+#ifndef USERPROG_EXCEPTION_H
+#define USERPROG_EXCEPTION_H
+
+/* Page fault error code bits that describe the cause of the exception. */
+#define PF_P 0x1 /* 0: not-present page. 1: access rights violation. */
+#define PF_W 0x2 /* 0: read, 1: write. */
+#define PF_U 0x4 /* 0: kernel, 1: user process. */