RVCT supports an interface to the status word in the floating-point
environment. This is called __ieee_status and
it is generally the most efficient function to use for modifying
the status word for VFP. __ieee_status is defined
in fenv.h.
__ieee_status has the prototype:
unsigned int __ieee_status(unsigned int mask, unsigned int flags);
The function modifies the writable parts of the status word according to the parameters, and returns the previous value of the whole word.
The writable bits are modified by setting them to:
new = (old & ~mask) ^ flags;
Four different operations can be performed on each bit of the status word, depending on the corresponding bits in mask and flags (see Table 4.6).
Table 4.6. Status word bit modification
| Bit of mask | Bit of flags | Effect |
|---|---|---|
| 0 | 0 | Leave alone |
| 0 | 1 | Toggle |
| 1 | 0 | Set to 0 |
| 1 | 1 | Set to 1 |
The layout of the status word as seen by __ieee_status is
shown in Figure 4.1.
The fields in Figure 4.1 are as follows:
-
Bits 0 to 4 (values
0x1to0x10, respectively) are the sticky flags, or cumulative flags, for each exception. The sticky flag for an exception is set to 1 whenever that exception happens and is not trapped. Sticky flags are never cleared by the system, only by the user. The mapping of exceptions to bits is:bit 0 (
0x01) is for the Invalid Operation exceptionbit 1 (
0x02) is for the Divide by Zero exceptionbit 2 (
0x04) is for the Overflow exceptionbit 3 (
0x08) is for the Underflow exceptionbit 4 (
0x10) is for the Inexact Result exception.
Bits 8 to 12 (values
0x100to0x1000) are the exception masks. These control whether each exception is trapped or not. If a bit is set to 1, the corresponding exception is trapped. If a bit is set to 0, the corresponding exception sets its sticky flag and returns a plausible result, as described in Exceptions.Bits 16 to 18, and bits 20 and 21, are used by VFP hardware to control the VFP vector capability. The
__ieee_statuscall does not let you modify these bits.-
Bits 22 and 23 control the rounding mode (Table 4.7).
Table 4.7. Rounding mode control
Bits Rounding mode 00Round to nearest 01Round up 10Round down 11Round toward zero Note
The standard fplib libraries
f*support only the Round to nearest rounding mode. If you require support for the other rounding modes, you must use the full IEEEg*libraries. See Library naming conventions. -
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.
Note
The standard fplib libraries do not support the FZ mode. Instead, each library either always flushes to zero or never flushes to zero, and you choose the library to use at build time. Only the VFP hardware honors the FZ mode if bit 24 is set.
However, this means that functions that are not provided in the hardware (and so are supplied in software fplib) do not support the FZ mode even when compiling for hard VFP. As a result, the behavior of fplib is not consistent across all functions when you change the FZ mode dynamically.
Bit 27 indicates that saturation has occurred in an advanced SIMD saturating integer operation. This is accessible through the
__ieee_statuscall.Bits marked R are reserved. They cannot be written to by the
__ieee_statuscall, and you must ignore anything you find in them.
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);
To untrap the Inexact Result exception:
__ieee_status(FE_IEEE_MASK_INEXACT, 0);
To clear the Underflow sticky flag:
__ieee_status(FE_IEEE_UNDERFLOW, 0);