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Floating-Point Unit
6-14 M68060 USER’S MANUAL MOTOROLA
handled in this manner. If the destination data format is extended and there is a difference
between the infinitely precise intermediate result and the round-to-nearest result, the rela-
tive difference is 2–64 (the value of the guard bit). This error is equal to one-half of the least
significant bit’s value and is the worst case error that can be introduced when using the RN
Figure 6-9. Rounding Algorithm Flowchart
Result Integer 63-Bit Fraction Guard Round Sticky
Intermediate x xxx…x00 1 0 0
Rounded-to-Nearest x xxx…x00 0 0 0
ENTRY
INEX2
1
GUARD
0
ROUND
0
STICKY
0
EXIT EXIT
GUARD, ROUND,
AND STICKY ARE
CHOPPED
SHIFT MANTISSA
RIGHT 1 BIT,
ADD 1 TO EXPONENT
ADD 1 TO
LSB
SELECT ROUNDING MODE
GUARD AND LSB = 1,
ROUND AND STICKY = 0
OR
GUARD = 1
ROUND OR STICKY = 1
INTERMEDIATE
RESULT
OVERFLOW = 1
GUARD, ROUND,
AND STICKY BITS = 0
EXACT RESULT
RPRMRN RZ
ADD 1 TO
LSB
INTERMEDIATE
RESULT
POS NEG POS NEG
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