IBM Assembler Interview Question:

HOW MANY MAXIMUM BASE REGISTERS WE CAN HAVE IN A PROGRAM AND ALSO HOW MANY MAXIMUM BASE REGISTERS WE CAN HAVE IN A SINGLE PROGRAM?

Submitted by: Administrator

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).
Submitted by: Administrator

Discounting the first answers implied pushing and popping registers with USING and DROP, the more practical answer is 13. In addition to the standard usage of R0, some instructions like TRT also alter the contents of R1 and R2.
Submitted by: Howy61