For a JVM microarchitecture (Mic-1) why is it that in the control portion the B bus register is encoded in a 4 bit field within the MIR, sent to the decoder and then the appropriate register is put on the B bus, while the C bus register is bit mapped directly to what register the ALU result is intended to be stored/written to? I just don't see why the C bus register couldn't also be encoder in a 4 bit field.