From 2fd18851754e715bd55e7040aaace2590ac0d8cb Mon Sep 17 00:00:00 2001
From: ds22x <45218067+ds22x@users.noreply.github.com>
Date: Sat, 20 Jul 2024 16:12:24 +0200
Subject: [PATCH] Add Force VDP Mode option & update option description (#565)

---
 libretro/libretro.c              | 81 ++++++++++++++++++++++++++++++++
 libretro/libretro_core_options.h | 15 ++++++
 2 files changed, 96 insertions(+)

diff --git a/libretro/libretro.c b/libretro/libretro.c
index c632d64f..a581879b 100644
--- a/libretro/libretro.c
+++ b/libretro/libretro.c
@@ -1455,6 +1455,87 @@ static void check_variables(bool first_run)
     }
   }
 
+  var.key = "genesis_plus_gx_vdp_mode";
+  environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var);
+  {
+    orig_value = config.vdp_mode;
+    if (var.value && !strcmp(var.value, "60hz"))
+      config.vdp_mode = 1;
+    else if (var.value && !strcmp(var.value, "50hz"))
+      config.vdp_mode = 2;
+    else
+      config.vdp_mode = 0;
+
+    if (orig_value != config.vdp_mode)
+    {
+      if (system_hw)
+      {
+        get_region(NULL);
+
+        if ((system_hw == SYSTEM_MCD) || ((system_hw & SYSTEM_SMS) && config.bios))
+        {
+          /* system with region BIOS should be reinitialized */
+          reinit = true;
+        }
+        else
+        {
+          static const uint16 vc_table[4][2] = 
+          {
+            /* NTSC, PAL */
+            {0xDA , 0xF2},  /* Mode 4 (192 lines) */
+            {0xEA , 0x102}, /* Mode 5 (224 lines) */
+            {0xDA , 0xF2},  /* Mode 4 (192 lines) */
+            {0x106, 0x10A}  /* Mode 5 (240 lines) */
+          };
+
+          /* framerate might have changed, reinitialize audio timings */
+          audio_set_rate(44100, 0);
+
+          /* reinitialize I/O region register */
+          if (system_hw == SYSTEM_MD)
+          {
+            io_reg[0x00] = 0x20 | region_code | (config.bios & 1);
+          }
+          else if (system_hw == SYSTEM_MCD)
+          {
+            io_reg[0x00] = region_code | (config.bios & 1);
+          }
+          else
+          {
+            io_reg[0x00] = 0x80 | (region_code >> 1);
+          }
+
+          /* reinitialize VDP timings */
+          lines_per_frame = vdp_pal ? 313 : 262;
+
+          /* reinitialize NTSC/PAL mode in VDP status */
+          if (system_hw & SYSTEM_MD)
+          {
+            status = (status & ~1) | vdp_pal;
+          }
+
+          /* reinitialize VC max value */
+          switch (bitmap.viewport.h)
+          {
+            case 192:
+              vc_max = vc_table[0][vdp_pal];
+              break;
+            case 224:
+              vc_max = vc_table[1][vdp_pal];
+              break;
+            case 240:
+              vc_max = vc_table[3][vdp_pal];
+              break;
+          }
+
+          update_viewports = true;
+        }
+
+        update_frameskip = true;
+      }
+    }
+  }
+
   var.key = "genesis_plus_gx_force_dtack";
   environ_cb(RETRO_ENVIRONMENT_GET_VARIABLE, &var);
   {
diff --git a/libretro/libretro_core_options.h b/libretro/libretro_core_options.h
index 55928c10..d1cef8a2 100644
--- a/libretro/libretro_core_options.h
+++ b/libretro/libretro_core_options.h
@@ -131,6 +131,21 @@ struct retro_core_option_v2_definition option_defs_us[] = {
       },
       "auto"
    },
+   {
+      "genesis_plus_gx_vdp_mode",
+      "Force VDP Mode",
+      NULL,
+      "Overrides the VDP mode to force it to run at either NTSC 60Hz or PAL 50Hz, regardless of system region.",
+      NULL,
+      "system",
+      {
+         { "auto",  "Disabled" },
+         { "60hz",  "NTSC (60Hz)" },
+         { "50hz",  "PAL (50Hz)" },
+         { NULL, NULL },
+      },
+      "auto"
+   },
    {
       "genesis_plus_gx_bios",
       "System Boot ROM",