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Fixed-point multiplication, strange results

Honored Contributor II

Hello everybody, 


I made DSPBuilder-to-Avalon-ST-Interface-Test which uses fixed-point multiplications (see attached picture for a oversimplified DSPBuilder-Model). 


However I get the following results, when trying to multiplicate the input with 1.5: 

0x00040000 ---> 0x00060000, correct: 4 * 1.5 = 6 

0x00030000 ---> 0x80040000, this isn't correct, is it? I expected 4.5 as result, in 2's complement fixed-point format it should look like this: 0x00048000 


It seems somehow rotated, but not exactly. What's going on? 



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