add an example of overriding an exception

closes #2

examples/exception-override.rs

@@ -0,0 +1,42 @@
+//! Overriding an exception
+//!
+//! **NOTE** You have to disable the `cortex-m-rt` crate "exceptions" feature to
+//! make this work.
+
+#![feature(used)]
+#![no_std]
+
+extern crate cortex_m;
+extern crate cortex_m_rt;
+extern crate {{name}};
+
+use core::ptr;
+
+use cortex_m::{asm, exception};
+use {{name}}::interrupt;
+
+fn main() {
+    unsafe {
+        // Invalid memory access
+        ptr::read_volatile(0x2FFF_FFFF as *const u32);
+    }
+}
+
+extern "C" fn hard_fault(_: exception::HardFault) {
+    // You'll hit this breakpoint rather than the one in cortex-m-rt
+    asm::bkpt()
+}
+
+#[allow(dead_code)]
+#[used]
+#[link_section = ".rodata.exceptions"]
+static EXCEPTIONS: exception::Handlers = exception::Handlers {
+    hard_fault: hard_fault,
+    ..exception::DEFAULT_HANDLERS
+};
+
+#[allow(dead_code)]
+#[used]
+#[link_section = ".rodata.interrupts"]
+static INTERRUPTS: interrupt::Handlers =
+    interrupt::Handlers { ..interrupt::DEFAULT_HANDLERS };