For arithmetic instructions, the lower 2 bits of <sop> encode addressing mode:
- 0?: arg_a = previous value of accumulator, arg_b = mem[x]
- 1?: arg_a = mem[y], arg_b = mem[x]
+ 0?: arg_a = mem[y], arg_b = mem[x]
+ 1?: arg_a = previous value of accumulator, arg_b = mem[x]
?0: store result to accumulator
?1: store result to both accumulator and mem[y]