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glibc  2.9
fesetround.c
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00001 /* Set current rounding direction.
00002    Copyright (C) 2001, 2005 Free Software Foundation, Inc.
00003    This file is part of the GNU C Library.
00004 
00005    The GNU C Library is free software; you can redistribute it and/or
00006    modify it under the terms of the GNU Lesser General Public
00007    License as published by the Free Software Foundation; either
00008    version 2.1 of the License, or (at your option) any later version.
00009 
00010    The GNU C Library is distributed in the hope that it will be useful,
00011    but WITHOUT ANY WARRANTY; without even the implied warranty of
00012    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00013    Lesser General Public License for more details.
00014 
00015    You should have received a copy of the GNU Lesser General Public
00016    License along with the GNU C Library; if not, write to the Free
00017    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
00018    02111-1307 USA.  */
00019 
00020 #include <fenv.h>
00021 
00022 int
00023 fesetround (int round)
00024 {
00025   unsigned short int cw;
00026   int mxcsr;
00027 
00028   if ((round & ~0xc00) != 0)
00029     /* ROUND is no valid rounding mode.  */
00030     return 1;
00031 
00032   /* First set the x87 FPU.  */
00033   asm ("fnstcw %0" : "=m" (*&cw));
00034   cw &= ~0xc00;
00035   cw |= round;
00036   asm ("fldcw %0" : : "m" (*&cw));
00037 
00038   /* And now the MSCSR register for SSE, the precision is at different bit
00039      positions in the different units, we need to shift it 3 bits.  */
00040   asm ("stmxcsr %0" : "=m" (*&mxcsr));
00041   mxcsr &= ~ 0x6000;
00042   mxcsr |= round << 3;
00043   asm ("ldmxcsr %0" : : "m" (*&mxcsr));
00044 
00045   return 0;
00046 }
00047 libm_hidden_def (fesetround)