Hello from Southern Mississippi:
Subroutine Calls are initiated with a simple JST (Jump and Store Return) instruction. This is the mechanism:
LDA Money To pass an argument
DO PZE Initially Zero
My design executes the LDA and JST in parallel. The JST stores the incremented PC in DO, and executes the instruction DO+1.
Upon completion of SUBR Called (DO), an Indirect Jump is executed, JMPN.
JMPN DO Return to JST + 1
Priority Interrupts are handled in the same manner; as an Interrupt is nothing more that a JST
I don't have to consider what Register does what, as I don't have any. My registers are in Operand (Local Memory) . I potentially have 256K of Registers.
Have a good day
Erin Greene & Assiciates
Business Information Management Systems