Integer-To-Fractional Data Conversion; Limited Data Values - Motorola DSP56000 Manual

24-bit digital signal processor

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Memory Reference

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.

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

DATA REPRESENTATION AND ROUNDING

Table 3-1 Limited Data Values

S = SIGN BIT

3F = HEXADECIMAL DATA TO BE CONVERTED

and the most positive long word is $7FFFFFFFFFFF

DATA ARITHMETIC LOGIC UNIT

Limited Value (Hexadecimal)

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