first user application

user.c

@@ -0,0 +1,22 @@
+#include "user.h"
+
+extern char __stack_top[];
+
+__attribute__((noreturn)) void exit(void) {
+    for(;;);
+}
+
+void putchar(char ch) {
+    // TODO
+}
+
+__attribute__((section(".text.start")))
+__attribute__((naked))
+void start(void) {
+    __asm__ __volatile__(
+        "mv sp, %[stack_top]\n"
+        "call main\n"
+        "call exit\n"
+        :: [stack_top] "r" (__stack_top)
+    );
+}