diff --git a/HISTORY.txt b/HISTORY.txt
index e3a085b..58de36f 100644
--- a/HISTORY.txt
+++ b/HISTORY.txt
@@ -54,6 +54,7 @@ Genesis Plus GX 1.7.5 (xx/xx/xxxx) (Eke-Eke)
 * added support for more X-in-1 pirate mappers
 * added support for some new unlicensed games with copy protection (Thunderbolt II, Tom Clown, Chaoji Puke / Super Poker, Rock Heaven, Rock World)
 * added support for Everdrive extended SSF mapper
+* added support for MegaSD CD hardware overlay (MD+ hacks) and extended SSF2 / ROM write mappers
 * added (very basic) emulation of Flashkit MD hardware
 * added emulation of Micro Machines USA on-board TMSS bypass logic hardware
 * added SRAM support for games larger than 8MB
diff --git a/builds/genesis_plus_gx_libretro.dll b/builds/genesis_plus_gx_libretro.dll
index 8b2b316..a2c4475 100644
Binary files a/builds/genesis_plus_gx_libretro.dll and b/builds/genesis_plus_gx_libretro.dll differ
diff --git a/builds/genplus_cube.dol b/builds/genplus_cube.dol
index f96379a..3ecdbfa 100644
Binary files a/builds/genplus_cube.dol and b/builds/genplus_cube.dol differ
diff --git a/builds/genplus_wii.dol b/builds/genplus_wii.dol
index ae96736..550bcbf 100644
Binary files a/builds/genplus_wii.dol and b/builds/genplus_wii.dol differ
diff --git a/core/cart_hw/md_cart.c b/core/cart_hw/md_cart.c
index 6ff9e9f..694c525 100644
--- a/core/cart_hw/md_cart.c
+++ b/core/cart_hw/md_cart.c
@@ -44,6 +44,7 @@
 #include "shared.h"
 #include "eeprom_i2c.h"
 #include "eeprom_spi.h"
+#include "megasd.h"
 
 /* Cart database entry */
 typedef struct
@@ -518,6 +519,21 @@ void md_cart_init(void)
     /* cartridge ROM mapping is reinitialized on /VRES */
     cart.hw.bankshift = 1;
   }
+  else if ((strstr(rominfo.consoletype,"SEGA SSF2") != NULL) && (cart.romsize <= 0x800000))
+  {
+    /* MegaSD enhanced SSF2 mapper (max. 8MB ROM) */
+    cart.special |= HW_MEGASD;
+    cart.hw.time_w = megasd_enhanced_ssf2_mapper_w;
+
+    /* cartridge ROM mapping is reinitialized on /VRES */
+    cart.hw.bankshift = 1;
+  }
+  else if ((strstr(rominfo.consoletype,"SEGA MEGASD") != NULL) && (cart.romsize <= 0x400000))
+  {
+    /* MegaSD ROM write mapper (max. 4MB ROM) */
+    cart.special |= HW_MEGASD;
+    cart.hw.time_w = megasd_rom_mapper_w;
+  }
   else if (strstr(rominfo.domestic,"SUPER STREET FIGHTER2"))
   {
     /* SSF2 mapper */
@@ -760,6 +776,26 @@ void md_cart_init(void)
       break;
     }
   }
+
+  /**********************************************
+        MEGASD ADD-ON
+  ***********************************************/
+  /* enable MegaSD overlay for cartridge ROM (max. 8MB) when Mega CD hardware is disabled and either MegaSD add-on is forced enabled or automatic add-on detection is enabled and MegaSD compatible disc image is loaded */
+  if ((cart.romsize <= 0x800000) && (system_hw == SYSTEM_MD) && ((config.add_on == HW_ADDON_MEGASD) || ((config.add_on | cdd.loaded) == HW_ADDON_MEGASD)))
+  {
+    cart.special |= HW_MEGASD;
+  }
+
+  /* force Mega CD sound hardware initialization when MegaSD overlay is enabled (if not already initialized)  */
+  if ((cart.special & HW_MEGASD) && !snd.blips[1] && !snd.blips[2])
+  {
+    /* allocate blip buffers for PCM and CD-DA audio streams */
+    snd.blips[1] = blip_new(snd.sample_rate / 10);
+    snd.blips[2] = blip_new(snd.sample_rate / 10);
+
+    /* initialize PCM and CD-DA audio */
+    audio_set_rate(snd.sample_rate, snd.frame_rate);
+  }
 }
 
 /* hardware that need to be reseted on power on */
@@ -775,7 +811,13 @@ void md_cart_reset(int hard_reset)
       m68k.memory_map[i].base = cart.rom + ((i<<16) & cart.mask);
     }
   }
-  
+
+  /* MegaSD hardware */
+  if (cart.special & HW_MEGASD)
+  {
+    megasd_reset();
+  }
+
   /* SVP chip */
   if (svp)
   {
@@ -828,7 +870,7 @@ int md_cart_context_save(uint8 *state)
   {
     /* get base address */
     base = m68k.memory_map[i].base;
-      
+
     if (base == sram.sram)
     {
       /* SRAM */
@@ -857,6 +899,12 @@ int md_cart_context_save(uint8 *state)
     save_param(&svp->ssp1601,sizeof(ssp1601_t));
   }
 
+  /* MegaSD hardware */
+  if (cart.special & HW_MEGASD)
+  {
+    bufferptr += megasd_context_save(&state[bufferptr]);
+  }
+
   return bufferptr;
 }
 
@@ -913,6 +961,12 @@ int md_cart_context_load(uint8 *state)
     load_param(&svp->ssp1601,sizeof(ssp1601_t));
   }
 
+  /* MegaSD hardware */
+  if (cart.special & HW_MEGASD)
+  {
+    bufferptr += megasd_context_load(&state[bufferptr]);
+  }
+
   return bufferptr;
 }
 
diff --git a/core/cart_hw/md_cart.h b/core/cart_hw/md_cart.h
index b34fb8f..7bdfcae 100644
--- a/core/cart_hw/md_cart.h
+++ b/core/cart_hw/md_cart.h
@@ -55,9 +55,16 @@
 #define TYPE_AR 0x02  /* (Pro) Action Replay */
 #define TYPE_SK 0x03  /* Sonic & Knuckles */
 
+/* CD hardware add-on (MD mode) */
+#define HW_ADDON_AUTO    0x00
+#define HW_ADDON_MEGACD  0x01
+#define HW_ADDON_MEGASD  0x02
+#define HW_ADDON_NONE    0x03
+
 /* Special hardware (0x01 & 0x02 reserved for Master System 3-D glasses & Terebi Oekaki) */
 #define HW_J_CART   0x04
 #define HW_LOCK_ON  0x08
+#define HW_MEGASD   0x10
 
 /* Cartridge extra hardware */
 typedef struct
diff --git a/core/cd_hw/cdd.c b/core/cd_hw/cdd.c
index 43be53c..3eac694 100644
--- a/core/cd_hw/cdd.c
+++ b/core/cd_hw/cdd.c
@@ -36,6 +36,7 @@
  *
  ****************************************************************************************/
 #include "shared.h"
+#include "megasd.h"
 
 #if defined(USE_LIBTREMOR) || defined(USE_LIBVORBIS)
 #define SUPPORTED_EXT 20
@@ -363,8 +364,9 @@ int cdd_load(char *filename, char *header)
   char *ptr, *lptr;
   cdStream *fd;
   
-  /* assume CD image file by default */
+  /* assume normal CD image file by default */
   int isCDfile = 1;
+  int isMSDfile = 0;
 
   /* first unmount any loaded disc */
   cdd_unload();
@@ -395,7 +397,7 @@ int cdd_load(char *filename, char *header)
 
     /* retrieve CHD header */
     head = chd_get_header(cdd.chd.file);
- 
+
     /* detect invalid hunk size */
     if ((head->hunkbytes == 0) || (head->hunkbytes % CD_FRAME_SIZE))
     {
@@ -526,7 +528,7 @@ int cdd_load(char *filename, char *header)
       cdd.toc.tracks[cdd.toc.last].start = cdd.toc.end;
 
       /* CD mounted */
-      cdd.loaded = 1;
+      cdd.loaded = HW_ADDON_MEGACD;
       return 1;
     }
 
@@ -537,8 +539,9 @@ int cdd_load(char *filename, char *header)
   }
 #endif
 
-  /* save a copy of base filename */
-  strncpy(fname, filename, 256);
+  /* save a copy of base filename (max. 255 characters) */
+  strncpy(fname, filename, 255);
+  fname[256] = 0;
 
   /* check loaded file extension */
   if (memcmp("cue", &filename[strlen(filename) - 3], 3) && memcmp("CUE", &filename[strlen(filename) - 3], 3))
@@ -794,6 +797,28 @@ int cdd_load(char *filename, char *header)
         }
       }
 
+      /* decode REM LOOP xxx command (MegaSD specific command) */
+      else if (sscanf(lptr, "REM LOOP %d", &bb) == 1)
+      {
+        cdd.toc.tracks[cdd.toc.last].loopEnabled = 1;
+        cdd.toc.tracks[cdd.toc.last].loopOffset = bb;
+        isMSDfile = 1;
+      }
+
+      /* decode REM LOOP command (MegaSD specific command) */
+      else if (strstr(lptr,"REM LOOP"))
+      {
+        cdd.toc.tracks[cdd.toc.last].loopEnabled = 1;
+        isMSDfile = 1;
+      }
+
+      /* decode REM NOLOOP command (MegaSD specific command) */
+      else if (strstr(lptr,"REM NOLOOP"))
+      {
+        cdd.toc.tracks[cdd.toc.last].loopEnabled = -1;
+        isMSDfile = 1;
+      }
+
       /* decode PREGAP commands */
       else if (sscanf(lptr, "PREGAP %02d:%02d:%02d", &mm, &ss, &bb) == 3)
       {
@@ -1211,7 +1236,7 @@ int cdd_load(char *filename, char *header)
     cdd.toc.tracks[cdd.toc.last].start = cdd.toc.end;
 
     /* CD mounted */
-    cdd.loaded = 1;
+    cdd.loaded = isMSDfile ? HW_ADDON_MEGASD : HW_ADDON_MEGACD;
 
     /* Automatically try to open associated subcode data file */
     memcpy(&fname[strlen(fname) - 4], ".sub", 4);
@@ -1617,6 +1642,12 @@ void cdd_read_audio(unsigned int samples)
     /* save last audio output for next frame */
     cdd.audio[0] = prev_l;
     cdd.audio[1] = prev_r;
+
+    /* check CD-DA track end (Mega SD add-on specific)*/
+    if (cart.special & HW_MEGASD)
+    {
+      megasd_update_cdda(samples);
+    }
   }
   else
   {
diff --git a/core/cd_hw/cdd.h b/core/cd_hw/cdd.h
index dae50c2..c3d6c80 100644
--- a/core/cd_hw/cdd.h
+++ b/core/cd_hw/cdd.h
@@ -86,6 +86,8 @@ typedef struct
   int start;
   int end;
   int type;
+  int loopEnabled;
+  int loopOffset;
 } track_t; 
 
 /* CD TOC */
diff --git a/core/cd_hw/pcm.c b/core/cd_hw/pcm.c
index e068d70..80ae3e6 100644
--- a/core/cd_hw/pcm.c
+++ b/core/cd_hw/pcm.c
@@ -2,7 +2,7 @@
  *  Genesis Plus
  *  PCM sound chip (315-5476A) (RF5C164 compatible)
  *
- *  Copyright (C) 2012-2016  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2012-2021  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -226,10 +226,10 @@ void pcm_update(unsigned int samples)
   pcm.cycles = 0;
 }
 
-void pcm_write(unsigned int address, unsigned char data)
+void pcm_write(unsigned int address, unsigned char data, unsigned int cycles)
 {
-  /* synchronize PCM chip with SUB-CPU */
-  int clocks = s68k.cycles - pcm.cycles;
+  /* synchronize PCM chip with CPU */
+  int clocks = cycles - pcm.cycles;
   if (clocks > 0)
   {
     /* number of internal clocks (samples) to run */
@@ -350,10 +350,10 @@ void pcm_write(unsigned int address, unsigned char data)
   }
 }
 
-unsigned char pcm_read(unsigned int address)
+unsigned char pcm_read(unsigned int address, unsigned int cycles)
 {
   /* synchronize PCM chip with SUB-CPU */
-  int clocks = s68k.cycles - pcm.cycles;
+  int clocks = cycles - pcm.cycles;
   if (clocks > 0)
   {
     /* number of internal clocks (samples) to run */
diff --git a/core/cd_hw/pcm.h b/core/cd_hw/pcm.h
index eef847d..03e1a59 100644
--- a/core/cd_hw/pcm.h
+++ b/core/cd_hw/pcm.h
@@ -2,7 +2,7 @@
  *  Genesis Plus
  *  PCM sound chip (315-5476A) (RF5C164 compatible)
  *
- *  Copyright (C) 2012-2016  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2012-2021  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -70,8 +70,8 @@ extern void pcm_reset(void);
 extern int pcm_context_save(uint8 *state);
 extern int pcm_context_load(uint8 *state);
 extern void pcm_update(unsigned int samples);
-extern void pcm_write(unsigned int address, unsigned char data);
-extern unsigned char pcm_read(unsigned int address);
+extern void pcm_write(unsigned int address, unsigned char data, unsigned int cycles);
+extern unsigned char pcm_read(unsigned int address, unsigned int cycles);
 extern void pcm_ram_dma_w(unsigned int words);
 
 #endif
diff --git a/core/cd_hw/scd.c b/core/cd_hw/scd.c
index d776e45..edb728d 100644
--- a/core/cd_hw/scd.c
+++ b/core/cd_hw/scd.c
@@ -482,7 +482,7 @@ static unsigned int scd_read_byte(unsigned int address)
     /* get /LDS only */
     if (address & 1)
     {
-      return pcm_read((address >> 1) & 0x1fff);
+      return pcm_read((address >> 1) & 0x1fff, s68k.cycles);
     }
 
     return s68k_read_bus_8(address);
@@ -601,7 +601,7 @@ static unsigned int scd_read_word(unsigned int address)
   if (!(address & 0x8000))
   {
     /* get /LDS only */
-    return pcm_read((address >> 1) & 0x1fff);
+    return pcm_read((address >> 1) & 0x1fff, s68k.cycles);
   }
 
 #ifdef LOG_SCD
@@ -764,7 +764,7 @@ static void scd_write_byte(unsigned int address, unsigned int data)
     /* get /LDS only */
     if (address & 1)
     {
-      pcm_write((address >> 1) & 0x1fff, data);
+      pcm_write((address >> 1) & 0x1fff, data, s68k.cycles);
       return;
     }
 
@@ -1028,7 +1028,7 @@ static void scd_write_word(unsigned int address, unsigned int data)
   if (!(address & 0x8000))
   {
     /* get /LDS only */
-    pcm_write((address >> 1) & 0x1fff, data);
+    pcm_write((address >> 1) & 0x1fff, data & 0xff, s68k.cycles);
     return;
   }
 
diff --git a/core/loadrom.c b/core/loadrom.c
index 9f14c3f..f92f40d 100644
--- a/core/loadrom.c
+++ b/core/loadrom.c
@@ -3,7 +3,7 @@
  *  ROM Loading Support
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2020  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2021  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -58,20 +58,21 @@
 #define ROMMEMO       456
 #define ROMCOUNTRY    496
 
-#define P3BUTTONS   1
-#define P6BUTTONS   2
-#define PKEYBOARD   4
-#define PPRINTER    8
-#define PBALL       16
-#define PFLOPPY     32
-#define PACTIVATOR  64
-#define PTEAMPLAYER 128
-#define PMSYSTEMPAD 256
-#define PSERIAL     512
-#define PTABLET     1024
-#define PPADDLE     2048
-#define PCDROM      4096
-#define PMOUSE      8192
+#define P3BUTTONS   0x0001
+#define P6BUTTONS   0x0002
+#define PKEYBOARD   0x0004
+#define PPRINTER    0x0008
+#define PBALL       0x0010
+#define PFLOPPY     0x0020
+#define PACTIVATOR  0x0040
+#define PTEAMPLAYER 0x0080
+#define PMSYSTEMPAD 0x0100
+#define PSERIAL     0x0200
+#define PTABLET     0x0400
+#define PPADDLE     0x0800
+#define PCDROM      0x1000
+#define PMOUSE      0x2000
+#define PMENACER    0x4000
 
 #define MAXCOMPANY 64
 #define MAXPERIPHERALS 15
@@ -288,7 +289,7 @@ void getrominfo(char *romheader)
     /* Supported peripherals */
     rominfo.peripherals = 0;
     for (i = 0; i < 14; i++)
-      for (j=0; j < 14; j++)
+      for (j=0; j < MAXPERIPHERALS; j++)
         if (romheader[ROMIOSUPPORT+i] == peripheralinfo[j].pID[0])
           rominfo.peripherals |= (1 << j);
   }
@@ -581,9 +582,6 @@ int load_rom(char *filename)
   {
     /* enable CD hardware */
     system_hw = SYSTEM_MCD;
-
-    /* boot from CD hardware */
-    scd.cartridge.boot = 0x00;
   }
   else
   {
@@ -710,10 +708,10 @@ int load_rom(char *filename)
 
   /* Save auto-detected system hardware  */
   romtype = system_hw;
-  
+
   /* CD image file */
   if (system_hw == SYSTEM_MCD)
-  {   
+  {
     /* try to load CD BOOTROM for selected region */
     if (!load_bios(SYSTEM_MCD))
     {
@@ -723,6 +721,9 @@ int load_rom(char *filename)
       /* error booting from CD */
       return (0);
     }
+
+    /* boot from CD hardware */
+    scd.cartridge.boot = 0x00;
   }
 
   /* CD BOOTROM */
@@ -744,44 +745,26 @@ int load_rom(char *filename)
     system_bios = (system_bios & 0xf0) | (region_code >> 4);
   }
 
-  /* ROM cartridge (max. 8MB) with CD loaded */
-  else if ((cart.romsize <= 0x800000) && cdd.loaded)
+  /* 16-bit ROM cartridge (max. 8MB) with optional CD hardware add-on support enabled */
+  else if ((system_hw == SYSTEM_MD) && (cart.romsize <= 0x800000) && (config.add_on != HW_ADDON_NONE))
   {
-    /* try to load CD BOOTROM */
-    if (load_bios(SYSTEM_MCD))
+    if ((rominfo.peripherals & PCDROM) || (strstr(rominfo.domestic,"FLUX") != NULL) ||
+        (config.add_on == HW_ADDON_MEGACD) || ((config.add_on | cdd.loaded) == HW_ADDON_MEGACD))
     {
-      /* enable CD hardware */
-      system_hw = SYSTEM_MCD;
-
-      /* boot from cartridge */
-      scd.cartridge.boot = 0x40;
-    }
-    else
-    {
-      /* unmount CD image */
-      cdd_unload();
-    }    
-  }
-  
-  /* ROM cartridge with CD support */
-  else if ((strstr(rominfo.domestic,"FLUX") != NULL) ||
-           (strstr(rominfo.domestic,"WONDER LIBRARY") != NULL) ||
-           (strstr(rominfo.product,"T-5740") != NULL))
-  {
-    /* check if console hardware is set to AUTO */
-    if (!config.system)
-    {
-      /* try to load CD BOOTROM */
+      /* try to load CD BOOTROM for selected region */
       if (load_bios(SYSTEM_MCD))
       {
-        char fname[256];
-        int len = strlen(filename);
-
-        /* automatically try to load associated .iso file */
-        while ((len && (filename[len] != '.')) || (len > 251)) len--;
-        strncpy(fname, filename, len);
-        strcpy(&fname[len], ".iso");
-        cdd_load(fname, (char *)cdc.ram);
+        /* automatically try to load associated .iso file if no CD image loaded yet */
+        if (!cdd.loaded)
+        {
+          char fname[256];
+          int len = strlen(filename);
+          while ((len && (filename[len] != '.')) || (len > 251)) len--;
+          strncpy(fname, filename, len);
+          strcpy(&fname[len], ".iso");
+          fname[len+4] = 0;
+          cdd_load(fname, (char *)cdc.ram);
+        }
 
         /* enable CD hardware */
         system_hw = SYSTEM_MCD;
@@ -789,6 +772,11 @@ int load_rom(char *filename)
         /* boot from cartridge */
         scd.cartridge.boot = 0x40;
       }
+      else
+      {
+        /* unmount any loaded CD image */
+        cdd_unload();
+      }
     }
   }
 
diff --git a/core/loadrom.h b/core/loadrom.h
index b3efb4e..ce9b5f4 100644
--- a/core/loadrom.h
+++ b/core/loadrom.h
@@ -3,7 +3,7 @@
  *  ROM Loading Support
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2020 Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2021 Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
diff --git a/core/system.c b/core/system.c
index 358262e..8ca2ed9 100644
--- a/core/system.c
+++ b/core/system.c
@@ -5,7 +5,7 @@
  *  Support for 16-bit & 8-bit hardware modes
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2018  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2021  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -130,8 +130,8 @@ void audio_set_rate(int samplerate, double framerate)
   /* resampled to desired rate at the end of each frame, using Blip Buffer.            */
   blip_set_rates(snd.blips[0], mclk, samplerate);
 
-  /* Mega CD sound hardware */
-  if (system_hw == SYSTEM_MCD)
+  /* Mega CD sound hardware enabled ? */
+  if (snd.blips[1] && snd.blips[2])
   {
     /* number of SCD master clocks run per second */
     mclk = (mclk / system_clock) * SCD_CLOCK;
@@ -195,8 +195,8 @@ int audio_update(int16 *buffer)
   /* run sound chips until end of frame */
   int size = sound_update(mcycles_vdp);
 
-  /* Mega CD specific */
-  if (system_hw == SYSTEM_MCD)
+  /* Mega CD sound hardware enabled ? */
+  if (snd.blips[1] && snd.blips[2])
   {
     /* sync PCM chip with other sound chips */
     pcm_update(size);
diff --git a/core/system.h b/core/system.h
index ac72727..d7ca95f 100644
--- a/core/system.h
+++ b/core/system.h
@@ -5,7 +5,7 @@
  *  Support for 16-bit & 8-bit hardware modes
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2018  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2021  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
diff --git a/gcw0/Makefile b/gcw0/Makefile
index 2400056..3d4d95e 100644
--- a/gcw0/Makefile
+++ b/gcw0/Makefile
@@ -88,7 +88,8 @@ OBJECTS	+=      $(OBJDIR)/sram.o        \
 		$(OBJDIR)/eeprom_i2c.o  \
 		$(OBJDIR)/eeprom_spi.o  \
 		$(OBJDIR)/md_cart.o	\
-		$(OBJDIR)/sms_cart.o	
+		$(OBJDIR)/sms_cart.o	\
+		$(OBJDIR)/megasd.o
 		
 OBJECTS	+=      $(OBJDIR)/scd.o	\
 		$(OBJDIR)/cdd.o	\
diff --git a/gcw0/config.c b/gcw0/config.c
index 7ed0265..3c9c941 100644
--- a/gcw0/config.c
+++ b/gcw0/config.c
@@ -62,7 +62,8 @@ void set_config_defaults(void)
     config.force_dtack    = 0;
     config.addr_error     = 1;
     config.bios           = 0;
-    config.lock_on        = 0; /* = OFF (can be TYPE_SK, TYPE_GG & TYPE_AR) */
+    config.lock_on        = 0; /* = OFF (or TYPE_SK, TYPE_GG & TYPE_AR) */
+    config.add_on         = 0; /* = HW_ADDON_AUTO (or HW_ADDON_MEGACD, HW_ADDON_MEGASD & HW_ADDON_NONE) */
     config.ntsc           = 0;
     config.lcd            = 0; /* 0.8 fixed point */
 
diff --git a/gcw0/config.h b/gcw0/config.h
index 9c9f1eb..116d8b8 100644
--- a/gcw0/config.h
+++ b/gcw0/config.h
@@ -40,6 +40,7 @@ typedef struct
   uint8 addr_error;
   uint8 bios;
   uint8 lock_on;
+  uint8 add_on;
   uint8 hot_swap;
   uint8 invert_mouse;
   uint8 gun_cursor[2];
diff --git a/gx/config.c b/gx/config.c
index 209ddfc..3ab7faa 100644
--- a/gx/config.c
+++ b/gx/config.c
@@ -122,6 +122,7 @@ void config_default(void)
   config.addr_error     = 1;
   config.bios           = 0;
   config.lock_on        = 0;
+  config.add_on         = HW_ADDON_AUTO;
   config.hot_swap       = 0;
 
   /* video options */
diff --git a/gx/config.h b/gx/config.h
index 353007d..034aae7 100644
--- a/gx/config.h
+++ b/gx/config.h
@@ -71,6 +71,7 @@ typedef struct
   uint8 addr_error;
   uint8 bios;
   uint8 lock_on;
+  uint8 add_on;
   uint8 hot_swap;
   uint8 invert_mouse;
   uint8 gun_cursor[2];
diff --git a/gx/gui/menu.c b/gx/gui/menu.c
index 186030f..f5e7bf0 100644
--- a/gx/gui/menu.c
+++ b/gx/gui/menu.c
@@ -3,7 +3,7 @@
  *
  *  Genesis Plus GX menu
  *
- *  Copyright Eke-Eke (2009-2019)
+ *  Copyright Eke-Eke (2009-2021)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -385,6 +385,7 @@ static gui_item items_system[] =
   {NULL,NULL,"System Boot: BIOS&CART",  "Select system booting method",                56,132,276,48},
   {NULL,NULL,"System Lockups: ON",      "Enable/Disable original system lock-ups",     56,132,276,48},
   {NULL,NULL,"68k Address Error: ON",   "Enable/Disable 68k address error exceptions", 56,132,276,48},
+  {NULL,NULL,"CD Add-on: MEGA/SEGA CD",    "Select cartridge mode CD hardware add-on",    56,132,276,48},
   {NULL,NULL,"Lock-on: OFF",            "Select Lock-On cartridge type",               56,132,276,48},
   {NULL,NULL,"Cartridge Swap: OFF",     "Enable/Disable cartridge hot swap",           56,132,276,48},
   {NULL,NULL,"BIOS & Lock-On ROM paths","Configure BIOS & Lock-On ROM paths",          56,132,276,48},
@@ -1311,25 +1312,34 @@ static void systemmenu ()
   sprintf (items[5].text, "System Lockups: %s", config.force_dtack ? "OFF" : "ON");
   sprintf (items[6].text, "68k Address Error: %s", config.addr_error ? "ON" : "OFF");
 
-  if (config.lock_on == TYPE_GG)
-    sprintf (items[7].text, "Lock-On: GAME GENIE");
-  else if (config.lock_on == TYPE_AR)
-    sprintf (items[7].text, "Lock-On: ACTION REPLAY");
-  else if (config.lock_on == TYPE_SK)
-    sprintf (items[7].text, "Lock-On: SONIC&KNUCKLES");
+  if (config.add_on == HW_ADDON_AUTO)
+    sprintf (items[7].text, "CD Add-on: AUTO");
+  else if (config.add_on == HW_ADDON_MEGACD)
+    sprintf (items[7].text, "CD Add-on: MEGA/SEGA CD");
+  else if (config.add_on == HW_ADDON_MEGASD)
+    sprintf (items[7].text, "CD Add-on: MEGASD");
   else
-    sprintf (items[7].text, "Lock-On: OFF");
+    sprintf (items[7].text, "CD Add-on: NONE");
 
-  sprintf (items[8].text, "Cartridge Swap: %s", (config.hot_swap & 1) ? "ON":"OFF");
+  if (config.lock_on == TYPE_GG)
+    sprintf (items[8].text, "Lock-On: GAME GENIE");
+  else if (config.lock_on == TYPE_AR)
+    sprintf (items[8].text, "Lock-On: ACTION REPLAY");
+  else if (config.lock_on == TYPE_SK)
+    sprintf (items[8].text, "Lock-On: SONIC&KNUCKLES");
+  else
+    sprintf (items[8].text, "Lock-On: OFF");
+
+  sprintf (items[9].text, "Cartridge Swap: %s", (config.hot_swap & 1) ? "ON":"OFF");
 
   if (svp)
   {
-    sprintf (items[10].text, "SVP Cycles: %d", SVP_cycles);
-    m->max_items = 11;
+    sprintf (items[11].text, "SVP Cycles: %d", SVP_cycles);
+    m->max_items = 12;
   }
   else
   {
-    m->max_items = 10;
+    m->max_items = 11;
   }
 
   GUI_InitMenu(m);
@@ -1520,17 +1530,31 @@ static void systemmenu ()
         break;
       }
 
-      case 7:  /*** Cart Lock-On ***/
+      case 7:  /*** CD add-on ***/
+      {
+        config.add_on = (config.add_on + 1) % (HW_ADDON_NONE + 1);
+        if (config.add_on == HW_ADDON_AUTO)
+          sprintf (items[7].text, "CD Add-on: AUTO");
+        else if (config.add_on == HW_ADDON_MEGACD)
+          sprintf (items[7].text, "CD Add-on: MEGA/SEGA CD");
+        else if (config.add_on == HW_ADDON_MEGASD)
+          sprintf (items[7].text, "CD Add-on: MEGASD");
+        else
+          sprintf (items[7].text, "CD Add-on: NONE");
+        break;
+      }
+
+      case 8:  /*** Cart Lock-On ***/
       {
         config.lock_on = (config.lock_on + 1) % (TYPE_SK + 1);
         if (config.lock_on == TYPE_GG)
-          sprintf (items[7].text, "Lock-On: GAME GENIE");
+          sprintf (items[8].text, "Lock-On: GAME GENIE");
         else if (config.lock_on == TYPE_AR)
-          sprintf (items[7].text, "Lock-On: ACTION REPLAY");
+          sprintf (items[8].text, "Lock-On: ACTION REPLAY");
         else if (config.lock_on == TYPE_SK)
-          sprintf (items[7].text, "Lock-On: SONIC&KNUCKLES");
+          sprintf (items[8].text, "Lock-On: SONIC&KNUCKLES");
         else
-          sprintf (items[7].text, "Lock-On: OFF");
+          sprintf (items[8].text, "Lock-On: OFF");
 
         if ((system_hw == SYSTEM_MD) || (system_hw == SYSTEM_PICO))
         {
@@ -1564,14 +1588,14 @@ static void systemmenu ()
         break;
       }
 
-      case 8:  /*** Cartridge Hot Swap ***/
+      case 9:  /*** Cartridge Hot Swap ***/
       {
         config.hot_swap ^= 1;
-        sprintf (items[8].text, "Cartridge Swap: %s", (config.hot_swap & 1) ? "ON":"OFF");
+        sprintf (items[9].text, "Cartridge Swap: %s", (config.hot_swap & 1) ? "ON":"OFF");
         break;
       }
 
-      case 9:  /*** System ROM paths ***/
+      case 10:  /*** System ROM paths ***/
       {
         GUI_DeleteMenu(m);
         rompathmenu();
@@ -1579,10 +1603,10 @@ static void systemmenu ()
         break;
       }
 
-      case 10:  /*** SVP cycles per line ***/
+      case 11:  /*** SVP cycles per line ***/
       {
         GUI_OptionBox(m,0,"SVP Cycles",(void *)&SVP_cycles,1,1,1500,1);
-        sprintf (items[10].text, "SVP Cycles: %d", SVP_cycles);
+        sprintf (items[11].text, "SVP Cycles: %d", SVP_cycles);
         break;
       }
 
diff --git a/gx/gui/menu.h b/gx/gui/menu.h
index ac3b36a..d2425f2 100644
--- a/gx/gui/menu.h
+++ b/gx/gui/menu.h
@@ -3,7 +3,7 @@
  *
  *  Genesis Plus GX menus
  *
- *  Copyright Eke-Eke (2009-2019)
+ *  Copyright Eke-Eke (2009-2021)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
diff --git a/libretro/libretro.c b/libretro/libretro.c
index 7d95f53..3ee1729 100644
--- a/libretro/libretro.c
+++ b/libretro/libretro.c
@@ -3,7 +3,7 @@
  *
  *  Genesis Plus GX libretro port
  *
- *  Copyright Eke-Eke (2007-2020)
+ *  Copyright Eke-Eke (2007-2021)
  *
  *  Copyright Daniel De Matteis (2012-2016)
  *
@@ -608,6 +608,7 @@ static void config_default(void)
    config.addr_error     = 1;
    config.bios           = 0;
    config.lock_on        = 0;
+   config.add_on         = HW_ADDON_AUTO;
    config.lcd            = 0; /* 0.8 fixed point */
 #ifdef HAVE_OVERCLOCK
    config.overclock      = 100;
@@ -1104,6 +1105,21 @@ static void check_variables(void)
       m68k.aerr_enabled = config.addr_error = 0;
   }
 
+  var.key = "genesis_plus_gx_add_on";
+  environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var);
+  {
+    orig_value = config.add_on;
+    if (var.value && !strcmp(var.value, "sega/mega cd"))
+      config.add_on = HW_ADDON_MEGACD;
+    else if (var.value && !strcmp(var.value, "megasd"))
+      config.add_on = HW_ADDON_MEGASD;
+    else if (var.value && !strcmp(var.value, "none"))
+      config.add_on = HW_ADDON_NONE;
+    else
+      config.add_on = HW_ADDON_AUTO;
+    /* note: game needs to be reloaded for change to take effect */
+  }
+
   var.key = "genesis_plus_gx_lock_on";
   environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var);
   {
@@ -1986,6 +2002,7 @@ void retro_set_environment(retro_environment_t cb)
       { "genesis_plus_gx_bios", "System bootrom; disabled|enabled" },
       { "genesis_plus_gx_bram", "CD System BRAM; per bios|per game" },
       { "genesis_plus_gx_addr_error", "68k address error; enabled|disabled" },
+      { "genesis_plus_gx_add_on", "CD add-on (MD mode); auto|sega/mega cd|megasd|none" },
       { "genesis_plus_gx_lock_on", "Cartridge lock-on; disabled|game genie|action replay (pro)|sonic & knuckles" },
       { "genesis_plus_gx_ym2413", "Master System FM (YM2413); auto|disabled|enabled" },
 #ifdef HAVE_OPLL_CORE
diff --git a/libretro/osd.h b/libretro/osd.h
index 8f83c83..e8c7461 100644
--- a/libretro/osd.h
+++ b/libretro/osd.h
@@ -3,7 +3,7 @@
  *
  *  Genesis Plus GX libretro port
  *
- *  Copyright Eke-Eke (2007-2016)
+ *  Copyright Eke-Eke (2007-2021)
  *
  *  Copyright Daniel De Matteis (2012-2016)
  *
@@ -118,6 +118,7 @@ typedef struct
   uint8 addr_error;
   uint8 bios;
   uint8 lock_on;
+  uint8 add_on;
   uint8 overscan;
   uint8 aspect_ratio;
   uint8 ntsc;
diff --git a/psp2/Makefile b/psp2/Makefile
index 6c6a743..81230c8 100644
--- a/psp2/Makefile
+++ b/psp2/Makefile
@@ -86,7 +86,8 @@ OBJECTS	+=      $(OBJDIR)/sram.o        \
 		$(OBJDIR)/eeprom_i2c.o  \
 		$(OBJDIR)/eeprom_spi.o  \
 		$(OBJDIR)/md_cart.o	\
-		$(OBJDIR)/sms_cart.o
+		$(OBJDIR)/sms_cart.o	\
+		$(OBJDIR)/megasd.o
 
 OBJECTS	+=      $(OBJDIR)/scd.o	\
 		$(OBJDIR)/cdd.o	\
diff --git a/psp2/config.c b/psp2/config.c
index 208e078..22ca60d 100644
--- a/psp2/config.c
+++ b/psp2/config.c
@@ -32,6 +32,7 @@ void set_config_defaults(void)
   config.addr_error     = 1;
   config.bios           = 0;
   config.lock_on        = 0; /* = OFF (can be TYPE_SK, TYPE_GG & TYPE_AR) */
+  config.add_on         = 0; /* = HW_ADDON_AUTO (or HW_ADDON_MEGACD, HW_ADDON_MEGASD & HW_ADDON_NONE) */
   config.ntsc           = 0;
   config.lcd            = 0; /* 0.8 fixed point */
 
diff --git a/psp2/config.h b/psp2/config.h
index 657d6ef..cce1455 100644
--- a/psp2/config.h
+++ b/psp2/config.h
@@ -39,6 +39,7 @@ typedef struct
   uint8 addr_error;
   uint8 bios;
   uint8 lock_on;
+  uint8 add_on;
   uint8 hot_swap;
   uint8 invert_mouse;
   uint8 gun_cursor[2];
diff --git a/sdl/Makefile.sdl1 b/sdl/Makefile.sdl1
index 4783878..f83f52a 100644
--- a/sdl/Makefile.sdl1
+++ b/sdl/Makefile.sdl1
@@ -96,7 +96,8 @@ OBJECTS	+=      $(OBJDIR)/sram.o        \
 		$(OBJDIR)/eeprom_i2c.o  \
 		$(OBJDIR)/eeprom_spi.o  \
 		$(OBJDIR)/md_cart.o	\
-		$(OBJDIR)/sms_cart.o	
+		$(OBJDIR)/sms_cart.o	\
+		$(OBJDIR)/megasd.o
 		
 OBJECTS	+=      $(OBJDIR)/scd.o	\
 		$(OBJDIR)/cdd.o	\
diff --git a/sdl/Makefile.sdl2 b/sdl/Makefile.sdl2
index 488d341..8ecbfaf 100644
--- a/sdl/Makefile.sdl2
+++ b/sdl/Makefile.sdl2
@@ -96,7 +96,8 @@ OBJECTS	+=      $(OBJDIR)/sram.o        \
 		$(OBJDIR)/eeprom_i2c.o  \
 		$(OBJDIR)/eeprom_spi.o  \
 		$(OBJDIR)/md_cart.o	\
-		$(OBJDIR)/sms_cart.o	
+		$(OBJDIR)/sms_cart.o	\
+		$(OBJDIR)/megasd.o
 		
 OBJECTS	+=      $(OBJDIR)/scd.o	\
 		$(OBJDIR)/cdd.o	\
diff --git a/sdl/config.c b/sdl/config.c
index 482ce46..1acf53b 100644
--- a/sdl/config.c
+++ b/sdl/config.c
@@ -34,7 +34,8 @@ void set_config_defaults(void)
   config.force_dtack    = 0;
   config.addr_error     = 1;
   config.bios           = 0;
-  config.lock_on        = 0; /* = OFF (can be TYPE_SK, TYPE_GG & TYPE_AR) */
+  config.lock_on        = 0; /* = OFF (or TYPE_SK, TYPE_GG & TYPE_AR) */
+  config.add_on         = 0; /* = HW_ADDON_AUTO (or HW_ADDON_MEGACD, HW_ADDON_MEGASD & HW_ADDON_ONE) */
   config.ntsc           = 0;
   config.lcd            = 0; /* 0.8 fixed point */
 
diff --git a/sdl/config.h b/sdl/config.h
index f39642d..200701f 100644
--- a/sdl/config.h
+++ b/sdl/config.h
@@ -39,6 +39,7 @@ typedef struct
   uint8 addr_error;
   uint8 bios;
   uint8 lock_on;
+  uint8 add_on;
   uint8 hot_swap;
   uint8 invert_mouse;
   uint8 gun_cursor[2];