28th, 2007 2:40 AM

Help with simple program

Okay I'm done with school for a while and I decided to toy with systems programming after a crash course in assembly I decided to try something. The program boots but doesn't execute properly. I've examined my code but I haven't found any problem. So if you could help me I'd really appreciate. It was assembled in NASM.

[BITS 16]

ORG 0

mov AH,0x0

mov AL,0x0

int 0x10; enter 40x25 B/W text video mode

continue:

int 0x16;read key strokes

cmp AH,0x1C0D;test for an enter key stroke

je exit

mov AL,AH

int 10;display character

jmp continue

exit:

mov AH,0x0

int 19;warm boot

times 510-($-$$) db 0x0;fill up empty bytes

dw 0xAA55;make bootable

The program boots initializes the video mode but on a key press does nothing after a few key strokes it starts beeping with the keystrokes. The enter key does not trigger a warm boot. Any suggestions would be appreciated

grimpirate

May 28th, 2007 5:26 PM

I'll refer you to the following posts I started here where I had similar inquiries and problems and also to a tutorial I posted on another forum:

Hope that helps ya, also I like to use the FASM compiler but that's a personal preference, however, their forum has lots of info
http://flatassembler.net/

teencoder

May 29th, 2007 2:28 AM

Thanks I tried to fix a few things but it's still broke and I have no clue where. Guess this is what I get for diving in headfirst if anyone could take a look I'd appreciate it.

[BITS 16]

ORG 0

jmp initiate

nop

times 59 db 0xFF

initiate:

mov AH,0x0

mov AL,0x0

int 0x10;set video

jmp main

main:

continue:

int 0x16;read keyboard 

cmp AH,0x5A; test for enter key 

je end; close 

mov AL,AH

mov AH,0x0E

mov BH,1

int 10;display character 

mov AH,0

jmp continue; 

end:

jmp terminate

terminate:

mov AH,0x0;

int 19;

times 510-($-$$) db 0x0

dw 0xAA55;make bootable

grimpirate

May 29th, 2007 12:44 PM

Where exactly does the program fail? Does it not accept the enter key at all? I suggest before trying to accept input, trying to output something beforehand. Also, some of your labels and instructions like jmp main and main: are unecessary, as is the jmp terminate with the terminate: label right after.
Also check out this post at the flatassembler page
http://board.flatassembler.net/topic.php?t=6088
That program prints Hello World! waits for a key press and when a key is entered it powers down, which I think is sort of what you're trying to achieve.

DaWei

May 29th, 2007 1:40 PM

end:

jmp terminate

terminate:

Look at this and apply a little thought. Execution normally proceeds to the next instruction. A branch or call modifies that. Jumping to the next instruction is just a waste (as mentioned in Grim's post).

Now suppose you remove the worthless jump. The end and terminate labels now represent the same address. That doesn't consume any code or take any time, but what the hell good is it, except for introducing confusion?

I don't care how many trees you kill. Use a pencil and paper to solidify your thoughts. Trace lines from here to there. Disentangle spaghetti. Better to kill a tree than an innocent person when your critical code goes south.

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