feat(port_std): support interrupts

I feel the current implementation is under-documented but it should be
at least enough for running tests.

Author: yvt

README.md

@@ -20,7 +20,7 @@ Constance is a proof-of-concept of a static RTOS that utilizes Rust's compile-ti
 | ☐ Mutexes        | **Tools**       | **Boards**        |
 | ☐ Timer          | ☑︎ Test Harness  | ☑︎ Hosted          |
 | ☐ Alarm          | ☑︎ Test Suite    | ☐ F401RE          |
-| ☐ Interrupts     | ☑︎ Configurator  |                   |
+| ☑︎ Interrupts     | ☑︎ Configurator  |                   |
 | ☐ CPU Exceptions |                 |                   |
 | ☐ Panicking      |                 |                   |
 

src/constance_port_std/src/lib.md

@@ -30,3 +30,26 @@ fn task_body(_: usize) {
     log::warn!("yay");
 }
 ```
+
+# Interrupts
+
+This port fully supports [the standard interrupt handling framework].
+
+ - The full range of priority values is available. The default priority is `0`.
+ - The simulated hardware exposes `1024` (= [`NUM_INTERRUPT_LINES`]) interrupt
+   lines.
+ - Smaller priority values are prioritized.
+ - Negative priority values are considered unmanaged.
+
+[the standard interrupt handling framework]: ::constance#interrupt-handling-framework
+[`NUM_INTERRUPT_LINES`]: crate::NUM_INTERRUPT_LINES
+
+## Implementation
+
+All interrupt handlers execute in the main thread. Whenever an interrupt is pended or enabled, preemption checking code will run, and under the right condition, will yield the control to the dispatcher.
+
+The dispatcher loop handles top-level interrupts and calls the interrupt handlers directly.
+
+In an interrupt handler, the preemption checking code handles nested interrupts and calls their interrupt handlers.
+
+**To be implemented:** True asynchronous interrupts aren't supported yet.