Datasheet

Floating-point Support
ARM DUI 0067D Copyright © 1999-2001 ARM Limited. All rights reserved. 5-9
• Bits 22 and 23 control the rounding mode (Table 5-5).
• Bit 24 enables FZ (Flush to Zero) mode if it is set. In FZ mode, denormals are
forced to zero to speed up processing (because denormals can be difficult to work
with and slow down floating-point systems). Setting this bit reduces accuracy but
might increase speed.
• Bits marked R are reserved.
In addition to defining the
__ieee_status call itself, fenv.h also defines some constants
to be used for the arguments:
#define FE_IEEE_FLUSHZERO (0x01000000)
#define FE_IEEE_ROUND_TONEAREST (0x00000000)
#define FE_IEEE_ROUND_UPWARD (0x00400000)
#define FE_IEEE_ROUND_DOWNWARD (0x00800000)
#define FE_IEEE_ROUND_TOWARDZERO (0x00C00000)
#define FE_IEEE_ROUND_MASK (0x00C00000)
#define FE_IEEE_MASK_INVALID (0x00000100)
#define FE_IEEE_MASK_DIVBYZERO (0x00000200)
#define FE_IEEE_MASK_OVERFLOW (0x00000400)
#define FE_IEEE_MASK_UNDERFLOW (0x00000800)
#define FE_IEEE_MASK_INEXACT (0x00001000)
#define FE_IEEE_MASK_ALL_EXCEPT (0x00001F00)
#define FE_IEEE_INVALID (0x00000001)
#define FE_IEEE_DIVBYZERO (0x00000002)
#define FE_IEEE_OVERFLOW (0x00000004)
#define FE_IEEE_UNDERFLOW (0x00000008)
#define FE_IEEE_INEXACT (0x00000010)
#define FE_IEEE_ALL_EXCEPT (0x0000001F)
For example, to set the rounding mode to round down, you would do:
__ieee_status(FE_IEEE_ROUND_MASK, FE_IEEE_ROUND_DOWNWARD);
To trap the Invalid Operation exception and untrap all other exceptions:
__ieee_status(FE_IEEE_MASK_ALL_EXCEPT, FE_IEEE_MASK_INVALID);
Table 5-5 Rounding mode control
Bits Rounding mode
00 Round to nearest
01 Round up
10 Round down
11 Round toward zero