Solution:

Current Auxiliary Register:

To select a specific auxiliary register, load the 3-bit auxiliary register pointer (ARP) of status register ST0 with a value from 0 to 7.

The ARP can be loaded as a primary operation by the MAR instruction or by the LST instruction. The ARP can be loaded as a secondary operation by any instruction that supports indirect addressing.

The register pointed to by the ARP is referred to as the current auxiliary register or current AR. During the processing of an instruction, the content of the current auxiliary register is used as the address at which the data-memory access occurs.

The ARAU passes this address to the data-read address bus (DRAB) if the instruction requires a read from data memory, or it passes the address to the data-write address bus (DWAB) if the instruction requires a write-to-data memory.

After the instruction uses the data value, the contents of the current auxiliary register can be incremented or decremented by the ARAU, which implements unsigned 16-bit arithmetic.

Normally, the ARAU performs its arithmetic operations in the decode phase of the pipeline (when the instruction specifies the operations is being decoded).

This allows the address to be generated before the decode phase of the next instruction.

There is an exception to this rule: during the processing of the NORM instruction, the auxiliary register and/or ARP modification is done during the execute phase of the pipeline.