Freescale Semiconductor MCF5480 - Manual (Page 210)

MCF548x Reference Manual, Rev. 3
6-20 Freescale Semiconductor
6.6.1.2 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 6-13 .
6.6.1.3 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 6-14 .
Note that the FPU never generates denormalized numbers. If necessary, software can create them in the
underflow exception handler.
Table 6-12. 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 6-13. 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.
Table 6-14. IDE Exception Enabled/Disabled Results
Condition IDE Description
Exception
disabled
0 Any denormalized operand is treated as zero, FPSR[INEX] is set, and the operation
proceeds.
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.
FPSR[INEX] is not set to allow the handler to set it appropriately.