add code in ch1-3mini-rt-usrland

guide-code/ch1-3mini-rt-usrland/.cargo/config

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+[build]
+target = "riscv64gc-unknown-none-elf"

guide-code/ch1-3mini-rt-usrland/Cargo.toml

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+[package]
+name = "os"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

guide-code/ch1-3mini-rt-usrland/README.md

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+# rust-no-std-examples
+
+### Usage
+
+```bash
+cargo build
+qemu-riscv64 target/riscv64gc-unknown-none-elf/debug/os; echo $?
+# Hello world!
+# 9
+```

guide-code/ch1-3mini-rt-usrland/src/console.rs

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+use core::fmt::{Write, Arguments, Result};
+use crate::sys_write;
+
+struct Stdout;
+
+impl Write for Stdout {
+    fn write_str(&mut self, s: &str) -> Result {
+        sys_write(1, s.as_bytes());
+        Ok(())
+    }
+}
+
+pub fn print(args: Arguments) {
+    Stdout.write_fmt(args).unwrap();
+}
+
+macro_rules! print {
+    ($fmt: literal $(, $($arg: tt)+)?) => {
+        $crate::console::print(format_args!($fmt $(, $($arg)+)?));
+    }
+}
+
+macro_rules! println {
+    ($fmt: literal $(, $($arg: tt)+)?) => {
+        $crate::console::print(format_args!(concat!($fmt, "\n") $(, $($arg)+)?));
+    }
+}

guide-code/ch1-3mini-rt-usrland/src/lang_item.rs

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+use core::panic::PanicInfo;
+
+#[panic_handler]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}

guide-code/ch1-3mini-rt-usrland/src/main.rs

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+// os/src/main.rs
+#![no_std]
+#![no_main]
+#![feature(panic_info_message)]
+
+mod lang_item;
+#[macro_use]
+mod console;
+
+const SYSCALL_EXIT: usize = 93;
+const SYSCALL_WRITE: usize = 64;
+
+fn syscall(id: usize, args: [usize; 3]) -> isize {
+    let mut ret;
+    unsafe {
+        core::arch::asm!(
+            "ecall",
+            inlateout("x10") args[0] => ret,
+            in("x11") args[1],
+            in("x12") args[2],
+            in("x17") id,
+        );
+    }
+    ret
+}
+
+pub fn sys_exit(xstate: i32) -> isize {
+    syscall(SYSCALL_EXIT, [xstate as usize, 0, 0])
+}
+
+pub fn sys_write(fd: usize, buffer: &[u8]) -> isize {
+    syscall(SYSCALL_WRITE, [fd, buffer.as_ptr() as usize, buffer.len()])
+}
+
+#[no_mangle]
+extern "C" fn _start() {
+    print!("Hello ");
+    println!("world!");
+    sys_exit(9);
+}