[pjc50]

This is one of those "everyone doing this kind of work knows" that's rather hard to source, but: this is basically the point of volatile. Especially for reads rather than writes, where you may want to read some location that is being written into by a different piece of hardware.

People used to use it for thread synchronization before proper memory barrier primitives (see https://mariadb.org/wp-content/uploads/2017/11/2017-11-Memor... ) were available. It was not entirely reliable for this purpose.

[tialaramex]

Yeah. I could have sworn that I've read somewhere an anecdote from the Bell Labs era in which this comes up, but I can't find it and might be misremembering. The whole volatile keyword doesn't exist in K&R C as released, there are no "type qualifiers" at all in that language, both volatile and const are introduced in C89.

Duff's famous Device, often misunderstood as some insight about memory copying or something silly, was an MMIO hack, it doesn't look like an MMIO hack to us because it doesn't say volatile, but that's because Duff's compiler did not have that keyword, the reason Duff doesn't change the destination pointer is that it's pointing at hardware and the hardware isn't going anywhere, writing different bytes to the same address is I/O.

[pjmlp]

No idea about volatile, but I do remember function prototypes and const came as influence from C++, well CFront.