ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Application Level Programmers’ Model
The criteria for the Underflow exception to occur are different in Flush-to-zero mode. For
details, see Flush-to-zero on page A2-39.
IXC Inexact. The flag is set to 1 if the result of an operation is not equivalent to the value that
would be produced if the operation were performed with unbounded precision and exponent
range.
The criteria for the Inexact exception to occur are different in Flush-to-zero mode. For
details, see Flush-to-zero on page A2-39.
IDC Input Denormal. The flag is set to 1 if a denormalized input operand is replaced in the
computation by a zero, as described in Flush-to-zero on page A2-39.
With the Advanced SIMD extension and the VFPv3 extension these are non-trapping exceptions and the
data-processing instructions do not generate any trapped exceptions.
With the VFPv2 and VFPv3U extensions:
These exceptions can be trapped, by setting trap enable flags in the FPSCR, see VFPv3U on
page A2-31. Trapped floating-point exceptions are delivered to user code in an IMPLEMENTATION
DEFINED fashion.
The definitions of the floating-point exceptions change as follows:
— if the Underflow exception is trapped, it occurs if the absolute value of the result of an
operation, produced before rounding, is less than the minimum positive normalized number
for the destination precision, regardless of whether the rounded result is inexact
— higher priority trapped exceptions can prevent lower priority exceptions from occurring, as
described in Combinations of exceptions on page A2-44.
Table A2-7 shows the default results of the floating-point exceptions:
Table A2-7 Floating-point exception default results
Exception type Default result for positive sign Default result for negative sign
IOC, Invalid Operation Quiet NaN Quiet NaN
DZC, Division by Zero +∞ (plus infinity) –∞ (minus infinity)
OFC, Overflow RN, RP:
RM, RZ:
+∞ (plus infinity)
+MaxNorm
RN, RM:
RP, RZ:
UFC, Underflow Normal rounded result Normal rounded result
IXC, Inexact Normal rounded result Normal rounded result
IDC, Input Denormal Normal rounded result Normal rounded result
–∞ (minus infinity)
–MaxNorm
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. A2-43