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Floating-Point Unit
MOTOROLA
M68060 USER’S MANUAL
6-7
6.1.4 Floating-Point Instruction Address Register (FPIAR)
For the subset of the floating-point instructions that generate exception traps, the FPU loads
the 32-bit FPIAR with the logical address of the instruction before executing the instruction.
Because the integer unit can execute instructions while the FPU executes floating-point
instructions, the program counter (PC) value stacked by the MC68060 in response to a float-
ing-point exception handler may not point to the offending instruction. Therefore, a floating-
point exception handler uses the address in the FPIAR to locate a floating-point instruction
that has caused an exception. Since the FMOVE to/from the FPCR, FPSR, or FPIAR and
FMOVEM instructions cannot generate floating-point exceptions, these instructions do not
modify the FPIAR. However, they can be used to read the FPIAR in an exception handler
without changing the previous value. A reset or a restore operation of the null state clears
the FPIAR.
6.2 FLOATING-POINT DATA FORMATS AND DATA TYPES
The M68000 floating-point model (MC68881, MC68882, MC68040, and MC68060) supports
the following floating-point data formats: single precision, double precision, extended
precision, and packed decimal. The M68000 floating-point model supports the following
data types: normalized, zeros, infinities, unnormalized numbers, denormalized numbers,
and NANs. The MC68060 supports part of the M68000 floating-point model in hardware.
Table 6-3 lists the floating-point data formats and data types supported by the MC68060.
Table 6-4 through Table 6-7 summarize the floating-point data formats and data types
details.
*
Data Format/Type Supported by On-Chip MC68060 FPU Hardware
Data Format/Type Supported by Software (M68060SP)
New AEXC Bit = Old AEXC Bit + EXC Bits
IOP = IOP + (BSUN
+
SNAN
+
OPERR)
OVFL = OVFL + (OVFL)
UNFL = UNFL + (UNFL • INEX2)
DZ = DZ + (DZ)
INEX = INEX + (INEX1
+
INEX2
+
OVFL)
Table 6-3. MC68060 FPU Data Formats and Data Types
Number Types
Data Formats
Single-
Precision
Real
Double-
Precision
Real
Extended-
Precision
Real
Packed-
Decimal
Real
Byte
Integer
Word
Integer
Long-Word
Integer
Normalized * * * * * *
Zero * * * * * *
Infinity * * * — —
NAN * * * — —
Denormalized † † †
Unnormalized
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