nasm assembler; output numbers?

infinity42 · 11-11-2005, 05:51 PM

Probably a very simple question - I am just starting assembler. If I have something like:

Code:

...
mov eax,20 ;20==getpid
int 0x80
...

now eax should contain my processes' PID I believe? My question is this: how do I print this. I have been using:

Code:

...
mov eax,4 ;4==write
mov ebx,1 ;1==stdout
mov ecx,buf ;some memory address holding the string
mov edx,len ;length of string
int 0x80
...

which works fine for ascii strings. I can get the PID out of eax an into memory, but do I then have to convert the number into ascii, or is there a simpler way?

Thanks

Wim Sturkenboom · 11-13-2005, 11:42 PM

My assembly knowledge is limited to microcontrollers and at this stage very rusty.
AFAIK you have to do the conversion yourself.

Below some pseudo-code that might help you on the way:

Code:

// first digit
buf[0]=(pid%10)+0x30;
pid/=10;
// second digit
buf[1]=(pid%10)+0x30;
pid/=10;
 .
 .
 .

The trick is to separate each digit and convert it to an ascii value. Separation is done by using the modulo-10.
Conversion is done by simply adding 30hex.
After the conversion, we divide the pid by 10 so we can use the modulo-10 on the next digit.

Please note that the above example places the digits in reverse order in the buffer.
Hope this helps you on the way.

bastl · 11-14-2005, 11:20 AM

O.K. the code above you got posted is a little difficult to convert to NASM and would than be code of
(I guess) 600 lines.
And yes, you have to convert the value to ascii-numbers you can read.
In assembler you have all to do on your own in assembler there are no were lying
headers and libraries around like with c++.

Now here a procedure to convert the value of eax
from my homepage: beckware.de - INFO - software - NASM - examples
or direct: http://people.freenet.de/beckware/na...xtostdout.html
there are also other useful things.
Have also a look to the NASM forum:
http://sourceforge.net/forum/?group_id=6208

Code:

;
-------------------------------------------------
;Writes the value of eax to the standard output as decimal.
;-------------------------------------------------

stdout:
section .data

.buf		db	'0000000000',10,0

section .text
	pushad
	
	mov	ebx,.buf+9
	mov	ecx,10
	mov	edi,10
.a:
	mov	edx,0
	div	edi
	add	edx,48
	mov	[ebx],dl
	dec	ebx
	loop	.a

	mov	eax,4			;schreibe
	mov	ebx,1			;auf Terminal 
	mov	ecx,.buf
	mov	edx,11		;11 Bytes
	int	$80

	popad
	ret

What happens here:
With div edi, eax is divided by 10 and edx holds the rest. Ascii ´0´ is decimal 48 and that is added to edx and stored into .buf back-wards. Now .buf is written to STDOUT that´s all.

infinity42 · 11-15-2005, 01:02 PM

Ah thank you. That works great, thanks for the explaination to, I understand how it is done now! I did try to convert Wim Sturkenboom's code to NASM, but I couldn't quite do it. The DIV operator confused me somewhat, but after reading the manual carefully and looking at your code/explaination i think i understand it now.

Thanks a lot.

ntj · 05-26-2018, 01:22 PM

Bumping there for telling you this solution still working perfectly.
Is clean and easy to understand.

Thank you.