Created ECHO system call for testing purposes.

Furthermore, a quick way to get one's own process number.

kernel/ipc.h

@@ -0,0 +1,14 @@
+/* Masks and flags for system calls. */
+#define SYSCALL_FUNC	0x0F	/* mask for system call function */
+#define SYSCALL_FLAGS   0xF0    /* mask for system call flags */
+#define NON_BLOCKING    0x10	/* prevent blocking, return error */
+#define FRESH_ANSWER    0x20	/* ignore pending notifications as answer */
+				/* (default behaviour for SENDREC calls) */
+
+/* System calls (numbers passed when trapping to the kernel) */
+#define ECHO		 0	/* function code for echoing messages */
+#define SEND		 1	/* function code for sending messages */
+#define RECEIVE		 2	/* function code for receiving messages */
+#define SENDREC	 	 3  	/* function code for SEND + RECEIVE */
+#define NOTIFY		 4	/* function code for notifications */
+