Analog Devices ADSP-BF535 Blackfin, Hardware Reference Manual

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Using Data Formats
2-4 ADSP-BF535 Blackfin Processor Hardware Reference
Unsigned
Unsigned binary numbers may be thought of as positive and having nearly
twice the magnitude of a signed number of the same length. The processor
treats the least significant words of multiple precision numbers as
unsigned numbers.
Signed Numbers: Two’s-Complement
In Blackfin processor arithmetic, the word signed refers to two’s-comple-
ment numbers. Most Blackfin processor family operations presume or
support two’s-complement arithmetic.
Fractional Representation: 1.15
Blackfin processor arithmetic is optimized for numerical values in a frac-
tional binary format denoted by 1.15 (“one dot fifteen”). In the 1.15
format, one sign bit (the MSB) and fifteen fractional bits represent values
from –1 up to one LSB less than +1.
Figure 2-2 shows the bit weighting for 1.15 numbers. and some examples
of 1.15 numbers and their decimal equivalents.
Figure 2-2. Bit Weighting for 1.15 Numbers
2
0
2
–1
2
–2
2
–3
2
–4
2
–5
2
–6
2
–7
2
–8
2
–9
2
–10
2
–11
2
–12
2
–13
2
–14
2
–15
1.15 NUMBER (HEXADECIMAL)
0X0001
0X7FFF
0XFFFF
0X8000
DECIMAL EQUIVALENT
0.000031
0.999969
–0.000031
–1.000000