IBM 5-1/4" Diskette Drive Adapter Hardware Reference Manual page 9

unit's microprocessor. Because of this multi-byte interchange of
information between the FDC and the system unit's
microprocessor, it is convenient to consider each command as
consisting of three phases:
,~
Command Phase
The FDC receives all information required to perform a particular
operation from the system unit's microprocessor,
Execution Phase
The FDC performs the operation it was instructed to do.
Result Phase
After completion of the operation, status and other housekeeping
information are made available to the system unit's
microprocessor ,
The following tables define the symbols used in the command
summary. The command summary immediately follows these
tables,
Diskette Adapter 5