help converting C to fasm

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magicSqr

Joined: 27 Aug 2011
Posts: 105

magicSqr 10 Nov 2011, 13:03

Hi,

I came across a bit of code in C (I think)

Code:

       op = x;
     res = 0;

        /* "one" starts at the highest power of four <= than the argument. */

      one = 1 << 30;    /* second-to-top bit set */
 while (one > op) one >>= 2;

    while (one != 0) {
             if (op >= res + one) {
                      op = op - (res + one);
                      res = res +  2 * one;
               }
              res /= 2;
           one /= 4;
         }

I have no idea what << and >> are Sad

Could someone translate this into fasm?

Many thanks

magic²

10 Nov 2011, 13:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20343
Location: In your JS exploiting you and your system

revolution 10 Nov 2011, 13:25

"one = 1 << 30" ---> "mov eax,1 shl 30"
"one >>= 2" ---> "shr eax,2"

10 Nov 2011, 13:25

goldenspider

Joined: 16 May 2011
Posts: 38

goldenspider 10 Nov 2011, 13:35

Code:

xor edx,edx             ;res = 0;
mov eax,dword 1 shl 30  ;one = 1 << 30
...
shr eax,2               ;one >>= 2
...
lea edx,[edx+2*eax]     ;res = res +  2 * one; 
...
shr edx,1               ;res /= 2
shr eax,2               ;one /= 4

10 Nov 2011, 13:35

Rock_maniak_forever_

Joined: 17 Feb 2010
Posts: 15
Location: Invisible country.

Rock_maniak_forever_ 10 Nov 2011, 15:25

magicSqr Compile and try to look in disassembler or debugger. I suppose it's could more be easy.

10 Nov 2011, 15:25

magicSqr

Joined: 27 Aug 2011
Posts: 105

magicSqr 10 Nov 2011, 17:28

Many thanks for the help everyone Wink

10 Nov 2011, 17:28

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