I agree with Jay's idea of a background scanning routine, but I have (successfully!) avoided interrupts like the plague for my keyboards. BTW, I use membrane matrix keyboards a lot, which I expect have v. noisy contacts(?).
The easiest approach for a 0-key rollover deal is to use a state machine; just look at all 4 inputs (if 4x4) at the beginning of the routine; if any pressed, scan & find which & handle it; set next state & exit. Next time thru (100mS later), if not the 0 state, wait till all keys off.

If you must use a common-contact keyboard, use a bit of logic (8-to-3 priority devices or such), or Ben's thingy (almost a Belgian idea :-).

Regards,

Richard

Regards,

Richard