Add example usage

Author: bjoernQ

riscv-rt/src/lib.rs

@@ -607,6 +607,40 @@
 //!
 //! It is expected that the custom code does not clobber any registers.
 //!
+//! ### Example
+//!
+//! ```rust,no_run
+//! core::arch::global_asm!(
+//! r#"
+//! .section .trap.start, "ax"
+//! .extern _pre_default_start_trap_ret
+//! .global _pre_default_start_trap
+//!
+//! // with the pre-default-start-trap feature enabled this code is executed before
+//! // the code in _default_start_trap
+//! _pre_default_start_trap:
+//!     // move SP to some save place if it's pointing below the RAM
+//!     // otherwise we won't be able to do anything reasonable
+//!     // (since we don't have a useable stack otherwise)
+//!     //
+//!     // most probably we will just print something and halt in this case
+//!     // we actually can't do anything else
+//!     csrw mscratch, t0
+//!     la t0, _dram_origin
+//!     bge sp, t0, 1f
+//!
+//!     // set SP to the start of the stack
+//!     la sp, _stack_start
+//!
+//!     1:
+//!     // remember to not clobber any registers, restore t0 from mscratch
+//!     csrr t0, mscratch
+//!
+//!     // jump back to continue with _default_start_trap
+//!     j _pre_default_start_trap_ret
+//! "#
+//! );
+//! ```
 //! [attr-entry]: attr.entry.html
 //! [attr-exception]: attr.exception.html
 //! [attr-external-interrupt]: attr.external_interrupt.html