add comments

os/src/main.rs

@@ -35,16 +35,16 @@ fn clear_bss() {
 #[no_mangle]
 pub fn rust_main() -> ! {
     extern "C" {
-        fn stext();
-        fn etext();
-        fn srodata();
-        fn erodata();
-        fn sdata();
-        fn edata();
-        fn sbss();
-        fn ebss();
-        fn boot_stack();
-        fn boot_stack_top();
+        fn stext();               // begin addr of text segment
+        fn etext();               // end addr of text segment
+        fn srodata();             // start addr of Read-Only data segment
+        fn erodata();             // end addr of Read-Only data ssegment
+        fn sdata();               // start addr of data segment
+        fn edata()                // end addr of data segment
+        fn sbss();                // start addr of BSS segment
+        fn ebss();                // end addr of BSS segment
+        fn boot_stack();          // stack bottom
+        fn boot_stack_top();      // stack top
     }
     clear_bss();
     println!("Hello, world!");