dsp56000 Freescale Semiconductor, Inc, dsp56000 Datasheet - Page 41

• Manufacturer Part Number: dsp56000
• Description: 24-bit Digital Signal Processor Family Manual
• Manufacturer: Freescale Semiconductor, Inc

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.
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
Memory Reference
Destination
X and Y
L (X:Y)
X
Y
S
S
.
Figure 3-6 Integer-to-Fractional Data Conversion
DATA REPRESENTATION AND ROUNDING
Operand
Source
Freescale Semiconductor, Inc.
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
For More Information On This Product,
DATA ARITHMETIC LOGIC UNIT
Table 3-1 Limited Data Values
3F
3F
.
Go to: www.freescale.com
-23
Accumulator
and the most positive long word is $7FFFFFFFFFFF
Sign
3F = HEXADECIMAL DATA TO BE CONVERTED
+
+
+
-
-
-
+
+
+
-
-
-
S = SIGN BIT
Limited Value (Hexadecimal)
7FFFFF
7FFFFF
7FFFFF
7FFFFF
7FFFFF
800000
800000
800000
800000
800000
XDB
—
—
FFFFFF
7FFFFF
7FFFFF
7FFFFF
7FFFFF
800000
800000
800000
800000
000000
YDB
—
—
One 24 bit
One 24 bit
Two 24 bit
One 48 bit
Type of
Access
3 - 11