add os[1-8]-ref for os refereces, add guide, add README

os2-ref/src/syscall/fs.rs

@@ -0,0 +1,15 @@
+const FD_STDOUT: usize = 1;
+
+pub fn sys_write(fd: usize, buf: *const u8, len: usize) -> isize {
+    match fd {
+        FD_STDOUT => {
+            let slice = unsafe { core::slice::from_raw_parts(buf, len) };
+            let str = core::str::from_utf8(slice).unwrap();
+            print!("{}", str);
+            len as isize
+        }
+        _ => {
+            panic!("Unsupported fd in sys_write!");
+        }
+    }
+}

os2-ref/src/syscall/mod.rs

@@ -0,0 +1,16 @@
+const SYSCALL_WRITE: usize = 64;
+const SYSCALL_EXIT: usize = 93;
+
+mod fs;
+mod process;
+
+use fs::*;
+use process::*;
+
+pub fn syscall(syscall_id: usize, args: [usize; 3]) -> isize {
+    match syscall_id {
+        SYSCALL_WRITE => sys_write(args[0], args[1] as *const u8, args[2]),
+        SYSCALL_EXIT => sys_exit(args[0] as i32),
+        _ => panic!("Unsupported syscall_id: {}", syscall_id),
+    }
+}

os2-ref/src/syscall/process.rs

@@ -0,0 +1,6 @@
+use crate::batch::run_next_app;
+
+pub fn sys_exit(exit_code: i32) -> ! {
+    info!("[kernel] Application exited with code {}", exit_code);
+    run_next_app()
+}