When data are being put in or taken out of 8237's registers, the 8237 is a slave just like the system's I/O interfaces.

When both HRQ and \CS are low, the 8237 becomes a slave with the \IOR and \IOW being the input control pins.The CPU can read from or write to the internal registers of the controller by activating \IOR or \IOW.

The AEN, which is active when the controller is a master and is outputting an address, is 0 while the system is communicating with the controller's registers.

If the controller is the master, then it must supply the bus address. When it is master it puts the low-order byte of the address on the pins A7-A0 and the high-order byte on DB7-DB0, and sets AEN to 1.