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Hi,
I'm trying to implement HPET on my hobby OS. After I read this article: https://wiki.osdev.org/HPET, I want to ask one thing: Can you explain this line?

Not exactly that line, but the function write_register_64. I don't know how to implement it. Can anyone help me?

It's a function that writes a 64-bit value to a MMIO register.

If you're writing your kernel in C, you should probably write the function in assembly so you can ensure compiler optimizations won't reorder or remove the write.

That's the problem, I don't know what I should write. I have worked with registers, memory,... but this is my first time with MMIO register. In my case, is that I have to use port and outX/inX?

The clue is in the name Memory-Mapped Input/Output.

No, "memory mapped" means you read and write it as if it was memory. Literally just store or read a value from the correct address.

You should use a volatile pointer to avoid the compiler re-ordering, collapsing or eliding the loads and stores.

The C standard still gives compilers some flexibility in how they implement volatile. GCC seems to handle it in a way that's appropriate for MMIO, but I'm pretty sure Clang does not!

I know that volatile has historically been problematic in both GCC and Clang (https://llvm.org/pubs/2008-10-EMSOFT-Volatiles.pdf) but I'm not aware of any current bugs in code generation around use of volatile in Clang. I'd be interested to see any example.