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Floating Addition - Loosing data

Philip Garman

Hello,

I'm making use of floating math in my application and have run into a little problem with floating addition. It seems that the floating addition operation looses data at some point. I setup the following test program:

#include <intrins.h>

void main(void)
  {
  float t, v;
  unsigned int i;

  while(1)
    {
    // this block uses multiplication
    for(i=1; i<1000; i++)
      {
      t = 5964.1 * i;
      v = t/i;

      if(v != 5964.1)
        _nop_();
      }

    t = 0;

    // this block will continuously add  5964.1
    for(i=1; i<1000; i++)
      {
      t += 5964.1;
      v = t/i;

      if(v != 5964.1)
        _nop_();
      }
    }
  }

My project is setup using the P89C668 with all default options EXCEPT "bits to round for float compare" is set to 1. I tried all levels of optimization and the result was the same in all cases.

I then set a breakpoint on each nop. When I simulate the program and hit run, a break occurs on the second nop. This would indicate that the addition opperation lost some data.

Whats happening here? Why would the addition opperation not properly calculate the result while the multiplication does?

Thanks
Philip

Parents
  • 0 in reply to Andy Neil

    Could you use fixed-point instead?

    That's what I ALWAYS do in systems where accuracy is important. The following is probably obvious but I'll point it out anyway.

    An int can hold any of the following value ranges EASILY:

     -32768 to 32767
-3276.8 to 3276.7  (*10)
-327.68 to 327.67  (*100)
-32.768 to 32.767  (*1000)
-3.2768 to 3.2767  (*10000)

    This can be applied to char and long types as well.

    Jon

Reply
  • 0 in reply to Andy Neil

    Could you use fixed-point instead?

    That's what I ALWAYS do in systems where accuracy is important. The following is probably obvious but I'll point it out anyway.

    An int can hold any of the following value ranges EASILY:

     -32768 to 32767
-3276.8 to 3276.7  (*10)
-327.68 to 327.67  (*100)
-32.768 to 32.767  (*1000)
-3.2768 to 3.2767  (*10000)

    This can be applied to char and long types as well.

    Jon

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