Add documentation, test and changelog entry for zero-init-ram feature

Author: Norbert Fabritius

cortex-m-rt/CHANGELOG.md

@@ -7,14 +7,14 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
-## [v0.7.3]
-
 - Fixed a potential miscompilation caused by the initial stack pointer
   not being 8-byte aligned. This issue affected 0.7.1 and 0.7.2; for
   more details please see [the advisory] ([#467]).
 - A linker error is generated if the initial stack pointer is not 8-byte aligned ([#464]).
 - The initial stack pointer is now forced to be 8-byte aligned in the linker script,
   to defend against it being overridden outside of the cortex-m-rt linker script ([#465]).
+- Add `zero-init-ram` feature to initialize RAM with zeros on startup. This can be necessary on
+  safety-critical hardware to properly initialize memory integrity measures.
 
 [the advisory]: https://github.com/rust-embedded/cortex-m/discussions/469
 [#464]: https://github.com/rust-embedded/cortex-m/issues/464

cortex-m-rt/ci/script.sh

@@ -63,6 +63,8 @@ main() {
 
             cargo rustc --target "$TARGET" --example minimal --features "set-sp,${needed_features}" -- $linker
             cargo rustc --target "$TARGET" --example minimal --features "set-sp,${needed_features}" --release -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "zero-init-ram,${needed_features}" -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "zero-init-ram,${needed_features}" --release -- $linker
             cargo rustc --target "$TARGET" --example minimal --features "set-vtor,${needed_features}" -- $linker
             cargo rustc --target "$TARGET" --example minimal --features "set-vtor,${needed_features}" --release -- $linker
         done

cortex-m-rt/src/lib.rs

@@ -169,6 +169,13 @@
 //! `_stack_start` value from the linker script. This is not usually required, but some debuggers
 //! do not initialise SP when performing a soft reset, which can lead to stack corruption.
 //!
+//! ## `zero-init-ram`
+//!
+//! If this feature is enabled, RAM is initialized with zeros during startup from the `_ram_start`
+//! value to the `_ram_end` value from the linker script. This is not usually required, but might be
+//! necessary to properly initialize checksum-based memory integrity measures on safety-critical
+//! hardware.
+//!
 //! ## `set-vtor`
 //!
 //! If this feature is enabled, the vector table offset register (VTOR) is initialised in the reset