added support for the Commander X16 memory layout

c64/kernal/iostack/e50a.plot.s

@@ -0,0 +1,37 @@
+// #LAYOUT# STD *        #TAKE
+// #LAYOUT# *   KERNAL_0 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmers Reference Guide   - page 290
+// - [CM64] Computes Mapping the Commodore 64 - page 215
+//
+// CPU registers that has to be preserved (see [RG64]): none
+//
+
+
+PLOT:
+
+	bcs plot_get
+
+	// Note: the 'set' part has to start from $E50C, as this is a known address
+	// - https://www.lemon64.com/forum/viewtopic.php?t=3296&amp%3Bstart=15
+	// - https://sys64738.org/2019/05/c64-bedtime-coding-eng-printing-08/
+	// - 'Duotris' game calls $E50C during startup screen initialization
+
+plot_set:
+
+	sty PNTR
+	stx TBLX
+
+	jsr screen_calculate_pointers
+
+	// FALLTROUGH to save one byte on RTS
+
+plot_get:
+
+	ldy PNTR
+	ldx TBLX
+	rts