Crate riscv_rt [−] [src]
Minimal startup / runtime for RISCV 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).
A
panic_fmt
implementation that just calls abort that you can opt into through the "abort-on-panic" Cargo feature. If you don't use this feature you'll have to provide thepanic_fmt
lang item yourself. Documentation hereA minimal
start
lang item to support the standardfn main()
interface. (The processor goes to sleep after returning frommain
)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]
version = "0.1.0"
$ # tell Xargo which standard crates to build
$ edit Xargo.toml && cat $_
[dependencies.core]
stage = 0
[dependencies.compiler_builtins]
features = ["mem"]
stage = 1
$ # memory layout of the device
$ edit memory.x && cat $_
MEMORY
{
/* NOTE K = KiBi = 1024 bytes */
FLASH : ORIGIN = 0x08000000, LENGTH = 128K
RAM : ORIGIN = 0x20000000, LENGTH = 8K
}
$ edit src/main.rs && cat $_
#![no_std] extern crate riscv_rt; fn main() { // do something here }
$ cargo install xargo
$ xargo rustc --target riscv32-unknown-none -- \
-C link-arg=-Tlink.x -C linker=riscv32-unknown-elf-ld -Z linker-flavor=ld
$ riscv32-unknown-elf-objdump -Cd $(find target -name app) | head
Disassembly of section .text:
20400000 <_start>:
20400000: 800011b7 lui gp,0x80001
20400004: 80018193 addi gp,gp,-2048 # 80000800 <_stack_start+0xffffc800>
20400008: 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);
_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; // Size of the heap in bytes const SIZE: usize = 1024; extern "C" { static mut _sheap: u8; } fn main() { unsafe { let start_address = &mut _sheap as *mut u8; some_allocator::initialize(start_address, SIZE); } }
Enums
Exception |
Exception |
Interrupt |
Interrupt |
Trap |
Machine Cause CSR (mcause) is ReadOnly. Trap Cause |
Functions
start_rust |
Rust entry point (_start_rust) |
start_trap_rust |
Trap entry point rust (_start_trap_rust) |