Created a target and a library for the Commander X16 prototype computer.

libsrc/cx16/videomode.s

@@ -0,0 +1,30 @@
+;
+; 2009-09-07, Ullrich von Bassewitz
+; 2019-09-23, Greg King
+;
+; unsigned __fastcall__ videomode (unsigned Mode);
+; /* Set the video mode, return the old mode. */
+;
+
+        .export         _videomode
+        .import         SWAPPER
+
+        .include        "cx16.inc"
+
+
+.proc   _videomode
+        cmp     LLEN                    ; Do we have this mode already?
+        beq     @L9
+
+        lda     LLEN                    ; Get current mode ...
+        pha                             ; ... and save it
+
+        jsr     SWAPPER                 ; Toggle the mode
+
+        pla                             ; Get old mode into A
+
+; Done, old mode is in .A
+
+@L9:    ldx     #>$0000                 ; Clear high byte
+        rts
+.endproc