feat(adapter-riscv): boot on QEMU virt with UART hello world

crates/adapter-riscv/src/main.rs

@@ -0,0 +1,61 @@
+#![no_std]
+#![no_main]
+
+mod uart;
+
+use core::arch::global_asm;
+use uart::Uart;
+
+global_asm!(
+    r#"
+.section .text.entry
+.globl _start
+_start:
+    la sp, _stack_top
+    call kmain
+spin:
+    wfi
+    j spin
+"#
+);
+
+unsafe extern "C" {
+    static _bss_start: u8;
+    static _bss_end: u8;
+}
+
+fn zero_bss() {
+    unsafe {
+        let start = &_bss_start as *const u8 as *mut u8;
+        let end = &_bss_end as *const u8 as *mut u8;
+        let len = end as usize - start as usize;
+        core::ptr::write_bytes(start, 0, len);
+    }
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn kmain() -> ! {
+    zero_bss();
+
+    let uart = Uart::new();
+    uart.write_str("k-os: booted on RISC-V\n");
+
+    loop {
+        unsafe { core::arch::asm!("wfi") };
+    }
+}
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+    let uart = Uart::new();
+    uart.write_str("PANIC: ");
+    if let Some(location) = info.location() {
+        uart.write_str(location.file());
+        uart.put_char(b':');
+        uart.write_dec(location.line() as usize);
+    }
+    uart.put_char(b'\n');
+    loop {
+        unsafe { core::arch::asm!("wfi") };
+    }
+}