# Xous Stage 1 Loader

This contains the stage1 for Xous.  This program is responsible for
parsing the initial boot arguments, setting up memory maps, and
preparing the kernel and its initial programs to run.

This program runs in **Machine** mode and has access to the entire
system.  When it jumps to the kernel, the system will be in
**Supervisor** mode with no way to return to **Machine** mode.

## Building

To build the stage-1 module, you will need a `riscv32i-unknown-none-elf`
or `riscv32imac-unknown-none-elf` target for Rust.

1. Decide what target you want.  For simple, embedded systems this could
   be `riscv32i-unknown-none-elf`, and for more complex systems with
   compressed instructions you could use `riscv32imac-unknown-none-elf`.
2. Get Rust.  Go to https://rustup.rs/ and follow its instructions.
3. Install the proper toolchain: `rustup target add ${target_arch}`
4. Build the stage-1 loader: `cargo build --release --target
   ${target_arch}`
5. Get an executable binary: `riscv64-unknown-elf-gcc -O binary
   target/${target}/xous-stage1 xous-stage1.bin`
   
If you have made changes to the [assembly trampoline](https://github.com/betrusted-io/xous-core/blob/master/loader/src/asm.S) which sets up
the loader, you will need to manually rebuild this, by running [assemble.sh](https://github.com/betrusted-io/xous-core/blob/master/loader/assemble.sh).
This will replace a set of objects in the [`bin`](https://github.com/betrusted-io/xous-core/tree/master/loader/bin) directory.

## Using

This requires additional components in order to function:

1. A kernel to run, and optionally additional init processes
2. A kernel arguments structure to indicate memory regions
3. A stage-0 bootloader to set up the arguments

The stage-0 bootloader needs to load the kernel arguments structure into
memory at some address, then set register `$a0` (also known as `$x10` or
`R10` in RISC-V) to point there.  It then needs to jump to that address.

For example, in [Renode](https://github.com/renode/renode/), this is
accomplished with the following commands:

```
    sysbus LoadELF @../stage1/target/riscv32i-unknown-none-elf/debug/xous-stage1
    sysbus LoadBinary @../tools/args.bin 0x40800000
    # Set $a0 to point at the args binary
    cpu SetRegisterUnsafe 10 0x40800000
    cpu PC 0x20500000
```

There is no additional setup required.

## Testing

Because `xous-stage1` spends much of its time setting up pagetables for
RV32, it depends on a system with 32-bit pointers.  If you attempt to
run it on a system with 64-bit pointers, it will panic.

The easy solution is to install a 32-bit toolchain and test with that.
To get a list of possible toolchains, run `rustc --print target-list`
and pick one that starts with `i686-`, `armv7-`, or some other 32-bit
arch.

```sh
$ rustup target add i686-unknown-linux-musl
15:26:39 info: downloading component 'rust-std' for 'i686-unknown-linux-musl'
15:26:41 info: installing component 'rust-std' for 'i686-unknown-linux-musl'
$
```

To run tests, use `cargo --test`.  For example:

```sh
❯ cargo test --target i686-pc-windows-msvc
   Compiling lazy_static v1.4.0
   Compiling xous-stage1 v0.1.0 (D:\\Code\\Xous\\stage1)
    Finished test [unoptimized + debuginfo] target(s) in 6.32s
     Running target\\i686-pc-windows-msvc\\debug\\deps\\xous_stage1-4e16f995c7c21520.exe

running 6 tests
test test::copy_processes ... ok
test test::full_boot ... ok
test test::allocate_regions ... ok
test test::parse_args_bin ... ok
test test::read_initial_config ... ok
test test::tracker_sane ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

❯
```

## Contribution Guidelines

[![Contributor
Covenant](https://img.shields.io/badge/Contributor%20Covenant-v2.0%20adopted-ff69b4.svg)](../CODE_OF_CONDUCT.md)

Please see [CONTRIBUTING](../CONTRIBUTING.md) for details on how to make a
contribution.

Please note that this project is released with a [Contributor Code of
Conduct](../CODE_OF_CONDUCT.md). By participating in this project you agree
to abide its terms.

## License

Copyright © 2020

Licensed under the [Apache License
2.0](http://opensource.org/licenses/Apache-2.0) [LICENSE](LICENSE)