Multiple registers

We can use a struct to represent the memory layout of the UART’s registers, using types from the safe-mmio crate to wrap ones which can be read or written safely.

use safe_mmio::fields::{ReadPure, ReadPureWrite, ReadWrite, WriteOnly};

#[repr(C, align(4))]
pub struct Registers {
    dr: ReadWrite<u16>,
    _reserved0: [u8; 2],
    rsr: ReadPure<ReceiveStatus>,
    _reserved1: [u8; 19],
    fr: ReadPure<Flags>,
    _reserved2: [u8; 6],
    ilpr: ReadPureWrite<u8>,
    _reserved3: [u8; 3],
    ibrd: ReadPureWrite<u16>,
    _reserved4: [u8; 2],
    fbrd: ReadPureWrite<u8>,
    _reserved5: [u8; 3],
    lcr_h: ReadPureWrite<u8>,
    _reserved6: [u8; 3],
    cr: ReadPureWrite<u16>,
    _reserved7: [u8; 3],
    ifls: ReadPureWrite<u8>,
    _reserved8: [u8; 3],
    imsc: ReadPureWrite<u16>,
    _reserved9: [u8; 2],
    ris: ReadPure<u16>,
    _reserved10: [u8; 2],
    mis: ReadPure<u16>,
    _reserved11: [u8; 2],
    icr: WriteOnly<u16>,
    _reserved12: [u8; 2],
    dmacr: ReadPureWrite<u8>,
    _reserved13: [u8; 3],
}
  • #[repr(C)] tells the compiler to lay the struct fields out in order, following the same rules as C. This is necessary for our struct to have a predictable layout, as default Rust representation allows the compiler to (among other things) reorder fields however it sees fit.
  • There are a number of different crates providing safe abstractions around MMIO operations; we recommend the safe-mmio crate.
  • The difference between ReadPure or ReadOnly (and likewise between ReadPureWrite and ReadWrite) is whether reading a register can have side-effects which change the state of the device. E.g. reading the data register pops a byte from the receive FIFO. ReadPure means that reads have no side-effects, they are purely reading data.