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working with right shift operators in C

ruchi kataria

I am facing a problem in using the right shift operator in C with keil.
The instruction is:for(i=0;i<=8;i++) { j=sum>>i; }
where sum= 32 bit no.
the instruction is not executed if the syntax is as shown above. but if i write a numerical value instead of using the variable name 'sum' then the instruction is easily executed. i need to use the variable name only. how do i fix this problem?

Parents
  • 0 in reply to barry gordon

    My interpretation of the original post is that he wants to do something to each of the least significant 8 bits one at a time.

    for (i = 0; i < 8; ++i)
    {
    bit = (sum >> i) & 1;
    DoSomething(bit); // called for bit 0, bit 1... bit 7
    }

    This should work, though it's a little inefficient because it shifts too much. It also shifts a U32 unnecessarily, which will take time on an 8051. I'd probably write code more like:

    U8 temp = (U8)sum;
for (i = 0; i < 8; ++i)
    {
    DoSomething (temp & 1);
    temp >>= 1;
    }

    so that you only have to shift an 8-bit quantity, and you only do 8 1-bit shifts.

    So, if my interpretation of the OP is correct, the question is why (sum >> i) doesn't work in the posted code, though (sum >> 1) does.

    How about posting the assembly code output from the .cod file in both cases?

Reply
  • 0 in reply to barry gordon

    My interpretation of the original post is that he wants to do something to each of the least significant 8 bits one at a time.

    for (i = 0; i < 8; ++i)
    {
    bit = (sum >> i) & 1;
    DoSomething(bit); // called for bit 0, bit 1... bit 7
    }

    This should work, though it's a little inefficient because it shifts too much. It also shifts a U32 unnecessarily, which will take time on an 8051. I'd probably write code more like:

    U8 temp = (U8)sum;
for (i = 0; i < 8; ++i)
    {
    DoSomething (temp & 1);
    temp >>= 1;
    }

    so that you only have to shift an 8-bit quantity, and you only do 8 1-bit shifts.

    So, if my interpretation of the OP is correct, the question is why (sum >> i) doesn't work in the posted code, though (sum >> 1) does.

    How about posting the assembly code output from the .cod file in both cases?

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