Backend devices can tap into their frontend registers and read/write them at will. They can monitor EN-IN and EN-OUT to see when the frontend is reading or writing, and they can monitor the bits' Q values and overwrite their D values. And they supply read/write-ready to the RCL.

This is all completely independent of and asynchronous to the frontend's operation. For example, a temperature sensor could write the current temperature to the register, even if no frontend instruction ever reads it. If some program ever does get around to reading it, the latest value will be there.

(And never mind about race conditions: we'll deal with that when we discuss the ready bits in more detail.)