ch1: Support function call

src/entry.asm

@@ -1,4 +1,12 @@
     .section .text.entry
     .globl _start
 _start:
-    li x1, 100
+    la sp, boot_start_top
+    call rust_main
+
+    .section .bss.stack
+    .globl boot_stack_lower_bound
+boot_stack_lower_bound:
+    .space 4096 * 16
+    .globl boot_start_top
+boot_start_top:

src/main.rs

@@ -5,3 +5,19 @@ mod lang_items;
 
 use core::arch::global_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) }
+    });
+}