Complex sentence: Many of the extracodes were what we would probably call microcodes nowadays, they were simple arithmetic procedures which would have been too inefficient to implement in hardware, however about half of the codes were designated as Superviser functions which invoked operating system procedures.
Split into simpler sentences: Many of the extracodes were what we would probably call microcodes nowadays; they were simple arithmetic procedures which would have been too inefficient to implement in hardware. However, about half of the codes were designated as Supervisor functions which invoked operating system procedures.