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Motorola DSP56000 Manual: Integer-to-fractional Data Conversion; Limited Data Values

24-bit digital signal processor.
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Destination
Memory Reference
X
Y
X and Y
L (X:Y)
7 shows the bit weighting of words, long words, and accumulator operands for this repre-
sentation. The decimal points are all aligned and are left justified.
Data must be converted to a fractional number by scaling before being used by the DSP
or the user will have to be very careful in how the DSP manipulates the data. Moving $3F
to a 24-bit Data ALU register does not result in the contents being $00003F as might be
expected. Assuming numbers are fractional, the DSP left justifies rather than right justi-
fies. As a result, storing $3F in a 24-bit register results in the contents being $3F0000.
The simplest example of scaling is to convert all integer numbers to fractional numbers
by shifting the decimal 24 places to the left (see Figure 3-6). Thus, the data has not
changed; only the position of the decimal has moved.
S
.
S
Figure 3-6 Integer-to-Fractional Data Conversion
For words and long words, the most negative number that can be represented is -1
whose internal representation is $800000 and $800000000000, respectively. The most
positive word is $7FFFFF or 1 - 2
MOTOROLA
DATA REPRESENTATION AND ROUNDING
Table 3-1 Limited Data Values
Source
Accumulator
Operand
Sign
X:A
+
X:B
-
Y:A
+
Y:B
-
X:A Y:A
+
X:A Y:B
-
X:B Y:A
+
X:B Y:B
-
L:AB
+
L:BA
-
L:A
+
L:B
-
.
3F
S = SIGN BIT
3F = HEXADECIMAL DATA TO BE CONVERTED
3F
-23
and the most positive long word is $7FFFFFFFFFFF
DATA ARITHMETIC LOGIC UNIT
Limited Value (Hexadecimal)
XDB
YDB
7FFFFF
800000
7FFFFF
800000
7FFFFF
7FFFFF
800000
800000
7FFFFF
7FFFFF
800000
800000
7FFFFF
7FFFFF
800000
800000
7FFFFF
FFFFFF
800000
000000
Type of
Access
One 24 bit
One 24 bit
Two 24 bit
One 48 bit
3 - 11

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