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# riscv64i `#[no_std]` Linux user space example
This repository serves to my later self as reference to document some
`#[no_std]` and inline assembly features as well as cargo configurations.
It builds a `riscv64i` Linux user space binary which invokes some syscalls
according to the Linux [syscall(2)
ABI](https://man7.org/linux/man-pages/man2/syscall.2.html) for riscv.
```txt
Arch/ABI Instruction System Ret Ret Error Notes
call # val val2
───────────────────────────────────────────────────────────────────
riscv ecall a7 a0 a1 -
Arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
──────────────────────────────────────────────────────────────
riscv a0 a1 a2 a3 a4 a5 -
```
The syscalls are implemented in [lib.rs](src/lib.rs).
When building, cargo runs the [build.rs](build.rs) script before building the
crate itself, which in this case generates some rust code for the syscall
numbers from the riscv C headers.
Looking into [.cargo/config](.cargo/config) we actually build for the
`riscv64imac-unknown-none-elf` rather than the `riscv64gc-unknown-linux-gnu`
target, as our goal is to build a `risv64*i*` binary and that way we just have
to disable a few more extensions :^).
The binary can be automatically run in the QEMU user space emulator with `cargo
run`, this is because the `runner` is specified in
[.cargo/config](.cargo/config).
## Requirements
- `riscv64-linux-gnu-gcc` toolchain for parsing syscall numbers in
[build.rs](build.rs)
- `riscv64imac-unknown-none-elf` rustc target to generate riscv target code
(`rustup target add riscv64imac-unknown-none-elf`)
- `qemu-riscv64` QEMU userspace emulator
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