How does MMU deal with Memory mapped registers?

Question

Am I correct when I say that addresses of memory mapped registers are always physical addresses?

If yes then how does MMU deal with these addresses and decide not to do virtual to physical translations for them?

Answer 1

You are correct.

All registers and or memory locations within a processor's memory map are physical addresses.

Virtual to physical translations are done by the MMU and only occur within contiguous blocks of memory in which code can be executed from. i.e RAM or internal flash. No virtual to physical translations occur when other parts of the memory map are accessed, because they do not interact with the MMU.

Answer 2

The MMU doesn't decide anything. It merely maps addresses according to what it has been told by the OS: virtual addresses to physical ones, and/or interrupts the application program if the mapping for a particular physical address is marked as "invalid" or somehow inconsistent with the operation of the current machine instruction (e.g., for instruction fetches, "not executable", for stores, "read only", etc.).

The operating system establishes a set of rules and conventions that ensure that applications cannot create greif for one another. If writing to memory-mapped I/O devices is OK for this OS, then the OS will set MMU mappings (e.g. page map registers) to allow it; otherwise it will not set MMU pages to map to I/O devices.

For most general purpose OSes, allowing arbitrary programs to write to I/O registers is a definition of "causes grief" and they simply never set up such a mapping. This is how Windows acts from the point of user processes.

For special purpose OSes, have separate processes share I/O pages may be fine, especially if the processes running are trusted (e.g, part of the OS or pass some certification authority who asserts good quality). Then multiple trusted processes might share memory-mapped I/O devices safely and conveniently. Even untrusted processes can be run on such an OS; it simply doesn't give them access to I/O.

Back in 1972, I built a unique virtual memory 16 bit minicomputer. The MMU had two kinds of page mappings: mapping of virtual pages to physical (as you'd expect), and mapping of a page to a single 32 byte I/O device. What this means is that the OS can hand any process any device (not critical to OS function) safely.

In particular, it meant that each I/O driver has its own address space; if it screwed up, no problem. You could debug device drivers while running the OS without fear. (Windows suffered from I/O driver corruption destroying windows for years; still does I think but their quality control "trustedness checking" is wicked strong now).

Alas, it wasn't a commercial success. I was forced to go into software to make a living :-{