Motorola DSP56000 Manual: Integer/fractional Multiplication Comparison

24-bit digital signal processor.

INTEGER
S
SIGNED MULTIPLIER
S
S
MSP
2N — 1 PRODUCT
SIGN EXTENSION
2N BITS
Figure 3-9 Integer/Fractional Multiplication Comparison
and rounds down any value below one-half. The question arises as to which way one-
half should be rounded. If it is always rounded one way, the results will eventually be
biased in that direction. Convergent rounding solves the problem by rounding down if the
number is odd (LSB=0) and rounding up if the number is even (LSB=1). Figure 3-10
shows the four cases for rounding a number in the A1 (or B1) register. If scaling is set in
the status register, the resulting number will be rounded as it is put on the data bus. How-
ever, the contents of the register are not scaled.
3 - 14
DATA REPRESENTATION AND ROUNDING
SIGNED MULTIPLICATION N x N - 2N – 1 BITS
S
. .
.
LSP
DATA ARITHMETIC LOGIC UNIT
FRACTIONAL
S
SIGNED MULTIPLIER
. .
.
S•
MSP
2N — 1 PRODUCT
2N BITS
S
0
LSP
ZERO FILL
MOTOROLA

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