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Bit shifting via Visual C++ 5.0

huya
huya asked
on
Medium Priority
223 Views
Last Modified: 2010-04-02
Can anyone explain the reason behind the situation below :

the following code :

#include <stdio.h>

void main()
{
     unsigned int x,y;

     x = 0xAAAAAAAA;

    y = x >> 32;

   printf("%X",y)
}  



Produces:

      AAAAAAAA


Why is it that if I shift a 32 bit variable by 32 bits, I get the same number.  If I shift a number, say :

   y = 0xFFFFFFFF >> 32

then y=0.  ????

if I let x=FFFFFFFF and then do:

  y = X >> 31;
  y = y >> 1;

THEN I get y=0, but not if I do all the shifting at once.

Any help??????
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Commented:
The behavior of the shift operators with shift counts of or greater than the integral value's size is  undefined  (implementation defined).  In the x86 processor family (intel) this wiould be handled with the SAR instruction.  The behavior of the SAR instruction with counts that are of or larger than the operand size changed with the 286 model.  On 286's and later the SAR shifts by the count modulo 32.  Thus shifting by 33 is the same as shifting by 1.  However this is implimentation defined.  If you were to try this on a non-intel computer (or a different compiler) you might get different results.

Commented:
I've been looking though my manuals for the C++ rules in this area.  What I found so far is that the behavior of the shift operators is undefined if the shift is larger than the operand size, if the shift count is less than zero, or if the operand is less than zero.

I hope this helps.

Commented:
hey what happened to my post?  I said the same thing but my entry is totally gone.

Commented:
I answered this about an  hour ago

Commented:
Are you sure it actually went through?  Like did you see your answer in the page after you pressed submit?  If EE is too bussy you might get a message to that effect and your answer might not get recorded.

Commented:
Yeah i know it went thru cause I saw it after I pressed the submit, and then I got an email saying someone had added a comment.  It never told me that someone had posted an answer though.  I wrote a cool little program that shows this effect and then put the output and the same 'notice' you found as the answer.  Don't worry about it.  I figure it is probably a concurrent proccessing problem with EE.  I once got a free week at the Tropicana in Las Vegas for the same thing.  

Commented:
Lifes little things tend to balance out.  But a free week-long vacation and 50 pts?  It think you are coming out ahead.

Commented:
Yeah i know it went thru cause I saw it after I pressed the submit, and then I got an email saying someone had added a comment.  It never told me that someone had posted an answer though.  I wrote a cool little program that shows this effect and then put the output and the same 'notice' you found as the answer.  Don't worry about it.  I figure it is probably a concurrent proccessing problem with EE.  I once got a free week at the Tropicana in Las Vegas for the same thing.  

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Commented:
Thanks
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