Convergent Rounding - Motorola DSP56000 Manual

CASE I: IF A0 < $800000 (1/2), THEN ROUND DOWN (ADD NOTHING)
BEFORE ROUNDING
A2 A1
XX . . XX XXX . . . XXX0100 011XXX . . . . XXX
55 48 47
CASE II: IF A0 > $800000 (1/2), THEN ROUND UP (ADD 1 TO A1)
BEFORE ROUNDING
A2 A1
XX . . XX XXX . . . XXX0100 1110XX . . . . XXX
55 48 47
CASE III: IF A0 = $800000 (1/2), AND THE LSB OF A1 = 0,THEN ROUND DOWN (ADD NOTHING)
BEFORE ROUNDING
A2 A1
XX . . XX XXX . . . XXX0100 10000 . . . . . . 000
55 48 47
CASE IV: IF A0 = $800000 (1/2), AND THE LSB = 1, THEN ROUND UP (ADD 1 TO A1)
BEFORE ROUNDING
A2 A1
XX . . XX XXX . . . XXX0101 10000 . . . . . . 000
55 48 47
*A0 is always clear; performed during RND, MPYR, MACR
MOTOROLA
DATA REPRESENTATION AND ROUNDING
0
A0
24 23 0
1
A0
24 23 0
0
A0
24 23 0
1
A0
24 23 0
Figure 3-10 Convergent Rounding
DATA ARITHMETIC LOGIC UNIT
AFTER ROUNDING
A2 A1
XX . . XX XXX . . . XXX0100 000 . . . . . . . . 000
55 48 47
AFTER ROUNDING
A2 A1
XX . . XX XXX . . . XXX0101 000 . . . . . . . . 000
55 48 47
AFTER ROUNDING
A2 A1
XX . . XX XXX . . . XXX0100 000 . . . . . . . . 000
55 48 47
AFTER ROUNDING
A2 A1
XX . . XX XXX . . . XXX0110 000 . . . . . . . . 000
55 48 47 24 23
A0*
24 23 0
A0*
24 23 0
A0*
24 23 0
A0*
0
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