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. 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

"Thumb" here refers to a version of the Arm instruction set that has smaller instructions for reduced code size (it's a pun, see). The hf/F parts have hardware floating point acceleration. This will make numeric computations involving fractional ("floating decimal point") computations much faster.

For the micro:bit v2, we'll want the thumbv7em-none-eabihf target.

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:

$ rustup target add thumbv7em-none-eabihf

You only need to do the above step once; rustup will then update this target (re-installing a new standard library rust-std component that contains the core library we use) 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 mdbook/src/08-meet-your-software directory in the Git repo, then build. This initial code is an example, so we compile it as such.

$ cargo build --example init
   Compiling semver-parser v0.7.0
   Compiling proc-macro2 v1.0.86
   ...

    Finished dev [unoptimized + debuginfo] target(s) in 33.67s

NOTE Be sure to compile this crate without optimizations. The provided Cargo.toml file and build command above will ensure optimizations are off as long as you don't pass cargo the --release flag.

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. (The command below is equivalent to

readelf -h ../../../target/thumbv7em-none-eabihf/debug/examples/init

on systems that have readelf.)

$ cargo readobj --example init -- --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

If your numbers don't exactly match these, don't worry: a lot of this is quite dependent on the current build environment.

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

Discovery

Build it