Answer from cs61c-ck (Allen Lee 16583554) for Question 1
A Physical Address (PA) is an address that is used to directly reference the memory.  A Virtual Address (VA), however, is generated by the processor and can be translated to a PA through a combination of hardware and software.  VA's are mapped to PA's; it's possible for more than one VA to map to the same PA, which allows two different programs to share data or code.  VA's can also map to the disk.  To get a PA from a VA, we use the same page offset (lower bits), and pass the page number (upper bits) through some translation mechanism.