riscv64 的 M ModeM-mode(机器模式，缩写为 M 模式)是 RISC-V 中 hart(hardware thread,硬件线程)可以执行的最高权
限模式。在 M 模式下运行的 hart 对内存,I/O 和一些对于启动和配置系统来说必要的底层功能有着完全的
使用权。riscv64 的 S ModeS-mode(监管者模式，缩写为 S 模式)是支持现代类 Unix 操作系统的权限模式，支持现代类 Unix 操作系
统所需要的基于页面的虚拟内存机制是其核心。

# src/boot/entry_riscv64.asm.section .text.entry.global _start
_start:la sp, bootstacktopcall rust_main.section .bss.stack.align 12.global bootstack
bootstack:.space 4096 * 4.global bootstacktop
bootstacktop:

#![no_main]
#![no_std]use core::panic::PanicInfo;#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {loop{}
}#[no_mangle]
extern "C" fn rust_main() -> ! {loop{}
}

#![no_main]
#![no_std]use core::panic::PanicInfo;#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {loop{}
}#![feature(global_asm)]
global_asm!(include_str!("boot/entry_riscv64.asm"));#[no_mangle]
extern "C" fn rust_main() -> ! {loop{}
} 

error: an inner attribute is not permitted in this context--> src/main.rs:11:1|
11 | #![feature(global_asm)]| ^^^^^^^^^^^^^^^^^^^^^^^|= note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files. Outer attributes, like `#[test]`, annotate the item following them.error[E0658]: use of unstable library feature 'global_asm': `global_asm!` is not stable enough for use and is subject to change--> src/main.rs:13:1|
13 | global_asm!(include_str!("boot/entry_riscv64.asm"));| ^^^^^^^^^^|= note: see issue #35119 <https://github.com/rust-lang/rust/issues/35119> for more information= help: add `#![feature(global_asm)]` to the crate attributes to enableerror: aborting due to 2 previous errorsFor more information about this error, try `rustc --explain E0658`.
error: could not compile `rust_os`.

#![no_main]
#![no_std]
#![feature(global_asm)]use core::panic::PanicInfo;#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {loop{}
}global_asm!(include_str!("boot/entry_riscv64.asm"));#[no_mangle]
extern "C" fn rust_main() -> ! {loop{}
}