
4-22
ColdFire CF4e Core User’s Manual
For More Information On This Product,
Go to: www.freescale.com
FPU Programmer’s Model
4.3.9 Input Not-A-Number (INAN)
The INAN exception is a mechanism for handling a user-defined, non-IEEE data type. If
either input operand is a NAN, FPSR[INAN] is set. By enabling this exception, the user can
override the default action taken for NAN operands. Because FMOVEM, FMOVE FPCR,
and FSAVE instructions do not modify status bits, they cannot generate exceptions.
Therefore, these instructions are useful for manipulating INANs. See Table 4-13.
4.3.10 Input Denormalized Number (IDE)
The input denorm bit, FPCR[IDE], provides software support for denormalized operands.
When the IDE exception is disabled, the operand is treated as zero, FPSR[INEX] is set, and
the operation proceeds. When the IDE exception is enabled and an operand is
denormalized, an IDE exception is taken but FPSR[INEX] is not set to allow the handler to
set it appropriately. See Table 4-14.
Note that the FPU never generates denormalized numbers. If necessary, software can create
them in the underflow exception handler.
Table 4-12. BSUN Exception Enabled/Disabled Results
Condition
BSUN
Description
Exception
disabled
0
The floating-point condition is evaluated as if it were the equivalent IEEE-aware conditional
predicate. No exceptions are taken.
Exception
Enabled
1
The processor takes a floating-point pre-instruction exception.
The BSUN exception is unique in that the exception is taken before the conditional predicate is
evaluated. If the user BSUN exception handler fails to update the PC to the instruction after the
excepting instruction when returning, the exception executes again. Any of the following actions
prevent taking the exception again:
Clearing FPSR[NAN]
Disabling FPCR[BSUN]
Incrementing the stored PC in the stack bypasses the conditional instruction. This applies to
situations where fall-through is desired. Note that to accurately calculate the PC increment
requires knowledge of the size of the bypassed conditional instruction.
Table 4-13. INAN Exception Enabled/Disabled Results
Condition
INAN
Description
Exception
disabled
0
If the destination data format is single- or double-precision, a NAN is generated with a mantissa
of all ones and a sign of zero transferred to the destination. If the destination data format is B, W,
or L, a constant of all ones is written to the destination.
Exception
enabled
1
The result written to the destination is the same as the exception disabled case unless the
exception occurs on a FMOVE OUT, in which case the destination is unaffected.
F
Freescale Semiconductor, Inc.
n
.