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Floating-Point Unit
MOTOROLA
M68060 USER’S MANUAL
6-3
6.1.1 Floating-Point Data Registers (FP7–FP0)
The floating-point data registers are analogous to the integer data registers of the M68000
family. The floating-point data registers always contain extended-precision numbers. All
external operands, regardless of the data format, are converted to extended-precision val-
ues before being used in any calculation or stored in a floating-point data register. A reset
or a restore operation of the null state sets FP7–FP0 to positive, nonsignaling not-a-num-
bers (NANs).
6.1.2 Floating-Point Control Register (FPCR)
The FPCR (see Figure 6-3) contains an exception enable (ENABLE) byte that enables or
disables traps for each class of floating-point exceptions and a mode control (MODE) byte
that sets the user-selectable modes. The user can read or write to the FPCR. Motorola
reserves bits 31–16 for future definition; these bits are always read as zero and are ignored
during write operations. The reset function or a restore operation of the null state clears the
FPCR. When cleared, this register provides the IEEE 754 standard defaults.
6.1.2.1 EXCEPTION ENABLE BYTE.
Each bit of the ENABLE byte (see Figure 6-3) corre-
sponds to a floating-point exception class. The user can separately enable traps for each
class of floating-point exceptions.
6.1.2.2 MODE CONTROL BYTE.
The MODE byte (see Figure 6-3) controls the user-
selectable rounding modes and precisions. Zeros in this byte select the IEEE 754 standard
defaults. The rounding mode field (RND) specifies how inexact results are rounded, and the
rounding precision field (PREC) selects the boundary for rounding the mantissa.
Figure 6-2. Floating-Point User Programming Model
79 63 0
FP0
FP1
FP3
FP4
FP5
FP6
FP7
FP2
FLOATING-POINT
DATA REGISTERS
FPCR
FLOATING-POINT
CONTROL
REGISTER
FPSR
FLOATING-POINT
STATUS
REGISTER
FPIAR
FLOATING-POINT
INSTRUCTION
ADDRESS
REGISTER
071531
MODE
CONTROL
EXCEPTION
ENABLE
0
EXCEPTION
STATUS
CONDITION
CODE QUOTIENT ACCRUED
EXCEPTION
071531 23
031
16 8
8
16
24
64
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