The ARM® Compiler toolchain supports an interface to the status word in the floating-point environment. Some older versions of the ARM libraries implemented a function called
__fp_status() to provide this interface.
NoteThe ARM Compiler toolchain does not support floating-point exception trapping for AArch64 targets.
__fp_status() is the same as
__ieee_status() but it uses an older style of status word layout.
The compiler still supports the
__fp_status() function for
__fp_status() is defined in
The function prototype for
NoteThis function requires a floating-point model that supports exceptions. In ARM Compiler 6 this is disabled by default, and can be enabled by the armclang command-line option
The layout of the status word as seen by
__fp_status() is as follows:
Figure 5-1 Floating-point status word layout
The fields in the status word are as follows:
Bits 0 to 4 (values
0x10, 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 exception
- Bit 1 (
0x02) is for the Divide by Zero exception.
- Bit 2 (
0x04) is for the Overflow exception.
- Bit 3 (
0x08) is for the Underflow exception.
- Bit 4 (
0x10) is for the Inexact Result exception.
- Bit 0 (
- Bits 8 to 12 (values
0x1000) control various aspects of the Floating-Point Architecture (FPA). The FPA is obsolete and the ARM compilation tools do not support it. Any attempt to write to these bits is ignored.
- Bits 16 to 20 (values
0x100000) 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.
- Bits 24 to 31 contain the system ID that cannot be changed. It is set to
0x40for software floating-point, to
0x80or above for hardware floating-point, and to
1if a hardware floating-point environment is being faked by an emulator.
- Bits marked
Rare reserved. They cannot be written to by the
__fp_status()call, and you must ignore anything you find in them.
The rounding mode cannot be changed with the
In addition to defining the
itself, stdlib.h also defines the following constants
to be used for the arguments:
#define __fpsr_IXE 0x100000 #define __fpsr_UFE 0x80000 #define __fpsr_OFE 0x40000 #define __fpsr_DZE 0x20000 #define __fpsr_IOE 0x10000 #define __fpsr_IXC 0x10 #define __fpsr_UFC 0x8 #define __fpsr_OFC 0x4 #define __fpsr_DZC 0x2 #define __fpsr_IOC 0x1
For example, to trap the Invalid Operation exception and untrap
all other exceptions, you would call
the following input parameters:
__fp_status(_fpsr_IXE | _fpsr_UFE | _fpsr_OFE | _fpsr_DZE | _fpsr_IOE, _fpsr_IOE);
To untrap the Inexact Result exception:
To clear the Underflow sticky flag: