Build it

The first step is to build our "binary" crate. Because the microcontroller has a different architecture than your computer we'll have to cross compile. Cross compiling in Rust land is as simple as passing an extra --target flag to rustcor Cargo. The complicated part is figuring out the argument of that flag: the name of the target.

As we already know the microcontroller on the micro:bit v2 has a Cortex-M4F processor in it, the one on v1 a Cortex-M0. rustc knows how to cross-compile to the Cortex-M architecture and provides several different targets that cover the different processors families within that architecture:

thumbv6m-none-eabi, for the Cortex-M0 and Cortex-M1 processors
thumbv7m-none-eabi, for the Cortex-M3 processor
thumbv7em-none-eabi, for the Cortex-M4 and Cortex-M7 processors
thumbv7em-none-eabihf, for the Cortex-M4F and Cortex-M7F processors
thumbv8m.main-none-eabi, for the Cortex-M33 and Cortex-M35P processors
thumbv8m.main-none-eabihf, for the Cortex-M33F and Cortex-M35PF processors

For the micro:bit v2, we'll use the thumbv7em-none-eabihf target, for v1 the thumbv6m-none-eabi one. Before cross-compiling you have to download a pre-compiled version of the standard library (a reduced version of it, actually) for your target. That's done using rustup:

# For micro:bit v2
$ rustup target add thumbv7em-none-eabihf
# For micro:bit v1
$ rustup target add thumbv6m-none-eabi

You only need to do the above step once; rustup will re-install a new standard library (rust-std component) whenever you update your toolchain. Therefore you can skip this step, if you have already added the necessary target while verifying your setup.

With the rust-std component in place you can now cross compile the program using Cargo:

# make sure you are in the `src/05-led-roulette` directory

# For micro:bit v2
$ cargo build --features v2 --target thumbv7em-none-eabihf
   Compiling semver-parser v0.7.0
   Compiling typenum v1.12.0
   Compiling cortex-m v0.6.3
   (...)
   Compiling microbit-v2 v0.10.1
    Finished dev [unoptimized + debuginfo] target(s) in 33.67s

# For micro:bit v1
$ cargo build --features v1 --target thumbv6m-none-eabi
   Compiling fixed v1.2.0
   Compiling syn v1.0.39
   Compiling cortex-m v0.6.3
   (...)
   Compiling microbit v0.10.1
	Finished dev [unoptimized + debuginfo] target(s) in 22.73s

NOTE Be sure to compile this crate without optimizations. The provided Cargo.toml file and build command above will ensure optimizations are off.

OK, now we have produced an executable. This executable won't blink any LEDs, it's just a simplified version that we will build upon later in the chapter. As a sanity check, let's verify that the produced executable is actually an ARM binary:

# For micro:bit v2
# equivalent to `readelf -h target/thumbv7em-none-eabihf/debug/led-roulette`
$ cargo readobj --features v2 --target thumbv7em-none-eabihf --bin led-roulette -- --file-headers
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0x117
  Start of program headers:          52 (bytes into file)
  Start of section headers:          793112 (bytes into file)
  Flags:                             0x5000400
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         4
  Size of section headers:           40 (bytes)
  Number of section headers:         21
  Section header string table index: 19

# For micro:bit v1
# equivalent to `readelf -h target/thumbv6m-none-eabi/debug/led-roulette`
$ cargo readobj --features v1 --target thumbv6m-none-eabi --bin led-roulette -- --file-headers
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0xC1
  Start of program headers:          52 (bytes into file)
  Start of section headers:          693196 (bytes into file)
  Flags:                             0x5000200
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         4
  Size of section headers:           40 (bytes)
  Number of section headers:         22
  Section header string table index: 20

Next, we'll flash the program into our microcontroller.

Discovery

Build it