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Your understanding is correct.
There are further subtleties that make me say “like an i/o device”. For testing, programs often need to have repeatable random number sequences, so that it is important that a program be able to glom onto a RNG and take all the numbers generated, without some other thread or process grabbing one in the middle of the sequence. That is, the RNG needs to be (able to be) single-user exclusive use. Most of the other bulk-algorithm hardware has the same characteristic: you don’t want someone else getting the result of the MD4 that you started, and so on.
This exclusive use capability is characteristic of many i/o devices. Even those we often think of as having multiple concurrent users (a disk for example) actually has only one user (the driver) when you look closely at the hardware.
In contrast, operations are usable by all, and the hardware does the save-restore that gives the illusion that everybody has their own belt. It is impractical for hardware to provide the illusion that everyone has their own RNG. So the Mill provides an interface that supports exclusive use (using MMIO and the standard memory protection system) and asynchronous (or semi-synchronous) access.