What's the easiest way to have the opposite of a number ?

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vanzan

Joined: 27 Apr 2010
Posts: 14

vanzan 29 Apr 2010, 14:50

Hi !

I need to know a simple method to remplace a variable for his opposite number...

For example:

ttest = 1
;TREATEMENT
ttest = -1

I've tried to use IMUL but i failed to do it works...

Are there other solutions ?

My code

Code:

      MOV [tttest],10
         MOV AX,(-1)
         IMUL [tttest]   
                         
          CMP [tttest],(-10)
          JE cici   
          

         invoke WriteConsole, [stdout], "don't" , 5, NULL, NULL   
           
         cici: 
         invoke WriteConsole, [stdout], "work" , 4, NULL, NULL

29 Apr 2010, 14:50

a115433

Joined: 05 Mar 2010
Posts: 144

a115433 29 Apr 2010, 15:27

neg?
it will substract it from 0.

29 Apr 2010, 15:27

vanzan

Joined: 27 Apr 2010
Posts: 14

vanzan 29 Apr 2010, 15:28

Oh ! I've forget that the result was stock into BX --"

Correst code:

Code:

         MOV [tttest],10
         MOV AX,-1
         IMUL [tttest]   
         MOV  [tttest],BX
         
                        
          CMP [tttest],(-10)
          JE cici   
          

         invoke WriteConsole, [stdout], "don't" , 5, NULL, NULL   
           
         cici: 
         invoke WriteConsole, [stdout], "work" , 4, NULL, NULL

29 Apr 2010, 15:28

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 29 Apr 2010, 15:44

1) neg [ttest]
2A) not [ttest] \ inc [ttest]
2B) xor [ttest],-1 \ inc [ttest]
Actually mul puts the result into dx:ax or edx:eax or rdx:rax, but NOT in the BX
EDIT: my mistake removed

imul bx,[ttest],-1

imul bx,-1

etc.

but why would you use multiplication when its slow and hard to do? NEG-instructions is exactly designed for this!

Last edited by Madis731 on 29 Apr 2010, 16:18; edited 1 time in total

29 Apr 2010, 15:44

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 29 Apr 2010, 15:56

Madis731 wrote:

imul never touches more than the registers you give it.

Are you sure? imul r/m uses the same pairs as mul (mul doesn't have two- or three-operand encodings, so it uses them always).

29 Apr 2010, 15:56

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 29 Apr 2010, 16:04

Quote:

1) neg [ttest]
2) xor [ttest],-1

Perhaps I misunderstood your point but if that was supposed to show equivalence then this would be correct:
1) not [ttest]
2) xor [ttest], -1

29 Apr 2010, 16:04

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 29 Apr 2010, 16:19

I'm sorry - fixed - I got so excited about the multiplication usage that I wrote faster than I thought... Sad

sorry...

29 Apr 2010, 16:19

vanzan

Joined: 27 Apr 2010
Posts: 14

vanzan 29 Apr 2010, 18:30

Quote:

but why would you use multiplication when its slow and hard to do? NEG-instructions is exactly designed for this!

Because I didn't learnt this very useful "command" at my IT school ^^

Thanks, a lot ! It can't be more simple Wink

Edit: (I'm talking about "neg" operater)

29 Apr 2010, 18:30

edfed

Joined: 20 Feb 2006
Posts: 4333
Location: Now

edfed 29 Apr 2010, 19:52

in assembly, we don't say comand, but INSTRUCTION. say this to your teacher. Very Happy

another way:

Code:

xor eax,eax
sub eax,[ttest]

but franckly, none better than

Code:

neg [ttest]

29 Apr 2010, 19:52

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