The BMDC runs the machine. When you save and load, the save goes to the mass storage device (hard drive) the load goes to the BMDC memory which is 256KB.
The typical way that most of my customers DNC is to transfer the program to the hard drive of the machine by RS232 or other. Then under the run screen there is a DNC set up. I believe it is a F key.
Other communication: I have a few mold shops that have loaded WIN 3.11 for work groups, installed a ethernet board and transfer large programs to the hard drive very quickly for DNC.