MEGA65: allow OPL work on slow-dev space, too #380

Note, currently I don't know why the slow device space is used for OPL
why not the native I/O space, or the MEGA65 specific OPL regs
at $FE000xx which is intended for that very purpose!

btoschi on Discord has great success to make OPL working on MEGA65 but
requires this access mode, so this quick hack is intended to serve that
holy purpose :)

targets/mega65/cart.c

@@ -1,6 +1,6 @@
 /* A work-in-progess MEGA65 (Commodore 65 clone origins) emulator
    Part of the Xemu project, please visit: https://github.com/lgblgblgb/xemu
-   Copyright (C)2016-2023 LGB (Gábor Lénárt) <[email protected]>
+   Copyright (C)2016-2024 LGB (Gábor Lénárt) <[email protected]>
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
@@ -19,6 +19,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 #include "xemu/emutools.h"
 #include "cart.h"
 #include "xemu/emutools_files.h"
+#include "audio65.h"
 
 static Uint8 cart_mem[0x10000];
 static int loaded = 0;
@@ -35,13 +36,25 @@ Uint8 cart_read_byte  ( unsigned int addr )
 	Uint8 data = 0xFF;
 	if (addr < sizeof(cart_mem))
 		data = cart_mem[addr];
-	DEBUGPRINT("CART: reading byte ($%02X) at $%X" NL, data, addr + 0x4000000);
+	if (loaded && addr != 0x3FFDF60)	// see the comment about OPL at cart_write_byte()
+		DEBUGPRINT("CART: reading byte ($%02X) at $%X" NL, data, addr + 0x4000000);
 	return data;
 }
 
 
 void cart_write_byte ( unsigned int addr, Uint8 data )
 {
+	if (!loaded) {
+		// a hack OPL to be able to work in the "slow device area" [if no cartridge is loaded]
+		static Uint8 opl_reg_sel = 0;
+		if (addr == 0x3FFDF40) {
+			opl_reg_sel = data;
+			return;
+		} else if (addr == 0x3FFDF50) {
+			audio65_opl3_write(opl_reg_sel, data);
+			return;
+		}
+	}
 	DEBUGPRINT("CART: writing byte ($%02X) at $%X" NL, data, addr + 0x4000000);
 }
 

targets/mega65/cart.h

@@ -1,6 +1,6 @@
 /* A work-in-progess MEGA65 (Commodore 65 clone origins) emulator
    Part of the Xemu project, please visit: https://github.com/lgblgblgb/xemu
-   Copyright (C)2016-2023 LGB (Gábor Lénárt) <[email protected]>
+   Copyright (C)2016-2024 LGB (Gábor Lénárt) <[email protected]>
 
 This program is free software; you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by