Why does a 32-bit OS support 4 GB of RAM?

Question

Just reading some notes in a purdue lecture about OSs, and it says:

A program sees memory as an array of bytes that goes from address 0 to 2^32-1 (0

Answer 1

Mainly due to 32bit OS chosing to support only 2^32-1 addresses.

If the CPU has more than 32 address lines on the FSB, then the 32bit OS can choose to use a paging mechanism to access more than 4GiB. (For example Windows 2000 Advanced Server/Data Center editions on PAE supported Intel/AMD chips)