Runtime Instruction Modification

In an assembly language that does not support indirect addressing, such as the MU0 Processor, we have to be a little more creative in order to achieve equivalent behaviour.

Branch and Link §

We can emulate the function of the Branch and Link instruction by modifying the binary for an instruction at runtime:

label1 LDA label1
       ADD offset
	   ADD jmp_opcode
	   STA ret_addr
	   JMP subroutine
return ...
...
offset DEFW &0005 ; `return` is 5 instructions after `label1`
jmp_opcode DEFW &4000
ret_addr DEFW 0

This creates the binary representation of the instruction JMP return, and stores it in ret_addr. This allows the called subroutine (subroutine here) to return to its caller with JMP ret_addr.