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rtic/book/en/archive/by_example/tips/from_ram.md
Franz Dietrich 53ed7bf7ed
fix included examples and markdown(book) (#912) 
* fix included examples and markdown(book)

fixes: #911

* fix footnote pre_init

* more example link updates

* Restore pool example name

* Example: pool: Upgrade to heapless v0.8

* Example: pool: thumbv6 unsupported: wild cfg-if

Experiment with multi-backend example contained in the example

* Example: lm3s6965: Updated cargo.lock

* Book: Use cargo xtask for by-example

* Docs: Contributing: cargo xtask

---------

Co-authored-by: Henrik Tjäder <henrik@tjaders.com>
2024-04-03 22:01:46 +00:00

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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/lm3s6965/examples/ramfunc.rs}}

Running this program produces the expected output.

$ cargo xtask qemu --verbose --example ramfunc

{{#include ../../../../../ci/expected/lm3s6965/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/lm3s6965/ramfunc.run.grep.foo}}

$ cargo nm --example ramfunc  --target thumbv7m-none-eabi --release | grep '*bar::'

{{#include ../../../../../ci/expected/lm3s6965/ramfunc.run.grep.bar}}