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FPU Programmer’s Model
ARM DDI 0363E Copyright © 2009 ARM Limited. All rights reserved. 12-12
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• In default NaN mode, arithmetic
CDP
instructions involving NaN operands return the
default NaN regardless of the fractions of any NaN operands. SNaNs in an arithmetic
CDP
operation set the IOC flag, FPSCR[0]. NaN handling by data transfer and non-arithmetic
CDP
instructions is the same as in full-compliance mode.
Table 12-9 summarizes the effects of NaN operands on instruction execution.
Comparisons
Comparison results modify the flags in the FPSCR Register. You can use the
VMOV r15, FPSCR
instruction (formerly
FMSTAT
) to transfer the current flags from the FPSCR Register to the CPSR
Register. See the ARM Architecture Reference Manual for mapping of IEEE 754 standard
predicates to ARM conditions. The flags used are chosen so that subsequent conditional
execution of ARM instructions can test the predicates defined in the IEEE 754 standard.
Underflow
The Cortex-R4F FPU uses the before rounding form of tininess and the inexact result form of
loss of accuracy as described in the IEEE 754 standard to generate Underflow exceptions.
In flush-to-zero mode, results that are tiny before rounding, as described in the IEEE 754
standard, are flushed to a zero, and the UFC flag, FPSCR[3], is set. See the ARM Architecture
Reference Manual for information on flush-to-zero mode.
When the FPU is not in flush-to-zero mode, operations are performed on subnormal operands.
If the operation does not produce a tiny result, it returns the computed result, and the UFC flag,
FPSCR[3], is not set. The IXC flag, FPSCR[4], is set if the operation is inexact. If the operation
produces a tiny result, the result is a subnormal or zero value, and the UFC flag, FPSCR[3], is
set if the result was also inexact.
Table 12-9 QNaN and SNaN handling
Instruction
type
Default
NaN mode
With QNaN operand With SNaN operand
Arithmetic
CDP
Off The QNaN or one of the QNaN operands, if
there is more than one, is returned
according to the rules given in the ARM
Architecture Reference Manual.
IOC
a
set. The SNaN is quieted and the
result NaN is determined by the rules
given in the ARM Architecture
Reference Manual.
On Default NaN returns.
IOC
a
set. Default NaN returns.
Non-arithmetic
CDP
Off
NaN passes to destination with sign changed as appropriate.
On
FCMP(Z)
- Unordered compare. IOC set. Unordered compare.
FCMPE(Z)
- IOC set. Unordered compare. IOC set. Unordered compare.
Load/store
Off
All NaNs transferred.
On
a. IOC is the Invalid Operation exception flag, FPSCR[0].