ch1: Support function call

src/main.rs

@@ -4,4 +4,20 @@
 mod lang_items;
 
 use core::arch::global_asm;
-global_asm!(include_str!("entry.asm"));
+global_asm!(include_str!("entry.asm"));
+
+#[no_mangle]
+pub fn rust_main() -> ! {
+    clear_bss();
+    loop {}
+}
+
+fn clear_bss() {
+    extern "C" {
+        fn sbss();
+        fn ebss();
+    }
+    (sbss as usize..ebss as usize).for_each(|a| {
+        unsafe { (a as *mut u8).write_volatile(0) }
+    });
+}