implemented SCREEN routine

c64/kernal/screen.s

@@ -0,0 +1,11 @@
+; Function defined on p295 of C64 Programmers Reference Guide
+
+screen:
+
+	;; There are only 2 sane ways to implement this routine,
+	;; I hope this one is different than what Commodore picked :)
+
+	ldy #25 ; 25 columns
+	ldx #40 ; 40 rows
+
+	rts