making a processor

I am currently working on a processibng machine that will be programmed onto an altera fpga cyclone 2. I have completed the code and found an interesting issue. it is a 16 bit input and output. i can subtract 1 1 1 1 from 5 5 5 5 and get 4 4 4 4 but i cant subtract 1 1 1 1 from 0 0 0 0 and get F F F F here is the code:

i have omitted the clock debouncing modules