• State Not Answered
  • Locked Locked
  • Replies 2 replies
  • Subscribers 20 subscribers
  • Views 2292 views
  • Users 0 members are here
Options
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Problem with Assignment Operator Modulo

Graham Cole

According to K&R:

x *= y + 1

is equivalent to:

x = x * ( y + 1 )

And other assignment operators including modulo should be handled similarly.

However, the following function does not work as I expected.

void test() small
{
    unsigned char	x,y;

    x = 7;
    y = 7;

    x++;
    x %= 256;

    y = ( y + 1 ) % 256;

}

When executed, y is 8 (as expected) but x is set to zero.

An inspection of the assembly listing shows why:

             ; FUNCTION test (BEGIN)
                                           ; SOURCE LINE # 1002
                                           ; SOURCE LINE # 1006
;---- Variable 'x' assigned to Register 'R7' ----
0000 7F07              MOV     R7,#07H
                                           ; SOURCE LINE # 1007
;---- Variable 'y' assigned to Register 'R6' ----
0002 7E07              MOV     R6,#07H
                                           ; SOURCE LINE # 1009
0004 0F                INC     R7
                                           ; SOURCE LINE # 1010
0005 EF                MOV     A,R7
0006 75F000            MOV     B,#00H
0009 84                DIV     AB
000A AFF0              MOV     R7,B
                                           ; SOURCE LINE # 1012
000C EE                MOV     A,R6
000D 04                INC     A
000E FD                MOV     R5,A
000F 0E                INC     R6
                                           ; SOURCE LINE # 1014
0010 22                RET     
             ; FUNCTION test (END)
There is a division by zero, and where did that move to R5 come from.

Is this a compiler bug? It has caused me problems because my application had a modulus that was #define'd - when I changed my buffer length strange things started to happen. I got around the problem by avoiding the assignment operator.

0