uptime -- tells how long the system has been running

system.h

@@ -0,0 +1,12 @@
+/* system.h -- system services      */
+/* Copyright (c) 2020 Renaud Fivet  */
+
+extern volatile unsigned uptime ;   /* seconds elapsed since boot */
+
+int init( void) ;           /* System initialization, called once at startup */
+
+void kputc( unsigned char c) ;      /* character output */
+int  kputs( const char s[]) ;       /* string output */
+void yield( void) ;                 /* give way */
+
+/* end of system.h */