Crate riscv_rt

Minimal startup / runtime for RISC-V CPU's

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).

• An entry! macro to declare the entry point of the program.

• 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-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
entry!(main);

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);
    }
}

Macros

entry

Macro to define the entry point of the program

Functions

default_trap_handler	Default Trap Handler
start_rust	Rust entry point (_start_rust)
start_trap_rust	Trap entry point rust (_start_trap_rust)