Analog Devices ADSP-BF53x Blackfin Reference page 999

Binary Multiplication
In addition and subtraction, both operands must be in the same format
(signed or unsigned, radix point in the same location), and the result for-
mat is the same as the input format. Addition and subtraction are
performed the same way whether the inputs are signed or unsigned.
In multiplication, however, the inputs can have different formats, and the
result depends on their formats. The ADSP-BF53x/BF56x Blackfin family
assembly language allows you to specify whether the inputs are both
signed, both unsigned, or one of each (mixed-mode). The location of the
radix point in the result can be derived from its location in each of the
inputs. This is shown in
is a 32-bit number. If the inputs' formats are M.N and P.Q, the product
has the format (M + P).(N + Q). For example, the product of two 13.3
numbers is a 26.6 number. The product of two 1.15 numbers is a 2.30
number.
General Rule
M.N
x P.Q
(M + P).(N + Q)
Figure D-3. Format of Multiplier Result
ADSP-BF53x/BF56x Blackfin Processor Programming Reference
Figure D-3. The product of two 16-bit numbers
4-bit Example
1.111 (1.3 Format)
x 11.11 (2.2 Format)
1111
1111
1111
1111
111.00001 (3.5 Format = (1 + 2).(2 + 3) )
Numeric Formats
16-bit Examples
5.3
x 5.3
10.6
1.15
x 1.15
2.30
D-5