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using jb of a unsigned char in c

heyvaert vincent

hello,
I would like to do a bit test of a unsigned char variable. It's like a jb in assembler.

here is my code in c.


unsigned char displ;
unsigned char display3 = 0xA8;
unsigned char i;
sbit display_data = P1^0;

displ = display3;
for (i=0; i<7; i++) {
        if (displ^0 == 1){
        display_data = 0;
        }
        else {
        display_data = 1;
        }
        displ>>=1;
}


I don't have any warning or error, but the result of test is wrong.
is it possible to do in c?

tkx.

Parents
  • 0

    I don't have any warning or error, but the result of test is wrong.

    As Andy said - the ^ operator has special meaning only in sbit declarations. In all other cases, it is an XOR operator.

    is it possible to do in c?

    Yes, as long as the unsigned char is located in bdata memory and the appropriate bit within the char is declared with a sbit declaration. Refer to the compiler documentation, especially the chapter "Bit-Addressable Objects".

Reply
  • 0

    I don't have any warning or error, but the result of test is wrong.

    As Andy said - the ^ operator has special meaning only in sbit declarations. In all other cases, it is an XOR operator.

    is it possible to do in c?

    Yes, as long as the unsigned char is located in bdata memory and the appropriate bit within the char is declared with a sbit declaration. Refer to the compiler documentation, especially the chapter "Bit-Addressable Objects".

Children
  • 0 in reply to Christoph Franck

    The standard, portable way to test bits in C has been presented -- the bitwise and (&).

    On an 8051, you can also test bits directly if the bit happens to be in an SFR or bit-addressable RAM.

    if (TI) ...

    It's generally not worth moving a bit to bit-addressable RAM just to access its bits. If a variable lives there, fine; if you just have some byte, use &.

    On the all-important braces question:

    - I agree that consistency is more important than any of the styles

    - Lef to my own devices, I go for seperate lines and indented braces:

        if (TI)
        {
        }
    else
        {
        }

    - My favorite style is not the Pascal/C "block statement" syntax, but rather the Algol/Modula/Ada style where the grammar allows multiple statements in control structures:

       if (TI)
   else
       for ()
       end
   end

    The begin/end {/} delimiters are grammatically unnecessary and just visual clutter. But there's nothing to be done about the C grammar at this point. Unfortunately, C is popular enough that nearly all the minor languages feel obliged to be "C-like" so that people will use them.

  • 0 in reply to Drew Davis

    "On an 8051, you can also test bits directly if the bit happens to be in an SFR or bit-addressable RAM."

    To be precise, that should be "...in a bit-addressable SFR or bit-addressable RAM."

    Not all SFRs are bit-addressable.

  • 0 in reply to Andy Neil

    Thanks for the correction. Not all SFRs are bit addressable.