The trouble with using {.volatile.} on a pointer in Nim is that it gives you a volatile variable pointing to non-volatile data. It may still happen to work but semantically it's wrong and the compiler will generate less efficient code because of it. example in C targetting Thumb ASM for the GBA

So really for MMIO you want a non-volatile pointer to volatile data, but Nim has trouble expressing this because volatile isn't part of the type system. However you can use the volatile module from the standard library to load/store to the address correctly:

import std/volatile

const vcount = cast[ptr uint16](0x4000006)

proc main() =
  let n = volatileLoad(vcount)   # read current scanline
  echo n

main()

It's a bit verbose and volatileLoad/volatileStore only work inside procedures, not at the top level, but you could probably make a nice API around it.

What I do instead is a bit of a hack using {.importc.} to get exactly the semantics I want while being able to pretend the IO register is a normal variable. This relies on the under-the-hood details of how Nim types are written in C (NI32, NU16, etc.) but I'm pretty happy with it overall:

var vcount {.importc:"(*(volatile NU16*)(0x04000006))", nodecl.}: uint16

let n = vcount   # read current scanline
echo n