Loading...
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
Minimum Positive Normalized 1.7 × 10–4932
Minimum Positive Denormalized 1.7 × 10–4951
Table 6-7. Packed Decimal Real Format Summary
Data Type SM SE Y Y 3-Digit
Exponent
1-Digit
Integer 16-Digit Fraction
±Infinity 0/1 1 1 1 $FFF $XXXX $00…00
±NAN 0/1 1 1 1 $FFF $XXXX Nonzero
±SNAN 0/1 1 1 1 $FFF $XXXX Nonzero
+Zero 0 0/1 X X $000–$999 $XXX0 $00…00
–Zero 1 0/1 X X $000–$999 $XXX0 $00…00
+In-Range 0 0/1 X X $000–$999 $XXX0–$XXX9 $00…01–$99…99
–In-Range 1 0/1 X X $000–$999 $XXX0–$XXX9 $00…01–$99…99
Table 6-6. Extended-Precision Real Format Summary (Continued)
SM SE Y Y EXP2 (EXP3) INTEGERX X X X X X X XEXP1 EXP0
FRAC15 FRAC8
FRAC7 FRAC0
95 64
63 32
31 0
FRAC14 FRAC13 FRAC12 FRAC11 FRAC10 FRAC9
FRAC6 FRAC5 FRAC4 FRAC3 FRAC2 FRAC1
Loading...
Terms of Use | Privacy Policy | DMCA Policy
2006-2020 Rsmanuals.com