There are 15 MARIE instructions so with a 4-bit op code we can add one instruction.

provide the full MARIE RTN for the described new operation (include the fetch, decode, operand fetch (if necessary), execution and store result (if necessary) stages). X refers to a main memory address of the datum/pointer/branch location which is stored in IR[11..0], do not reference X in your RTN.

JumpBack X reset the PC to be PC [X] ([X] is the value stored at memory location X).