Add user program initproc/user_shell, allow user programs allocate data on heap.

user/src/console.rs

@@ -1,11 +1,15 @@
 use core::fmt::{self, Write};
-use crate::syscall::{STDOUT, sys_write};
+
+const STDIN: usize = 0;
+const STDOUT: usize = 1;
+
+use super::{read, write};
 
 struct Stdout;
 
 impl Write for Stdout {
     fn write_str(&mut self, s: &str) -> fmt::Result {
-        sys_write(STDOUT, s.as_bytes());
+        write(STDOUT, s.as_bytes());
         Ok(())
     }
 }
@@ -26,4 +30,10 @@ macro_rules! println {
     ($fmt: literal $(, $($arg: tt)+)?) => {
         $crate::console::print(format_args!(concat!($fmt, "\n") $(, $($arg)+)?));
     }
-}
+}
+
+pub fn getchar() -> u8 {
+    let mut c = [0u8; 1];
+    read(STDIN, &mut c);
+    c[0]
+}