Memory-Mapped Register Token Tags

Let's take a closer look at what exact type a register token has:

#![allow(unused)]
fn main() {
pub fn handler(reg: Regs, thr_init: ThrsInit) {
    let rcc_cr: reg::rcc::Cr<Srt> = reg.rcc_cr;
}
}

A register token tag has one generic parameter - a register tag. There are three possible register tags:

• Urt (for Unsynchronized register tag)
• Srt (for Synchronized register tag)
• Crt (for Copyable register tag)

The tags are crucial to eliminate data-races for read-modify-write operations and to control move-semantics of the tokens.

Here RegOwned is a kind of tag that doesn't implement the Copy trait, and RegAtomic makes all read-modify-write operations atomic.

Operations for register tokens and field tokens without an atomic tag (Urt) require exclusive (&mut) borrows. While atomic tokens (Srt, Crt) require shared (&) borrows. This eliminates any possibility of data-races by leveraging Rust compile-time checking. Despite Urt tagged tokens use more effective, but non-atomic processor instructions, it is impossible to use concurrently. A program with a possible data-race will be rejected by the compiler, and there are no additional checks in the run-time.

For the whole register tokens, the only affected operation in regard to atomicity is the modify method. However for the field tokens, all write operations incur additional cost if used with an atomic tag. Because field tokens could be shared between different threads.

Another property of a token is affinity (expressed by the RegOwned trait.) An affine type can't be copied nor cloned, and uses Rust move-semantics. If a token has an affine tag (Urt, Srt), it is guaranteed that there exists only one token for this particular register or field. Though such tokens could still have multiple shared borrows. Non-affine (Crt) tokens can be freely copied, because they implement the Copy trait. Copying of tokens is still zero-cost, because tokens are zero-sized. On the other hand copyable tokens are always atomic.

To switch between different tags of tokens, both whole register tokens and register field tokens provide the following three methods:

• into_unsync() - converts to unsynchronized token
• into_sync() - converts to synchronized token
• into_copy() - converts to copyable token

These methods take their tokens by-value, and return a new token of the same type but with a different tag. Not all conversions are possible. For example if a token is already Crt, there is no path backwards to Srt or Urt. Because we can't guarantee that all possible copies of the Crt token are dropped. For the details refer to the drone_core::reg documentation. As one might guess, these conversion methods are completely zero-cost.