C51: _CHKFLOAT_ RETURNS STRANGE VALUES

Information in this article applies to:

• C51 Version 7.10 and Earlier

SYMPTOM

The _chkfloat_ library routine does not detect floating-point numbers with the -INF value. The following program demonstrates this issue:

#include <reg51.h>
#include <intrins.h>
#include <math.h>
#include <stdio.h> /* for printf */

void tst_chkfloat (float f1) {
unsigned x;

x = _chkfloat_(f1);

  switch (x) {
    case 0:
      printf ("result is a number\n"); break;
    case 1:
      printf ("result is zero\n"); break;
    case 2:
      printf ("result is +INF\n"); break;
    case 3:
      printf ("result is -INF\n"); break;
    case 4:
      printf ("result is NaN\n"); break;
    default:
      printf ("ChkFloat = %u (invalid return code)\n", x);
      break;
  }
}

void main (void)
{
float f2, f3;

SCON  = 0x50;
TMOD |= 0x20;
TH1   = 221;
TR1   = 1;
TI    = 1;

f2 = 9.9e30;
f3 = 8.8e30;
tst_chkfloat(f2*f3);

f2 = 9.9e30;
f3 = -8.8e30;
tst_chkfloat(f2*f3);

tst_chkfloat(sqrt(-1.1));

tst_chkfloat(0.0);

f2 = 9.9e30;
f3 = 8.8e30;
tst_chkfloat(f2+f3);

while(1);
}

CAUSE

This issue is caused by a problem in the _chkfloat_ library routine.

RESOLUTION

The _chkfloat_ routine is corrected in the C51 V7.20 (and later) update. You may download the latest updates from the Keil Website.

MORE INFORMATION

• Refer to _chkfloat_ in the Cx51 User's Guide.
• Refer to Floating-point Numbers in the Cx51 User's Guide.

SEE ALSO

• C51: SUPPORT FOR FLOATING-POINT NUMBERS

Last Reviewed: Thursday, October 20, 2005