From: alaric AT abwillms DOT demon DOT co DOT uk (Alaric B. Williams)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: DJGPP stack when calling a function? Please help.
Date: Sun, 13 Apr 1997 19:13:03 GMT
Message-ID: <3350d7b0.6866403@news.demon.co.uk>
References: <19970413111500 DOT HAA07345 AT ladder01 DOT news DOT aol DOT com>
NNTP-Posting-Host: abwillms.demon.co.uk
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To: djgpp AT delorie DOT com
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On 13 Apr 1997 11:15:10 GMT, nitehawk91 AT aol DOT com wrote:

>Greetings !

Hello!

>parameters;
>return address; 4 bytes
>stack pointer ; 4 bytes

>I have a function that looks like this : func(short, short, short)
>Now the stack would look like :

>short 3; 2 byte
>short 2; 2 byte
>short 1; 2 byte
>return adress ; 4 byte
>stack pointer ; 4 bytes

This is providing you have stored EBP. 

>Now I tied to access the three parameters as following :
>
>mov eax, [esp+8] ; short 1 to eax
>mov ebx, [esp+10] ; short 2 to ebx
>mov ecx, [esp+12] ; short 3 to ecx

Ah, right. Those are short ints, but you're reading long ints from
them, as 32 bit registers. Try:

_my_func: 
   push ebp
   mov ebp,esp

   mov ax,[ebp+8]
   mov bx,[ebp+10]
   mov cx,[ebp+12]

   pop ebp
   ret

Note that I keep ebp as my frame pointer. This means that I can PUSH
and POP temporary values to my heart's content, and still keep track
of my stack frame amongst all that.

Also note that I am using the 16 bit registers ax, bx, and cx to store
the 16 bit values found in short ints.

>This compiles and the programm works fine, but I get idiotic results.
>(BTW : The assembler function doesn't return a value)

If it did, stick it in eax. Eg:

; extern "C" int add(int a,int b)

_add:
   push ebp
   mov ebp,esp

   mov eax,[ebp+8]
   add eax,[ebp+10]

   pop ebp
   ret

For little functions, feel free to stop storing ebp and all that,
instead using offsets on esp but minus 4 since we're not storing
ebp there any more.

>2. Pointer access problem :
>assume I have char string[] = {'h','e','l','l','o'}; 
>In C I could access the 'e' for example with string[1] or *(string+1).
>How can I make this in Nasm ?
>I have declared [EXTERN _string]
>Tried to access the 'e' with mov eax,[_string+1]
>This doesn't work also. Why ?

Similarly, you're using eax (32 bits) to access a char (8 bits). Try:

mov al,[_string+1]

Also make sure that _string really is the array - not a char* pointing
to it, as declared by:

char *string = "hello";

>Please help me.

"But now those days are gone, I'm not so self assured..."

>Thank's

NEtime :-)

>Matthias
><NiteHawk91 AT aol DOT com>

ABW
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Alaric B. Williams Internet : alaric AT abwillms DOT demon DOT co DOT uk
<A HREF="http://www.abwillms.demon.co.uk/">Hello :-)</A>