Steve, the Fanuc has two processors, the CNC and the PMC, the CNC looks after processing the part program together with the servo control, the PMC operates the Machine functions such as M,S & T codes and the Input/Output functions.
They communicate over a BMI bus using registers to comunicate, the F registers sent by the CNC are read by the PMC and the G registers sent by the PMC are read by the CNC.
For example the cycle start button is an input into the PMC, this is conditioned by the PMC however the machine tool builder wants to, (not Fanuc) it is then sent over to the CNC via a G register using a designated bit.
The CNC side is pre-writen, but the PMC is writen by the machine tool builder or Retro-fitter.
If you can view the PMC ladder you should be able to find the F register bit as a contact in a rung or a word in a register.