Answer from cs61c-ck (Allen Lee 16583554) for Question 3
Polling requires too much processor power.  In the example in the book, polling overhead for transfering data from floppy or hard disk requires 2-20% of the processor's clock cycles, which is not acceptable.  The solutions to the problems caused by polling are using interrupt-driven I/O devices which use interrupts to tell the processor that an I/O device needs attention, and a DMA (direct memory access) mechanism, which transfers data independent of the processor.