3. What would happen if you enabled an I/O interrupt (such as interrupts on keyboard keypresses) but did not initialize its interrupt vector?

Answer: The memory locations for the interrupt vector still contain some contents, we just haven't ensured that it's the right contents. When I/O activity occurs, the CPU will jump to whatever memory location is in the PC word of the interrupt vector and will load the PSW with whatever is in the PSW word in the interrupt vector, thus branching to an effectively random location (the PSW contents won't really be so important at this stage probably), which means that the computer is unlikely to function usefully after that until you turn it off and on (depending upon where it branches to).