Floating-Point Unit
MOTOROLA
M68060 USER’S MANUAL
6-11
NOTE: EXP3 is generated only during an FMOVE OUT if the source is too large to be represented
with a three-digit exponent. Otherwise, it is a don’t care.
6.3 COMPUTATIONAL ACCURACY
Whenever an attempt is made to represent a real number in a binary format of finite preci-
sion, there is a possibility that the number can not be represented exactly. This is commonly
referred to as a round-off error. Furthermore, when two inexact numbers are used in a cal-
culation, the error present in each number is reflected, and possibly aggravated, in the
result. All FPU calculations use an intermediate result. When the MC68060 performs an
operation, the calculation is carried out using extended-precision inputs, and the intermedi-
ate result is calculated as if to produce infinite precision. After the calculation is complete,
the intermediate result is rounded to the selected precision and stored in the destination.
The FPCR RND and PREC encodings (see Table 6-1 and Table 6-2) provide emulation for
devices that only support single and double precision. By setting the rounding precision to
single, the MC68060 will perform all calculations as if only 24 bits of precision were available
for the result. Setting the rounding precision to double does the same to 53 bits of precision.
The execution speed of all instructions is the same whether using single- or double-precision
rounding. When using these two forced rounding precisions, the MC68060 produces the
same results as any other device that conforms to the IEEE 754 standard, but does not sup-
port extended precision. The results are the same when performing the same operation in
extended precision and storing the results in single- or double-precision format.
Approximate Ranges
Maximum Positive Normalized
1.2
×
104932
1.7
×
10–4932
1.7
×
10–4951
Minimum Positive Normalized
Minimum Positive Denormalized
Table 6-7. Packed Decimal Real Format Summary
Data Type
SM
SE
Y
Y
3-Digit
Exponent
$FFF
$FFF
$FFF
$000–$999
$000–$999
$000–$999
$000–$999
1-Digit
Integer
$XXXX
$XXXX
$XXXX
$XXX0
$XXX0
16-Digit Fraction
±
Infinity
±
NAN
±
SNAN
+Zero
–Zero
+In-Range
–In-Range
0/1
0/1
0/1
0
1
0
1
1
1
1
1
1
1
X
X
X
X
1
1
1
X
X
X
X
$00…00
Nonzero
Nonzero
$00…00
$00…00
0/1
0/1
0/1
0/1
$XXX0–$XXX9
$XXX0–$XXX9
$00…01–$99…99
$00…01–$99…99
Table 6-6. Extended-Precision Real Format Summary (Continued)
SM SE Y Y
EXP2
(EXP3)
INTEGER
X X X X
X X X X
EXP1
EXP0
FRAC15
FRAC8
FRAC7
FRAC0
95
64
63
32
31
0
FRAC14
FRAC13
FRAC12
FRAC11
FRAC10
FRAC9
FRAC6
FRAC5
FRAC4
FRAC3
FRAC2
FRAC1