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Why this shift doesn't run?

emme emme

Hi,

I've the following question about this code fragment:

...
char  BuffRX
xdata UCHAR BuffRX[10];
idata unsigned short ValCRCcomp;
idata unsigned short ValCRCappo;
idata unsigned short ValCRCappo2;

BuffRX[1] = 0x11;
BufRX[2] = 0x2A;
...
ValCRCappo = (unsigned short) (BuffRX[1] << 8);
ValCRCappo2 = (unsigned short) (BuffRX[2] | 0x0000);
ValCRCcomp = (unsigned short)  (ValCRCappo | ValCRCappo2);
...

Even the values in BuffRX[1] and BuffRX[2] are differents from zero, ValCRCcomp is always zero!

Any hints?

Thanks

  • 0

    char BuffRX
    xdata UCHAR BuffRX[10];

    two vatiables with the same name, read the error message and act on it

    Erik

  • 0 

    char  BuffRX
xdata UCHAR BuffRX[10];
    I'm surprised you don't get a duplicate symbols error (or something to that effect).
    ValCRCappo = (unsigned short) (BuffRX[1] << 8);
    The result is undefined if the shift count is greater than or equal to the width (in bits) of the value shifted, so that should be:
    ValCRCappo = (unsigned short)BuffRX[1] << 8;
    ValCRCappo2 = (unsigned short) (BuffRX[2] | 0x0000);
    BuffRX[2] is never assigned to, so is probably zero.

    "ValCRCcomp is always zero!"

    Given what we've seen above, I can believe it.

  • 0

    Hi,

    sorry guys, the code posted previously contains some error, this is the right version:

    xdata UCHAR BuffRX[10];
idata unsigned short ValCRCcomp;
idata unsigned short ValCRCappo;
idata unsigned short ValCRCappo2;

BuffRX[1] = 0x11;
BuffRX[2] = 0x2A;
...
ValCRCappo = (unsigned short) (BuffRX[1] << 8);
ValCRCappo2 = (unsigned short) (BuffRX[2] | 0x0000);
ValCRCcomp = (unsigned short)  (ValCRCappo | ValCRCappo2);

  • 0 in reply to emme emme

    sorry guys, the code posted previously contains some error, this is the right version:

    That's OK, we are having sooo much fun trying to answer based on faulty posts.

    Erik

  • 0 in reply to emme emme 

    ValCRCappo = (unsigned short) (BuffRX[1] << 8);
    Fix this as I said in my previous post above; otherwise, all bets are off.
    ValCRCappo2 = (unsigned short) (BuffRX[2] | 0x0000);
    ValCRCappo2 should hold the value 0x002A at this point.
    ValCRCcomp = (unsigned short)  (ValCRCappo | ValCRCappo2);
    Because of the undefined shift result, ValCRCcomp could have any value, but I simply can't imagine it being zero despite the bad shift.