There are 16 registers, and ALL can be used as a base EXCEPT

for register 0, so the answer is AT LEAST 15. AT LEAST is

specified here because in any section of code, you can

"re-use" a previous base register once you are no longer

within the original address range ("scope") of that

particular base register. By re-using the registers, you can

have base registers that will cover ALL of the memory in the

machine - but not all at once - you have to "bite off" 15

base reg-at-a-time chunks of memory (all addressable memory

does not have to be contiguous - it can be scattered around

memory in 4K pieces).