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Fixed-point multiplication

Bruno Büsser

I want to multiply two fixed point numbers. After the multiplication I have to shift the result so that the binary point is correct.
Example:

int a;
int b;
int c;
c = (a * b) >> 10

The multiplication a*b produces a long int value in the MD register. After the shift operation the int result (the lower byte) should be stored in c.
My problem is how to use then 32-bit MD register. When I look at the assembler code only the lower byte of the MD register is used for the shift operation, but I first I want to shift the whole register and then take only the lower byte.
How can I implement that in C? Who knows an application note about fixed-point arithmetic with the C166?

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