20 replies to this topic

[dargueta]

Oh. Change 

A: db 10
B: db 2

to

A: dd 10
B: dd 2

and that should fix it. I'll explain why if it does.

[zika]

hhhhhhhhhhh loooooool it's works man thaaaaankx a looooot 

but why ?? db dosn't work and dd works ?

Edited by zika, 09 April 2015 - 10:44 AM.

[dargueta]

So here's the thing: db reserves a single byte in memory and assigns a value to it. dw reserves a word (two bytes, like ax), and dd reserves a doubleword (four bytes, like eax).

 

In C and other languages, doing something like this:

char a = 5;
int b = a;

would result in a == b. The processor (and therefore assembly language) has no concept of data types; all memory is the same. A processor can't look at a memory address and tell the difference between a 32-bit integer, a 32-bit pointer, a four-byte array, etc. It's just four bytes. Data types are a language feature.

 

The problem was that you were reserving a byte for A but then reading an entire dword. Thus, the processor loaded A plus the next three bytes, regardless of what they were, into eax instead of loading the first byte and zeroing out the upper 24 bits as you were expecting.

 

So eax and ebx were mostly filled with random values when you were comparing them, thus resulting in your problem.

Edited by dargueta, 09 April 2015 - 11:01 AM.

[zika]

emmmm ok thanks for clearfy ur answer.

so when I xor eax , eax the resault will be zero ok

but when I moved db to eax its take 1 byte and remain bytes it's feel by random  values that's what do u mean ?

[dargueta]

The rest is filled by whatever the next three bytes in memory happen to be. It might not be random. For example:

A: db 1
B: db 2
C: db 3
D: db 4

mov     eax, [A]

; EAX = 0x04030201

[zika]

thanx a lot Mr dargueta take this    u have no idea how much I smached me screen to do that

[dargueta]

Haha no problem. By the way, if you do need to do zero extension, you can use movzx, or movsx for signed numbers.

 

Take these examples:

unsigned char A = 5;
unsigned int i = A;

movzx   eax, BYTE [A]

signed char B = -5;
int i = B;

movsx   eax, BYTE [B]

You can also do the reverse:

al = -10;
signed short s = al;

movsx   WORD [s] = al

And between registers:

movzx   eax, bl

Edited by dargueta, 09 April 2015 - 08:54 PM.

[zika]

got it Mr dargueta thanx  a lot 

I'v changed dd to db and change mov eax , [A] to  mov al ,[A] and it's works . thanx I'll try to do it with movzx and movsx and see the output 

 

Haha no problem. By the way, if you do need to do zero extension, you can use movzx, or movsx for signed numbers.

 

Take these examples:

unsigned char A = 5;
unsigned int i = A;

movzx   eax, BYTE [A]

signed char B = -5;
int i = B;

movsx   eax, BYTE [B]

You can also do the reverse:

al = -10;
signed short s = al;

movsx   WORD [s] = al

And between registers:

movzx   eax, bl

yeap , I've changed my code and it's working like magic thanx , take this 

[dargueta]

You're welcome!