Motorola DSP56000 manual Limited Data Values, Integer-to-Fractional Data Conversion

DATA REPRESENTATION AND ROUNDING

Table 3-1 Limited Data Values

Destination	Source	Accumulator	Limited Value (Hexadecimal)		Type of
Memory Reference	Operand	Sign	XDB	YDB	Access
X	X:A	+	7FFFFF	—	One 24 bit
	X:B	-	800000	—
Y	Y:A	+	—	7FFFFF	One 24 bit
	Y:B	-	—	800000
	X:A Y:A	+	7FFFFF	7FFFFF
	X:A Y:B	-	800000	800000
X and Y	X:B Y:A	+	7FFFFF	7FFFFF	Two 24 bit
	X:B Y:B	-	800000	800000
	L:AB	+	7FFFFF	7FFFFF
	L:BA	-	800000	800000
L (X:Y)	L:A	+	7FFFFF	FFFFFF	One 48 bit
	L:B	-	800000	000000

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.

S3F.

S. 3F

S = SIGN BIT

3F = HEXADECIMAL DATA TO BE CONVERTED

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 - 2-23and the most positive long word is $7FFFFFFFFFFF

MOTOROLA DATA ARITHMETIC LOGIC UNIT 3 - 11