Building an Embedded Program

The default in a Cargo project is to compile for the host (native compilation). The nrf52-code/radio-app project has been configured for cross compilation to the ARM Cortex-M4 architecture. This configuration can be seen in the Cargo configuration file (.cargo/config):

# .cargo/config
[build]
target = "thumbv7em-none-eabihf" # = ARM Cortex-M4

The target thumbv7em-none-eabihf can be broken down as:

• thumbv7em - we generate instructions for the Armv7E-M architecture running in Thumb-2 mode (actually the only supported mode on this architecture)
• none - there is no Operating System
• eabihf - use the ARM Embedded Application Binary Interface, with Hard Float support
  • f32 and f64 can be passed to functions in FPU registers (like S0), instead of in integer registers (like R0)

✅ Inside the folder nrf52-code/radio-app, use the following command to cross compile the program to the ARM Cortex-M4 architecture.

cargo build --bin hello

It should be noted that building the application for this target also requires the thumbv7em-none-eabihf target. Normally, you would have to add this target to cargo by using rustup target add thumbv7em-none-eabihf, but we provide a rust-toolchain.toml that is used to cargo to determine which toolchains should be installed automatically.

The output of the compilation process will be an ELF (Executable and Linkable Format) file. The file will be placed in the target/thumbv7em-none-eabihf directory.

✅ Run $ file target/thumbv7em-none-eabihf/debug/hello and compare if your output is as expected.

Expected output:

$ file target/thumbv7em-none-eabihf/debug/hello
hello: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, with debug_info, not stripped