rtic/book/en/src/by-example/tips_from_ram.md

Running tasks from RAM

The main goal of moving the specification of RTIC applications to attributes in RTIC v0.4.0 was to allow inter-operation with other attributes. For example, the link_section attribute can be applied to tasks to place them in RAM; this can improve performance in some cases.

IMPORTANT: In general, the link_section, export_name and no_mangle attributes are powerful but also easy to misuse. Incorrectly using any of these attributes can cause undefined behavior; you should always prefer to use safe, higher level attributes around them like cortex-m-rt's interrupt and exception attributes.

In the particular case of RAM functions there's no safe abstraction for it in cortex-m-rt v0.6.5 but there's an RFC for adding a ramfunc attribute in a future release.

The example below shows how to place the higher priority task, bar, in RAM.

{{#include ../../../../examples/ramfunc.rs}}

Running this program produces the expected output.

$ cargo run --target thumbv7m-none-eabi --example ramfunc
{{#include ../../../../ci/expected/ramfunc.run}}

One can look at the output of cargo-nm to confirm that bar ended in RAM (0x2000_0000), whereas foo ended in Flash (0x0000_0000).

$ cargo nm --example ramfunc --release | grep ' foo::'
{{#include ../../../../ci/expected/ramfunc.run.grep.foo}}

$ cargo nm --example ramfunc --release | grep ' bar::'
{{#include ../../../../ci/expected/ramfunc.run.grep.bar}}