Crate riscv_minimal_rt

Source
Expand description

Minimal startup / runtime for RISC-V CPU’s

§Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.31 and up. It might compile with older versions but that may change in any new patch release. Note that riscv64imac-unknown-none-elf and riscv64gc-unknown-none-elf targets are not supported on stable yet.

§Features

This crate provides

  • Before main initialization of the .bss and .data sections.

  • Before main initialization of the FPU (for targets that have a FPU).

  • #[entry] to declare the entry point of the program

  • #[pre_init] to run code before static variables are initialized

  • A linker script that encodes the memory layout of a generic RISC-V microcontroller. This linker script is missing some information that must be supplied through a memory.x file (see example below).

  • A _sheap symbol at whose address you can locate a heap.

$ cargo new --bin app && cd $_

$ # add this crate as a dependency
$ edit Cargo.toml && cat $_
[dependencies]
riscv-minimal-rt = "0.4.0"
panic-halt = "0.2.0"

$ # memory layout of the device
$ edit memory.x && cat $_
MEMORY
{
  /* NOTE K = KiBi = 1024 bytes */
  FLASH : ORIGIN = 0x20000000, LENGTH = 16M
  RAM : ORIGIN = 0x80000000, LENGTH = 16K
}

$ edit src/main.rs && cat $_
ⓘ
#![no_std]
#![no_main]

extern crate panic_halt;

use riscv_rt::entry;

// use `main` as the entry point of this application
// `main` is not allowed to return
#[entry]
fn main() -> ! {
    // do something here
    loop { }
}
$ mkdir .cargo && edit .cargo/config && cat $_
[target.riscv32imac-unknown-none-elf]
rustflags = [
  "-C", "link-arg=-Tlink.x"
]

[build]
target = "riscv32imac-unknown-none-elf"
$ edit build.rs && cat $_
ⓘ
use std::env;
use std::fs::File;
use std::io::Write;
use std::path::Path;

/// Put the linker script somewhere the linker can find it.
fn main() {
    let out_dir = env::var("OUT_DIR").expect("No out dir");
    let dest_path = Path::new(&out_dir);
    let mut f = File::create(&dest_path.join("memory.x"))
        .expect("Could not create file");

    f.write_all(include_bytes!("memory.x"))
        .expect("Could not write file");

    println!("cargo:rustc-link-search={}", dest_path.display());

    println!("cargo:rerun-if-changed=memory.x");
    println!("cargo:rerun-if-changed=build.rs");
}
$ cargo build

$ riscv32-unknown-elf-objdump -Cd $(find target -name app) | head

Disassembly of section .text:

20000000 <_start>:
20000000:	800011b7          	lui	gp,0x80001
20000004:	80018193          	addi	gp,gp,-2048 # 80000800 <_stack_start+0xffffc800>
20000008:	80004137          	lui	sp,0x80004

§Symbol interfaces

This crate makes heavy use of symbols, linker sections and linker scripts to provide most of its functionality. Below are described the main symbol interfaces.

§memory.x

This file supplies the information about the device to the linker.

§MEMORY

The main information that this file must provide is the memory layout of the device in the form of the MEMORY command. The command is documented here, but at a minimum you’ll want to create two memory regions: one for Flash memory and another for RAM.

The program instructions (the .text section) will be stored in the memory region named FLASH, and the program static variables (the sections .bss and .data) will be allocated in the memory region named RAM.

§_stack_start

This symbol provides the address at which the call stack will be allocated. The call stack grows downwards so this address is usually set to the highest valid RAM address plus one (this is an invalid address but the processor will decrement the stack pointer before using its value as an address).

If omitted this symbol value will default to ORIGIN(RAM) + LENGTH(RAM).

§Example

Allocating the call stack on a different RAM region.

MEMORY
{
  /* call stack will go here */
  CCRAM : ORIGIN = 0x10000000, LENGTH = 8K
  FLASH : ORIGIN = 0x08000000, LENGTH = 256K
  /* static variables will go here */
  RAM : ORIGIN = 0x20000000, LENGTH = 40K
}

_stack_start = ORIGIN(CCRAM) + LENGTH(CCRAM);

§_heap_size

This symbol provides the size of a heap region. The default value is 0. You can set _heap_size to a non-zero value if you are planning to use heap allocations.

§_sheap

This symbol is located in RAM right after the .bss and .data sections. You can use the address of this symbol as the start address of a heap region. This symbol is 4 byte aligned so that address will be a multiple of 4.

§Example
extern crate some_allocator;

extern "C" {
    static _sheap: u8;
    static _heap_size: u8;
}

fn main() {
    unsafe {
        let heap_bottom = &_sheap as *const u8 as usize;
        let heap_size = &_heap_size as *const u8 as usize;
        some_allocator::initialize(heap_bottom, heap_size);
    }
}

§pre_init!

A user-defined function can be run at the start of the reset handler, before RAM is initialized. The macro pre_init! can be called to set the function to be run. The function is intended to perform actions that cannot wait the time it takes for RAM to be initialized, such as disabling a watchdog. As the function is called before RAM is initialized, any access of static variables will result in undefined behavior.

Functions§

Attribute Macros§

  • entry
    Attribute to declare the entry point of the program
  • pre_init
    Attribute to mark which function will be called at the beginning of the reset handler.