Backend devices can tap into their frontend registers at multiple points, 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.

Note that no actual logic is here -- we are just "tapping into" various nodes inside the register.

Also note that this is all completely independent of and asynchronous to the frontend's operation. For example, a temperature sensor could, every so often, at its own pace, write its 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.)