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Is this a C51 / uvision bug?

Hi all
Below is the result of some debugging. I have isolated some code from a bigger project and have put it into a stand-alone project.

I basically don't understand why I can do right bit-shifting and AND'ing on single line, when I can't do left bit-shifting and AND'ing on single line.

If it isn't a bug, then what have I missed?

I have included many comments to describe my problem

#include <ADUC832.H>
#include <string.h>

void main(void) {


char ascii[] = "MC";
unsigned char pdu[3];
int w=0, r=0, len;
char ch1, ch2, rr, rl;


 /*  This is what I want to do:

  while-loop run 1:
     1: Assign to var 'ch1':   ch1 = 'M' (= 0x4D = 0100 1101)
     2: Assign to var 'ch2':   ch2 = 'C' (= 0x43 = 0100 0011)
     3: Assign to var 'w'  :     w = 0
     4: OR together the following:
         ((ch1 >>(w%7))&0x7F) | ((ch2 <<(7-(w%7)))&0xFF);
     <=>     0100 1101        |       1000 0000
     <=>                  1100 1101
     <=>                     0xCD

  while-loop run 2:
     1: Assign to var 'ch1':   ch1 = 'C' (= 0x43 = 0100 0011)
     2: Assign to var 'ch2':   ch2 = 0x00
     3: Assign to var 'w'  :     w = 1
     4: OR together the following:
         ((ch1 >>(w%7))&0x7F) | ((ch2 <<(7-(w%7)))&0xFF);
     <=>     0010 0001        |       0000 0000
     <=>                  0010 0001
     <=>                     0x21

 */

len=strlen(ascii);

while (r<len) {

 // ------ First OR-part  -----------------------
 // -------Both versions below are OK  ----------

      // -- VER 1: OK
      //  ch1 = ascii[r];
      //  rr  = (w%7);
      //  ch1 = (ch1 >> rr) & 0x7F;

      // -- VER 2: OK
        ch1 = (ascii[r] >> (w%7)) & 0x7F;    // Bit-shifting and AND'ing
                                             // may be done in one line

 // ------  Second OR-part  -----------------------------
 //-------  Both versions below are NOT OK ??  ----------

      // -- VER 1: OK
        ch2 = ascii[r+1];
        rl = (7-(w%7));
        ch2 = (ch2 << rl) & ((char)0xFF);    // Bit shift and AND'ing can be
                                             // done in one line, IF type cast
                                             // is used - why?
      //  ch2 = ch2 & 0xFF;                  // If splitting into new line
                                             // type cast is not required?

      // -- VER 2: NOT OK
      //  ch2 = (ascii[r+1] << (7-(w%7))) & 0xFF;  // type cast doesn't help
      //  ch2 = ch2 & 0xFF;  // AND'ing must be on seperate line ?


    //----------------------------------------------------------------
    // IS THIS A BUG ??
    //----------------------------------------------------------------
    // Why can we bit-shift and do the AND'ing in a single line
    // for the first OR-part above, but cannot do it for the second
    // OR-part where bit-shifting and AND'ing must be on two seperate
    // lines ???
    //----------------------------------------------------------------

// ------ Do the actual OR'ing -------
        pdu[w]= (ch1 | ch2) ;

        if ((w%7)==6)
           r++;
        r++;
        w++;
    }
    pdu[w]=0; // terminator

    //----------------------------------------------------------------
    // Run to here in debugger and look at content of
    // local variable 'pdu'.
    // When using 'NOT OK' versions from above
    // pdu will contain {0x4D, 0x21, 0x00}
    // and not {0xCD, 0x21, 0x00} as the 'OK' versions
    // produce.
    //----------------------------------------------------------------

   while(1);

}

Parents
  • "Do you think you might be doing more harm than good?"

    There is some value to this discussion/argument and those we've read here in the past -- interview test cases. Some employment candidates have many years of experience and claim to know C. We have taken the basis of many of these arguments and test candidates on the root issue, looking for "Cignorance" and "Cidiocy" (to expand on Erik's vernacular). Since we develop deeply embedded systems, we don't test much related to the C libraries, but do test for core language competency.

    That said, if a veteran has some weaknesses in C, but is good in other areas, as long as they have not isolated themselves from education and still have an eagerness to (re)learn and improve themselves, we'll take 'em. Test for knowledge and attitude.

Reply
  • "Do you think you might be doing more harm than good?"

    There is some value to this discussion/argument and those we've read here in the past -- interview test cases. Some employment candidates have many years of experience and claim to know C. We have taken the basis of many of these arguments and test candidates on the root issue, looking for "Cignorance" and "Cidiocy" (to expand on Erik's vernacular). Since we develop deeply embedded systems, we don't test much related to the C libraries, but do test for core language competency.

    That said, if a veteran has some weaknesses in C, but is good in other areas, as long as they have not isolated themselves from education and still have an eagerness to (re)learn and improve themselves, we'll take 'em. Test for knowledge and attitude.

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