I think he used some software routines on the 6502 CPU to do the work of sending data to the drive. This was very timing sensitive so be partitioned out some of

I think he used some software routines on the 6502 CPU to do the work of sending data to the drive. This was very timing sensitive so be partitioned out some of the low level sequencing to the controller hardware while the rest was done by the very timing sensitive software routines. This technique is called bit-banging in general.