Add embassy-cortex-m crate.

- Move Interrupt and InterruptExecutor from `embassy` to `embassy-cortex-m`.
- Move Unborrow from `embassy` to `embassy-hal-common` (nothing in `embassy` requires it anymore)
- Move PeripheralMutex from `embassy-hal-common` to `embassy-cortex-m`.

7 files changed, 1270 insertions, 0 deletions

diff --git a/embassy-cortex-m/Cargo.toml b/embassy-cortex-m/Cargo.toml
new file mode 100644
index 000000000..8bd30403e
--- /dev/null
+++ b/embassy-cortex-m/Cargo.toml
@@ -0,0 +1,47 @@
+[package]
+name = "embassy-cortex-m"
+version = "0.1.0"
+authors = ["Dario Nieuwenhuis <[email protected]>"]
+edition = "2018"
+resolver = "2"
+
+[package.metadata.embassy_docs]
+src_base = "https://github.com/embassy-rs/embassy/blob/embassy-cortex-m-v$VERSION/embassy-cortex-m/src/"
+src_base_git = "https://github.com/embassy-rs/embassy/blob/master/embassy-cortex-m/src/"
+features = []
+flavors = [
+    { name = "thumbv6m-none-eabi",        target = "thumbv6m-none-eabi",         features = [] },
+    { name = "thumbv7m-none-eabi",        target = "thumbv7m-none-eabi",         features = [] },
+    { name = "thumbv7em-none-eabi",       target = "thumbv7em-none-eabi",        features = [] },
+    { name = "thumbv7em-none-eabihf",     target = "thumbv7em-none-eabihf",      features = [] },
+    { name = "thumbv8m.base-none-eabi",   target = "thumbv8m.base-none-eabi",    features = [] },
+    { name = "thumbv8m.main-none-eabi",   target = "thumbv8m.main-none-eabi",    features = [] },
+    { name = "thumbv8m.main-none-eabihf", target = "thumbv8m.main-none-eabihf",  features = [] },
+]
+
+[features]
+default = []
+
+# Define the number of NVIC priority bits.
+prio-bits-0 = []
+prio-bits-1 = []
+prio-bits-2 = []
+prio-bits-3 = []
+prio-bits-4 = []
+prio-bits-5 = []
+prio-bits-6 = []
+prio-bits-7 = []
+prio-bits-8 = []
+
+[dependencies]
+defmt = { version = "0.3", optional = true }
+log = { version = "0.4.14", optional = true }
+
+embassy = { version = "0.1.0", path = "../embassy"}
+embassy-macros = { version = "0.1.0", path = "../embassy-macros"}
+embassy-hal-common = { version = "0.1.0", path = "../embassy-hal-common"}
+atomic-polyfill = "0.1.5"
+critical-section = "0.2.5"
+cfg-if = "1.0.0"
+cortex-m = "0.7.3"
+
diff --git a/embassy-cortex-m/build.rs b/embassy-cortex-m/build.rs
new file mode 100644
index 000000000..6fe82b44f
--- /dev/null
+++ b/embassy-cortex-m/build.rs
@@ -0,0 +1,29 @@
+use std::env;
+
+fn main() {
+    let target = env::var("TARGET").unwrap();
+
+    if target.starts_with("thumbv6m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv6m");
+    } else if target.starts_with("thumbv7m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+    } else if target.starts_with("thumbv7em-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+        println!("cargo:rustc-cfg=armv7em"); // (not currently used)
+    } else if target.starts_with("thumbv8m.base") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_base");
+    } else if target.starts_with("thumbv8m.main") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_main");
+    }
+
+    if target.ends_with("-eabihf") {
+        println!("cargo:rustc-cfg=has_fpu");
+    }
+}
diff --git a/embassy-cortex-m/src/executor.rs b/embassy-cortex-m/src/executor.rs
new file mode 100644
index 000000000..63a1519cf
--- /dev/null
+++ b/embassy-cortex-m/src/executor.rs
@@ -0,0 +1,89 @@
+use core::marker::PhantomData;
+
+use crate::interrupt::{Interrupt, InterruptExt};
+use embassy::executor::{raw, SendSpawner};
+
+pub use embassy::executor::Executor;
+
+fn pend_by_number(n: u16) {
+    #[derive(Clone, Copy)]
+    struct N(u16);
+    unsafe impl cortex_m::interrupt::InterruptNumber for N {
+        fn number(self) -> u16 {
+            self.0
+        }
+    }
+    cortex_m::peripheral::NVIC::pend(N(n))
+}
+
+/// Interrupt mode executor.
+///
+/// This executor runs tasks in interrupt mode. The interrupt handler is set up
+/// to poll tasks, and when a task is woken the interrupt is pended from software.
+///
+/// This allows running async tasks at a priority higher than thread mode. One
+/// use case is to leave thread mode free for non-async tasks. Another use case is
+/// to run multiple executors: one in thread mode for low priority tasks and another in
+/// interrupt mode for higher priority tasks. Higher priority tasks will preempt lower
+/// priority ones.
+///
+/// It is even possible to run multiple interrupt mode executors at different priorities,
+/// by assigning different priorities to the interrupts. For an example on how to do this,
+/// See the 'multiprio' example for 'embassy-nrf'.
+///
+/// To use it, you have to pick an interrupt that won't be used by the hardware.
+/// Some chips reserve some interrupts for this purpose, sometimes named "software interrupts" (SWI).
+/// If this is not the case, you may use an interrupt from any unused peripheral.
+///
+/// It is somewhat more complex to use, it's recommended to use the thread-mode
+/// [`Executor`] instead, if it works for your use case.
+pub struct InterruptExecutor<I: Interrupt> {
+    irq: I,
+    inner: raw::Executor,
+    not_send: PhantomData<*mut ()>,
+}
+
+impl<I: Interrupt> InterruptExecutor<I> {
+    /// Create a new Executor.
+    pub fn new(irq: I) -> Self {
+        let ctx = irq.number() as *mut ();
+        Self {
+            irq,
+            inner: raw::Executor::new(|ctx| pend_by_number(ctx as u16), ctx),
+            not_send: PhantomData,
+        }
+    }
+
+    /// Start the executor.
+    ///
+    /// This initializes the executor, configures and enables the interrupt, and returns.
+    /// The executor keeps running in the background through the interrupt.
+    ///
+    /// This returns a [`SendSpawner`] you can use to spawn tasks on it. A [`SendSpawner`]
+    /// is returned instead of a [`Spawner`] because the executor effectively runs in a
+    /// different "thread" (the interrupt), so spawning tasks on it is effectively
+    /// sending them.
+    ///
+    /// To obtain a [`Spawner`] for this executor, use [`Spawner::for_current_executor`] from
+    /// a task running in it.
+    ///
+    /// This function requires `&'static mut self`. This means you have to store the
+    /// Executor instance in a place where it'll live forever and grants you mutable
+    /// access. There's a few ways to do this:
+    ///
+    /// - a [Forever](crate::util::Forever) (safe)
+    /// - a `static mut` (unsafe)
+    /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
+    pub fn start(&'static mut self) -> SendSpawner {
+        self.irq.disable();
+
+        self.irq.set_handler(|ctx| unsafe {
+            let executor = &*(ctx as *const raw::Executor);
+            executor.poll();
+        });
+        self.irq.set_handler_context(&self.inner as *const _ as _);
+        self.irq.enable();
+
+        self.inner.spawner().make_send()
+    }
+}
diff --git a/embassy-cortex-m/src/fmt.rs b/embassy-cortex-m/src/fmt.rs
new file mode 100644
index 000000000..f8bb0a035
--- /dev/null
+++ b/embassy-cortex-m/src/fmt.rs
@@ -0,0 +1,228 @@
+#![macro_use]
+#![allow(unused_macros)]
+
+#[cfg(all(feature = "defmt", feature = "log"))]
+compile_error!("You may not enable both `defmt` and `log` features.");
+
+macro_rules! assert {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert!($($x)*);
+        }
+    };
+}
+
+macro_rules! assert_eq {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert_eq!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert_eq!($($x)*);
+        }
+    };
+}
+
+macro_rules! assert_ne {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::assert_ne!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::assert_ne!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert_eq {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert_eq!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert_eq!($($x)*);
+        }
+    };
+}
+
+macro_rules! debug_assert_ne {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::debug_assert_ne!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug_assert_ne!($($x)*);
+        }
+    };
+}
+
+macro_rules! todo {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::todo!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::todo!($($x)*);
+        }
+    };
+}
+
+macro_rules! unreachable {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::unreachable!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::unreachable!($($x)*);
+        }
+    };
+}
+
+macro_rules! panic {
+    ($($x:tt)*) => {
+        {
+            #[cfg(not(feature = "defmt"))]
+            ::core::panic!($($x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::panic!($($x)*);
+        }
+    };
+}
+
+macro_rules! trace {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::trace!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::trace!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! debug {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::debug!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::debug!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! info {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::info!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::info!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! warn {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::warn!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::warn!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+macro_rules! error {
+    ($s:literal $(, $x:expr)* $(,)?) => {
+        {
+            #[cfg(feature = "log")]
+            ::log::error!($s $(, $x)*);
+            #[cfg(feature = "defmt")]
+            ::defmt::error!($s $(, $x)*);
+            #[cfg(not(any(feature = "log", feature="defmt")))]
+            let _ = ($( & $x ),*);
+        }
+    };
+}
+
+#[cfg(feature = "defmt")]
+macro_rules! unwrap {
+    ($($x:tt)*) => {
+        ::defmt::unwrap!($($x)*)
+    };
+}
+
+#[cfg(not(feature = "defmt"))]
+macro_rules! unwrap {
+    ($arg:expr) => {
+        match $crate::fmt::Try::into_result($arg) {
+            ::core::result::Result::Ok(t) => t,
+            ::core::result::Result::Err(e) => {
+                ::core::panic!("unwrap of `{}` failed: {:?}", ::core::stringify!($arg), e);
+            }
+        }
+    };
+    ($arg:expr, $($msg:expr),+ $(,)? ) => {
+        match $crate::fmt::Try::into_result($arg) {
+            ::core::result::Result::Ok(t) => t,
+            ::core::result::Result::Err(e) => {
+                ::core::panic!("unwrap of `{}` failed: {}: {:?}", ::core::stringify!($arg), ::core::format_args!($($msg,)*), e);
+            }
+        }
+    }
+}
+
+#[cfg(feature = "defmt-timestamp-uptime")]
+defmt::timestamp! {"{=u64:us}", crate::time::Instant::now().as_micros() }
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct NoneError;
+
+pub trait Try {
+    type Ok;
+    type Error;
+    fn into_result(self) -> Result<Self::Ok, Self::Error>;
+}
+
+impl<T> Try for Option<T> {
+    type Ok = T;
+    type Error = NoneError;
+
+    #[inline]
+    fn into_result(self) -> Result<T, NoneError> {
+        self.ok_or(NoneError)
+    }
+}
+
+impl<T, E> Try for Result<T, E> {
+    type Ok = T;
+    type Error = E;
+
+    #[inline]
+    fn into_result(self) -> Self {
+        self
+    }
+}
diff --git a/embassy-cortex-m/src/interrupt.rs b/embassy-cortex-m/src/interrupt.rs
new file mode 100644
index 000000000..df2aad0ec
--- /dev/null
+++ b/embassy-cortex-m/src/interrupt.rs
@@ -0,0 +1,735 @@
+use atomic_polyfill::{compiler_fence, AtomicPtr, Ordering};
+use core::mem;
+use core::ptr;
+use cortex_m::peripheral::NVIC;
+use embassy_hal_common::Unborrow;
+
+pub use embassy_macros::cortex_m_interrupt_take as take;
+
+/// Implementation detail, do not use outside embassy crates.
+#[doc(hidden)]
+pub struct Handler {
+    pub func: AtomicPtr<()>,
+    pub ctx: AtomicPtr<()>,
+}
+
+impl Handler {
+    pub const fn new() -> Self {
+        Self {
+            func: AtomicPtr::new(ptr::null_mut()),
+            ctx: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub(crate) struct NrWrap(pub(crate) u16);
+unsafe impl cortex_m::interrupt::InterruptNumber for NrWrap {
+    fn number(self) -> u16 {
+        self.0
+    }
+}
+
+pub unsafe trait Interrupt: Unborrow<Target = Self> {
+    fn number(&self) -> u16;
+    unsafe fn steal() -> Self;
+
+    /// Implementation detail, do not use outside embassy crates.
+    #[doc(hidden)]
+    unsafe fn __handler(&self) -> &'static Handler;
+}
+
+pub trait InterruptExt: Interrupt {
+    fn set_handler(&self, func: unsafe fn(*mut ()));
+    fn remove_handler(&self);
+    fn set_handler_context(&self, ctx: *mut ());
+    fn enable(&self);
+    fn disable(&self);
+    #[cfg(not(armv6m))]
+    fn is_active(&self) -> bool;
+    fn is_enabled(&self) -> bool;
+    fn is_pending(&self) -> bool;
+    fn pend(&self);
+    fn unpend(&self);
+    fn get_priority(&self) -> Priority;
+    fn set_priority(&self, prio: Priority);
+}
+
+impl<T: Interrupt + ?Sized> InterruptExt for T {
+    fn set_handler(&self, func: unsafe fn(*mut ())) {
+        compiler_fence(Ordering::SeqCst);
+        let handler = unsafe { self.__handler() };
+        handler.func.store(func as *mut (), Ordering::Relaxed);
+        compiler_fence(Ordering::SeqCst);
+    }
+
+    fn remove_handler(&self) {
+        compiler_fence(Ordering::SeqCst);
+        let handler = unsafe { self.__handler() };
+        handler.func.store(ptr::null_mut(), Ordering::Relaxed);
+        compiler_fence(Ordering::SeqCst);
+    }
+
+    fn set_handler_context(&self, ctx: *mut ()) {
+        let handler = unsafe { self.__handler() };
+        handler.ctx.store(ctx, Ordering::Relaxed);
+    }
+
+    #[inline]
+    fn enable(&self) {
+        compiler_fence(Ordering::SeqCst);
+        unsafe {
+            NVIC::unmask(NrWrap(self.number()));
+        }
+    }
+
+    #[inline]
+    fn disable(&self) {
+        NVIC::mask(NrWrap(self.number()));
+        compiler_fence(Ordering::SeqCst);
+    }
+
+    #[inline]
+    #[cfg(not(armv6m))]
+    fn is_active(&self) -> bool {
+        NVIC::is_active(NrWrap(self.number()))
+    }
+
+    #[inline]
+    fn is_enabled(&self) -> bool {
+        NVIC::is_enabled(NrWrap(self.number()))
+    }
+
+    #[inline]
+    fn is_pending(&self) -> bool {
+        NVIC::is_pending(NrWrap(self.number()))
+    }
+
+    #[inline]
+    fn pend(&self) {
+        NVIC::pend(NrWrap(self.number()))
+    }
+
+    #[inline]
+    fn unpend(&self) {
+        NVIC::unpend(NrWrap(self.number()))
+    }
+
+    #[inline]
+    fn get_priority(&self) -> Priority {
+        Priority::from(NVIC::get_priority(NrWrap(self.number())))
+    }
+
+    #[inline]
+    fn set_priority(&self, prio: Priority) {
+        unsafe {
+            let mut nvic: cortex_m::peripheral::NVIC = mem::transmute(());
+            nvic.set_priority(NrWrap(self.number()), prio.into())
+        }
+    }
+}
+
+impl From<u8> for Priority {
+    fn from(priority: u8) -> Self {
+        unsafe { mem::transmute(priority & PRIO_MASK) }
+    }
+}
+
+impl From<Priority> for u8 {
+    fn from(p: Priority) -> Self {
+        p as u8
+    }
+}
+
+#[cfg(feature = "prio-bits-0")]
+const PRIO_MASK: u8 = 0x00;
+#[cfg(feature = "prio-bits-1")]
+const PRIO_MASK: u8 = 0x80;
+#[cfg(feature = "prio-bits-2")]
+const PRIO_MASK: u8 = 0xc0;
+#[cfg(feature = "prio-bits-3")]
+const PRIO_MASK: u8 = 0xe0;
+#[cfg(feature = "prio-bits-4")]
+const PRIO_MASK: u8 = 0xf0;
+#[cfg(feature = "prio-bits-5")]
+const PRIO_MASK: u8 = 0xf8;
+#[cfg(feature = "prio-bits-6")]
+const PRIO_MASK: u8 = 0xfc;
+#[cfg(feature = "prio-bits-7")]
+const PRIO_MASK: u8 = 0xfe;
+#[cfg(feature = "prio-bits-8")]
+const PRIO_MASK: u8 = 0xff;
+
+#[cfg(feature = "prio-bits-0")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+}
+
+#[cfg(feature = "prio-bits-1")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x80,
+}
+
+#[cfg(feature = "prio-bits-2")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x40,
+    P2 = 0x80,
+    P3 = 0xc0,
+}
+
+#[cfg(feature = "prio-bits-3")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x20,
+    P2 = 0x40,
+    P3 = 0x60,
+    P4 = 0x80,
+    P5 = 0xa0,
+    P6 = 0xc0,
+    P7 = 0xe0,
+}
+
+#[cfg(feature = "prio-bits-4")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x10,
+    P2 = 0x20,
+    P3 = 0x30,
+    P4 = 0x40,
+    P5 = 0x50,
+    P6 = 0x60,
+    P7 = 0x70,
+    P8 = 0x80,
+    P9 = 0x90,
+    P10 = 0xa0,
+    P11 = 0xb0,
+    P12 = 0xc0,
+    P13 = 0xd0,
+    P14 = 0xe0,
+    P15 = 0xf0,
+}
+
+#[cfg(feature = "prio-bits-5")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x8,
+    P2 = 0x10,
+    P3 = 0x18,
+    P4 = 0x20,
+    P5 = 0x28,
+    P6 = 0x30,
+    P7 = 0x38,
+    P8 = 0x40,
+    P9 = 0x48,
+    P10 = 0x50,
+    P11 = 0x58,
+    P12 = 0x60,
+    P13 = 0x68,
+    P14 = 0x70,
+    P15 = 0x78,
+    P16 = 0x80,
+    P17 = 0x88,
+    P18 = 0x90,
+    P19 = 0x98,
+    P20 = 0xa0,
+    P21 = 0xa8,
+    P22 = 0xb0,
+    P23 = 0xb8,
+    P24 = 0xc0,
+    P25 = 0xc8,
+    P26 = 0xd0,
+    P27 = 0xd8,
+    P28 = 0xe0,
+    P29 = 0xe8,
+    P30 = 0xf0,
+    P31 = 0xf8,
+}
+
+#[cfg(feature = "prio-bits-6")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x4,
+    P2 = 0x8,
+    P3 = 0xc,
+    P4 = 0x10,
+    P5 = 0x14,
+    P6 = 0x18,
+    P7 = 0x1c,
+    P8 = 0x20,
+    P9 = 0x24,
+    P10 = 0x28,
+    P11 = 0x2c,
+    P12 = 0x30,
+    P13 = 0x34,
+    P14 = 0x38,
+    P15 = 0x3c,
+    P16 = 0x40,
+    P17 = 0x44,
+    P18 = 0x48,
+    P19 = 0x4c,
+    P20 = 0x50,
+    P21 = 0x54,
+    P22 = 0x58,
+    P23 = 0x5c,
+    P24 = 0x60,
+    P25 = 0x64,
+    P26 = 0x68,
+    P27 = 0x6c,
+    P28 = 0x70,
+    P29 = 0x74,
+    P30 = 0x78,
+    P31 = 0x7c,
+    P32 = 0x80,
+    P33 = 0x84,
+    P34 = 0x88,
+    P35 = 0x8c,
+    P36 = 0x90,
+    P37 = 0x94,
+    P38 = 0x98,
+    P39 = 0x9c,
+    P40 = 0xa0,
+    P41 = 0xa4,
+    P42 = 0xa8,
+    P43 = 0xac,
+    P44 = 0xb0,
+    P45 = 0xb4,
+    P46 = 0xb8,
+    P47 = 0xbc,
+    P48 = 0xc0,
+    P49 = 0xc4,
+    P50 = 0xc8,
+    P51 = 0xcc,
+    P52 = 0xd0,
+    P53 = 0xd4,
+    P54 = 0xd8,
+    P55 = 0xdc,
+    P56 = 0xe0,
+    P57 = 0xe4,
+    P58 = 0xe8,
+    P59 = 0xec,
+    P60 = 0xf0,
+    P61 = 0xf4,
+    P62 = 0xf8,
+    P63 = 0xfc,
+}
+
+#[cfg(feature = "prio-bits-7")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x2,
+    P2 = 0x4,
+    P3 = 0x6,
+    P4 = 0x8,
+    P5 = 0xa,
+    P6 = 0xc,
+    P7 = 0xe,
+    P8 = 0x10,
+    P9 = 0x12,
+    P10 = 0x14,
+    P11 = 0x16,
+    P12 = 0x18,
+    P13 = 0x1a,
+    P14 = 0x1c,
+    P15 = 0x1e,
+    P16 = 0x20,
+    P17 = 0x22,
+    P18 = 0x24,
+    P19 = 0x26,
+    P20 = 0x28,
+    P21 = 0x2a,
+    P22 = 0x2c,
+    P23 = 0x2e,
+    P24 = 0x30,
+    P25 = 0x32,
+    P26 = 0x34,
+    P27 = 0x36,
+    P28 = 0x38,
+    P29 = 0x3a,
+    P30 = 0x3c,
+    P31 = 0x3e,
+    P32 = 0x40,
+    P33 = 0x42,
+    P34 = 0x44,
+    P35 = 0x46,
+    P36 = 0x48,
+    P37 = 0x4a,
+    P38 = 0x4c,
+    P39 = 0x4e,
+    P40 = 0x50,
+    P41 = 0x52,
+    P42 = 0x54,
+    P43 = 0x56,
+    P44 = 0x58,
+    P45 = 0x5a,
+    P46 = 0x5c,
+    P47 = 0x5e,
+    P48 = 0x60,
+    P49 = 0x62,
+    P50 = 0x64,
+    P51 = 0x66,
+    P52 = 0x68,
+    P53 = 0x6a,
+    P54 = 0x6c,
+    P55 = 0x6e,
+    P56 = 0x70,
+    P57 = 0x72,
+    P58 = 0x74,
+    P59 = 0x76,
+    P60 = 0x78,
+    P61 = 0x7a,
+    P62 = 0x7c,
+    P63 = 0x7e,
+    P64 = 0x80,
+    P65 = 0x82,
+    P66 = 0x84,
+    P67 = 0x86,
+    P68 = 0x88,
+    P69 = 0x8a,
+    P70 = 0x8c,
+    P71 = 0x8e,
+    P72 = 0x90,
+    P73 = 0x92,
+    P74 = 0x94,
+    P75 = 0x96,
+    P76 = 0x98,
+    P77 = 0x9a,
+    P78 = 0x9c,
+    P79 = 0x9e,
+    P80 = 0xa0,
+    P81 = 0xa2,
+    P82 = 0xa4,
+    P83 = 0xa6,
+    P84 = 0xa8,
+    P85 = 0xaa,
+    P86 = 0xac,
+    P87 = 0xae,
+    P88 = 0xb0,
+    P89 = 0xb2,
+    P90 = 0xb4,
+    P91 = 0xb6,
+    P92 = 0xb8,
+    P93 = 0xba,
+    P94 = 0xbc,
+    P95 = 0xbe,
+    P96 = 0xc0,
+    P97 = 0xc2,
+    P98 = 0xc4,
+    P99 = 0xc6,
+    P100 = 0xc8,
+    P101 = 0xca,
+    P102 = 0xcc,
+    P103 = 0xce,
+    P104 = 0xd0,
+    P105 = 0xd2,
+    P106 = 0xd4,
+    P107 = 0xd6,
+    P108 = 0xd8,
+    P109 = 0xda,
+    P110 = 0xdc,
+    P111 = 0xde,
+    P112 = 0xe0,
+    P113 = 0xe2,
+    P114 = 0xe4,
+    P115 = 0xe6,
+    P116 = 0xe8,
+    P117 = 0xea,
+    P118 = 0xec,
+    P119 = 0xee,
+    P120 = 0xf0,
+    P121 = 0xf2,
+    P122 = 0xf4,
+    P123 = 0xf6,
+    P124 = 0xf8,
+    P125 = 0xfa,
+    P126 = 0xfc,
+    P127 = 0xfe,
+}
+
+#[cfg(feature = "prio-bits-8")]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[repr(u8)]
+pub enum Priority {
+    P0 = 0x0,
+    P1 = 0x1,
+    P2 = 0x2,
+    P3 = 0x3,
+    P4 = 0x4,
+    P5 = 0x5,
+    P6 = 0x6,
+    P7 = 0x7,
+    P8 = 0x8,
+    P9 = 0x9,
+    P10 = 0xa,
+    P11 = 0xb,
+    P12 = 0xc,
+    P13 = 0xd,
+    P14 = 0xe,
+    P15 = 0xf,
+    P16 = 0x10,
+    P17 = 0x11,
+    P18 = 0x12,
+    P19 = 0x13,
+    P20 = 0x14,
+    P21 = 0x15,
+    P22 = 0x16,
+    P23 = 0x17,
+    P24 = 0x18,
+    P25 = 0x19,
+    P26 = 0x1a,
+    P27 = 0x1b,
+    P28 = 0x1c,
+    P29 = 0x1d,
+    P30 = 0x1e,
+    P31 = 0x1f,
+    P32 = 0x20,
+    P33 = 0x21,
+    P34 = 0x22,
+    P35 = 0x23,
+    P36 = 0x24,
+    P37 = 0x25,
+    P38 = 0x26,
+    P39 = 0x27,
+    P40 = 0x28,
+    P41 = 0x29,
+    P42 = 0x2a,
+    P43 = 0x2b,
+    P44 = 0x2c,
+    P45 = 0x2d,
+    P46 = 0x2e,
+    P47 = 0x2f,
+    P48 = 0x30,
+    P49 = 0x31,
+    P50 = 0x32,
+    P51 = 0x33,
+    P52 = 0x34,
+    P53 = 0x35,
+    P54 = 0x36,
+    P55 = 0x37,
+    P56 = 0x38,
+    P57 = 0x39,
+    P58 = 0x3a,
+    P59 = 0x3b,
+    P60 = 0x3c,
+    P61 = 0x3d,
+    P62 = 0x3e,
+    P63 = 0x3f,
+    P64 = 0x40,
+    P65 = 0x41,
+    P66 = 0x42,
+    P67 = 0x43,
+    P68 = 0x44,
+    P69 = 0x45,
+    P70 = 0x46,
+    P71 = 0x47,
+    P72 = 0x48,
+    P73 = 0x49,
+    P74 = 0x4a,
+    P75 = 0x4b,
+    P76 = 0x4c,
+    P77 = 0x4d,
+    P78 = 0x4e,
+    P79 = 0x4f,
+    P80 = 0x50,
+    P81 = 0x51,
+    P82 = 0x52,
+    P83 = 0x53,
+    P84 = 0x54,
+    P85 = 0x55,
+    P86 = 0x56,
+    P87 = 0x57,
+    P88 = 0x58,
+    P89 = 0x59,
+    P90 = 0x5a,
+    P91 = 0x5b,
+    P92 = 0x5c,
+    P93 = 0x5d,
+    P94 = 0x5e,
+    P95 = 0x5f,
+    P96 = 0x60,
+    P97 = 0x61,
+    P98 = 0x62,
+    P99 = 0x63,
+    P100 = 0x64,
+    P101 = 0x65,
+    P102 = 0x66,
+    P103 = 0x67,
+    P104 = 0x68,
+    P105 = 0x69,
+    P106 = 0x6a,
+    P107 = 0x6b,
+    P108 = 0x6c,
+    P109 = 0x6d,
+    P110 = 0x6e,
+    P111 = 0x6f,
+    P112 = 0x70,
+    P113 = 0x71,
+    P114 = 0x72,
+    P115 = 0x73,
+    P116 = 0x74,
+    P117 = 0x75,
+    P118 = 0x76,
+    P119 = 0x77,
+    P120 = 0x78,
+    P121 = 0x79,
+    P122 = 0x7a,
+    P123 = 0x7b,
+    P124 = 0x7c,
+    P125 = 0x7d,
+    P126 = 0x7e,
+    P127 = 0x7f,
+    P128 = 0x80,
+    P129 = 0x81,
+    P130 = 0x82,
+    P131 = 0x83,
+    P132 = 0x84,
+    P133 = 0x85,
+    P134 = 0x86,
+    P135 = 0x87,
+    P136 = 0x88,
+    P137 = 0x89,
+    P138 = 0x8a,
+    P139 = 0x8b,
+    P140 = 0x8c,
+    P141 = 0x8d,
+    P142 = 0x8e,
+    P143 = 0x8f,
+    P144 = 0x90,
+    P145 = 0x91,
+    P146 = 0x92,
+    P147 = 0x93,
+    P148 = 0x94,
+    P149 = 0x95,
+    P150 = 0x96,
+    P151 = 0x97,
+    P152 = 0x98,
+    P153 = 0x99,
+    P154 = 0x9a,
+    P155 = 0x9b,
+    P156 = 0x9c,
+    P157 = 0x9d,
+    P158 = 0x9e,
+    P159 = 0x9f,
+    P160 = 0xa0,
+    P161 = 0xa1,
+    P162 = 0xa2,
+    P163 = 0xa3,
+    P164 = 0xa4,
+    P165 = 0xa5,
+    P166 = 0xa6,
+    P167 = 0xa7,
+    P168 = 0xa8,
+    P169 = 0xa9,
+    P170 = 0xaa,
+    P171 = 0xab,
+    P172 = 0xac,
+    P173 = 0xad,
+    P174 = 0xae,
+    P175 = 0xaf,
+    P176 = 0xb0,
+    P177 = 0xb1,
+    P178 = 0xb2,
+    P179 = 0xb3,
+    P180 = 0xb4,
+    P181 = 0xb5,
+    P182 = 0xb6,
+    P183 = 0xb7,
+    P184 = 0xb8,
+    P185 = 0xb9,
+    P186 = 0xba,
+    P187 = 0xbb,
+    P188 = 0xbc,
+    P189 = 0xbd,
+    P190 = 0xbe,
+    P191 = 0xbf,
+    P192 = 0xc0,
+    P193 = 0xc1,
+    P194 = 0xc2,
+    P195 = 0xc3,
+    P196 = 0xc4,
+    P197 = 0xc5,
+    P198 = 0xc6,
+    P199 = 0xc7,
+    P200 = 0xc8,
+    P201 = 0xc9,
+    P202 = 0xca,
+    P203 = 0xcb,
+    P204 = 0xcc,
+    P205 = 0xcd,
+    P206 = 0xce,
+    P207 = 0xcf,
+    P208 = 0xd0,
+    P209 = 0xd1,
+    P210 = 0xd2,
+    P211 = 0xd3,
+    P212 = 0xd4,
+    P213 = 0xd5,
+    P214 = 0xd6,
+    P215 = 0xd7,
+    P216 = 0xd8,
+    P217 = 0xd9,
+    P218 = 0xda,
+    P219 = 0xdb,
+    P220 = 0xdc,
+    P221 = 0xdd,
+    P222 = 0xde,
+    P223 = 0xdf,
+    P224 = 0xe0,
+    P225 = 0xe1,
+    P226 = 0xe2,
+    P227 = 0xe3,
+    P228 = 0xe4,
+    P229 = 0xe5,
+    P230 = 0xe6,
+    P231 = 0xe7,
+    P232 = 0xe8,
+    P233 = 0xe9,
+    P234 = 0xea,
+    P235 = 0xeb,
+    P236 = 0xec,
+    P237 = 0xed,
+    P238 = 0xee,
+    P239 = 0xef,
+    P240 = 0xf0,
+    P241 = 0xf1,
+    P242 = 0xf2,
+    P243 = 0xf3,
+    P244 = 0xf4,
+    P245 = 0xf5,
+    P246 = 0xf6,
+    P247 = 0xf7,
+    P248 = 0xf8,
+    P249 = 0xf9,
+    P250 = 0xfa,
+    P251 = 0xfb,
+    P252 = 0xfc,
+    P253 = 0xfd,
+    P254 = 0xfe,
+    P255 = 0xff,
+}
diff --git a/embassy-cortex-m/src/lib.rs b/embassy-cortex-m/src/lib.rs
new file mode 100644
index 000000000..143c56f39
--- /dev/null
+++ b/embassy-cortex-m/src/lib.rs
@@ -0,0 +1,8 @@
+#![no_std]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+pub mod executor;
+pub mod interrupt;
+pub mod peripheral;
diff --git a/embassy-cortex-m/src/peripheral.rs b/embassy-cortex-m/src/peripheral.rs
new file mode 100644
index 000000000..40277691c
--- /dev/null
+++ b/embassy-cortex-m/src/peripheral.rs
@@ -0,0 +1,134 @@
+use core::marker::PhantomData;
+use core::mem::MaybeUninit;
+use cortex_m::peripheral::scb::VectActive;
+use cortex_m::peripheral::{NVIC, SCB};
+
+use crate::interrupt::{Interrupt, InterruptExt, Priority};
+
+/// A type which can be used as state with `PeripheralMutex`.
+///
+/// It needs to be `Send` because `&mut` references are sent back and forth between the 'thread' which owns the `PeripheralMutex` and the interrupt,
+/// and `&mut T` is only `Send` where `T: Send`.
+pub trait PeripheralState: Send {
+    type Interrupt: Interrupt;
+    fn on_interrupt(&mut self);
+}
+
+pub struct StateStorage<S>(MaybeUninit<S>);
+
+impl<S> StateStorage<S> {
+    pub const fn new() -> Self {
+        Self(MaybeUninit::uninit())
+    }
+}
+
+pub struct PeripheralMutex<'a, S: PeripheralState> {
+    state: *mut S,
+    _phantom: PhantomData<&'a mut S>,
+    irq: S::Interrupt,
+}
+
+/// Whether `irq` can be preempted by the current interrupt.
+pub(crate) fn can_be_preempted(irq: &impl Interrupt) -> bool {
+    match SCB::vect_active() {
+        // Thread mode can't preempt anything.
+        VectActive::ThreadMode => false,
+        // Exceptions don't always preempt interrupts,
+        // but there isn't much of a good reason to be keeping a `PeripheralMutex` in an exception anyway.
+        VectActive::Exception(_) => true,
+        VectActive::Interrupt { irqn } => {
+            #[derive(Clone, Copy)]
+            struct NrWrap(u16);
+            unsafe impl cortex_m::interrupt::InterruptNumber for NrWrap {
+                fn number(self) -> u16 {
+                    self.0
+                }
+            }
+            NVIC::get_priority(NrWrap(irqn.into())) < irq.get_priority().into()
+        }
+    }
+}
+
+impl<'a, S: PeripheralState> PeripheralMutex<'a, S> {
+    /// Create a new `PeripheralMutex` wrapping `irq`, with `init` initializing the initial state.
+    ///
+    /// Registers `on_interrupt` as the `irq`'s handler, and enables it.
+    pub fn new(
+        irq: S::Interrupt,
+        storage: &'a mut StateStorage<S>,
+        init: impl FnOnce() -> S,
+    ) -> Self {
+        if can_be_preempted(&irq) {
+            panic!("`PeripheralMutex` cannot be created in an interrupt with higher priority than the interrupt it wraps");
+        }
+
+        let state_ptr = storage.0.as_mut_ptr();
+
+        // Safety: The pointer is valid and not used by anyone else
+        // because we have the `&mut StateStorage`.
+        unsafe { state_ptr.write(init()) };
+
+        irq.disable();
+        irq.set_handler(|p| unsafe {
+            // Safety: it's OK to get a &mut to the state, since
+            // - We checked that the thread owning the `PeripheralMutex` can't preempt us in `new`.
+            //   Interrupts' priorities can only be changed with raw embassy `Interrupts`,
+            //   which can't safely store a `PeripheralMutex` across invocations.
+            // - We can't have preempted a with() call because the irq is disabled during it.
+            let state = &mut *(p as *mut S);
+            state.on_interrupt();
+        });
+        irq.set_handler_context(state_ptr as *mut ());
+        irq.enable();
+
+        Self {
+            irq,
+            state: state_ptr,
+            _phantom: PhantomData,
+        }
+    }
+
+    pub fn with<R>(&mut self, f: impl FnOnce(&mut S) -> R) -> R {
+        self.irq.disable();
+
+        // Safety: it's OK to get a &mut to the state, since the irq is disabled.
+        let state = unsafe { &mut *self.state };
+        let r = f(state);
+
+        self.irq.enable();
+
+        r
+    }
+
+    /// Returns whether the wrapped interrupt is currently in a pending state.
+    pub fn is_pending(&self) -> bool {
+        self.irq.is_pending()
+    }
+
+    /// Forces the wrapped interrupt into a pending state.
+    pub fn pend(&self) {
+        self.irq.pend()
+    }
+
+    /// Forces the wrapped interrupt out of a pending state.
+    pub fn unpend(&self) {
+        self.irq.unpend()
+    }
+
+    /// Gets the priority of the wrapped interrupt.
+    pub fn priority(&self) -> Priority {
+        self.irq.get_priority()
+    }
+}
+
+impl<'a, S: PeripheralState> Drop for PeripheralMutex<'a, S> {
+    fn drop(&mut self) {
+        self.irq.disable();
+        self.irq.remove_handler();
+
+        // safety:
+        // - we initialized the state in `new`, so we know it's initialized.
+        // - the irq is disabled, so it won't preempt us while dropping.
+        unsafe { self.state.drop_in_place() }
+    }
+}