more work towards MEGA65 native mode screen support

src/kernal/screen/chrout_to_screen_code.s

@@ -0,0 +1,44 @@
+// #LAYOUT# STD *        #TAKE
+// #LAYOUT# *   KERNAL_0 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// CHROUT routine - convert PETSCII to screen code
+//
+
+
+chrout_to_screen_code:
+
+	// Codes $C0-$DF become $40-$5F
+	cmp #$E0
+	bcs !+
+	cmp #$C0
+	bcc !+
+	
+	and #$7F
+	rts                                // not high char
+!:
+	// Range $20-$3F is unchanged
+	cmp #$40
+	bcc !+                             // not high char
+
+	// Unshifted letters and symbols from $40-$5F
+	// all end up being -$40
+	// (C64 PRG p376)
+
+	// But anything >= $80 needs to be -$40
+	// (C64 PRG p380-381)
+	// And bit 7 should be cleared, only to be
+	// set by reverse video
+	sec
+	sbc #$40
+
+	// Fix shifted chars by adding $20 again
+	cmp #$20
+	bcc !+                             // not high char
+	cmp #$40
+	bcs !+                             // not high char
+	clc
+	adc #$20
+!:
+	rts