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Problem with Shift-Operator

Gunnar Bohlen

Hi,

I have a Problem with the Shift-Operator >> within my C-Code.
I'm using PK166 V4.2
The following code should set the Variable n to 0.

void main()
{
 unsigned char n;
 unsigned char p;  // dummy

 n=255;
 n=n>>22;

 p=0;		// dummy

}

With Simulator and real hardware n=3!

The Compiler generates a shift of 22, which is invalid. The Assembler Opcode only evaluates the lower 4 bits of the shift-operand.

Generated Assembler Code:
R4 contains the variable n:

 MOV R5, #0x16 // incorrect Opcode???
 SHR R4,R5

The lower 4 bits of 0x16=0x06, and that is what really happens. ( 255 >> 6 = 3 )

I get the same result with Compiler V5.04.

Is it invalid to shift a byte-variable more than 8 times?
If so, why does the compiler not generate a warning?

Parents
  • 0 in reply to erik malund

    "The Standard specifies that when shifting right an object having a signed type with a negative value, the resulting value is implementation-defined.
    It would be nice if that was CLEAR."

    Seems perfectly clear to to me. The standard goes to great lengths to write unambiguous English.

    "if I shift something with a value of x that means to me that I am shifting x places."

    Ludicrous:

    'Shift y right with x bits'

    'Shift y right by x bits'.

Reply
  • 0 in reply to erik malund

    "The Standard specifies that when shifting right an object having a signed type with a negative value, the resulting value is implementation-defined.
    It would be nice if that was CLEAR."

    Seems perfectly clear to to me. The standard goes to great lengths to write unambiguous English.

    "if I shift something with a value of x that means to me that I am shifting x places."

    Ludicrous:

    'Shift y right with x bits'

    'Shift y right by x bits'.

Children