Gets and sets the floating-point control word.
#include <float.h>
unsigned int _control87( unsigned int new, unsigned int mask );
| new | New control-word bit values | |
| mask | Mask for new control-word bits to set |
The _control87 function gets and sets the floating-point control word. The floating-point control word allows the program to change the precision, rounding, and infinity modes in the floating-point-math package. Floating-point exceptions can also be masked or unmasked using the _control87 function.
If the value for mask is equal to 0, then _control87 gets the floating-point control word. If mask is nonzero, then a new value for the control word is set in the following manner: for any bit that is on (equal to 1) in mask, the corresponding bit in new is used to update the control word. To put it another way,
fpcntrl = ((fpcntrl & ~mask) | (new & mask))
where fpcntrl is the floating-point control word.
The possible values for the mask constant (mask) and new control values (new) are shown in Table R.1.
Table R.1 Hex Values
| Mask | Hex Value | Constant | Hex Value |
| MCW_EM (Interrupt exception) | 0x003F | |||||
| _EM_INVALID | 0x0001 | |||||
| _EM_DENORMAL | 0x0002 | |||||
| _EM_ZERODIVIDE | 0x0004 | |||||
| _EM_OVERFLOW | 0x0008 | |||||
| _EM_UNDERFLOW | 0x0010 | |||||
| _EM_INEXACT | 0x0020 | |||||
| , | ||||||
Table R.1 Hex Values (continued)
| Mask | Hex Value | Constant | Hex Value |
| MCW_IC (Infinity control) | 0x1000 | , | ||||
| _IC_AFFINE | 0x1000 | |||||
| _IC_PROJECTIVE | 0x0000 | |||||
| , | ||||||
| MCW_RC (Rounding control) | 0x0C00 | , | ||||
| RC_CHOP | 0x0C00 | |||||
| RC_UP | 0x0800 | |||||
| _RC_DOWN | 0x0400 | |||||
| _RC_NEAR | 0x0000 | |||||
| , | ||||||
| MCW_PC (Precision control) | 0x0300 | , | ||||
| _PC_24 (24 bits) | 0x0000 | |||||
| _PC_53 (53 bits) | 0x0200 | |||||
| _PC_64 (64 bits) | 0x0300 | |||||
The bits in the value returned indicate the floating-point control state. See the FLOAT.H include file for a complete definition of the bits returned by _control87.
Standards:None
16-Bit:DOS, QWIN, WIN, WIN DLL
32-Bit:DOS32X
/* CNTRL87.C: This program uses _control87 to output the control word,
* set the precision to 24 bits, and reset the status to the default.
*/
#include <stdio.h>
#include <float.h>
void main( void )
{
double a = 0.1;
/* Show original control word and do calculation. */
printf( "Original: 0x%.4x\n", _control87( 0, 0 ) );
printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );
/* Set precision to 24 bits and recalculate. */
printf( "24-bit: 0x%.4x\n", _control87( _PC_24, MCW_PC ) );
printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );
/* Restore to default and recalculate. */
printf( "Default: 0x%.4x\n", _control87( CW_DEFAULT, 0xffff ) );
printf( "%1.1f * %1.1f = %.15e\n", a, a, a * a );
}
Original: 0x1332
0.1 * 0.1 = 1.000000000000000e-002
24-bit: 0x1332
0.1 * 0.1 = 9.999999776482582e-003
Default: 0x1032
0.1 * 0.1 = 1.000000000000000e-002