Freescale Semiconductor, Inc.
First, the results of each TBL are rounded with the TBLS round-to-nearest-even algorithm.
The following values would be returned by TBLS:
TBL # 1
TBL # 2
TBL # 3
0010 0000 .
0011 1111 .
0000 0001 .
Summing, the following result is obtained:
0010 0000 .
0011 1111 .
0000 0001 .
0110 0000 .
Now, using the same TBL results, the sum is first calculated and then rounded according
to the same algorithm:
0010 0000 . 0111 0000
0011 1111 . 0111 0000
0000 0001 . 0111 0000
0110 0001 . 0101 0000
Rounding yields:
0110 0001 .
The second result is preferred. The following code sequence illustrates how addition of a
series of table interpolations can be performed without loss of precision in the intermediate
results:
L0:
TBLSN.B
TBLSN.B
TBLSN.B
ADD.L
ea , Dx
ea , Dx
ea , Dl
Dx, Dm
Dm, Dl
#8, Dl
L1
Long addition avoids problems with carry
ADD.L
ASR.L
Move radix point
BCC.B
Fraction MSB in carry
ADDQ.B
#1, Dl
L1: . . .
MOTOROLA
MC68340 USER’S MANUAL
5- 35
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