Fixed-point multiplication, strange results

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?

Regards,

Sebastian