Question: How do I load a 24 bit address into an auxiliary register given that immediate addressing only allows 16 bit value?

Answer: Instead of using immediate addressing, use direct addressing. Store the address in a memory location using a ".word" directive. Then read the contents of the memory location by using direct addressing on the memory location that stores the address.

          EXAMPLE:       ADDSTORE   .WORD  address
                                    LDI   @ADDSTORE, AR0