From d1feaa8049003b6fa7580eab5317ecf941444383 Mon Sep 17 00:00:00 2001
From: EkeEke <ekeeke31@gmail.com>
Date: Thu, 27 Jun 2013 23:32:27 +0200
Subject: [PATCH] [Core/VDP] improved HVC latch behavior when not locked in
 hardware but forced for gun emulation ("Gunfight - 3 in 1" randomization
 issues when Justifier is enabled)

---
 core/input_hw/lightgun.c |  4 ++--
 core/input_hw/lightgun.h |  2 +-
 core/vdp_ctrl.c          | 17 +++++++++++++----
 3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/core/input_hw/lightgun.c b/core/input_hw/lightgun.c
index f9a6d5f..4ed0b76 100644
--- a/core/input_hw/lightgun.c
+++ b/core/input_hw/lightgun.c
@@ -2,7 +2,7 @@
  *  Genesis Plus
  *  Sega Light Phaser, Menacer & Konami Justifiers support
  *
- *  Copyright (C) 2007-2011  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2013  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -135,7 +135,7 @@ void lightgun_refresh(int port)
           m68k_update_irq(2);
         }
 
-        /* force HV Counter Latch (some games does not lock HV Counter but instead use larger offset value) */
+        /* HACK: force HV Counter latch (some games does not lock HV Counter but instead use larger offset value) */
         hvc_latch = 0x10000 | (y << 8);
         if (reg[12] & 1) 
         {
diff --git a/core/input_hw/lightgun.h b/core/input_hw/lightgun.h
index 4a67946..0021297 100644
--- a/core/input_hw/lightgun.h
+++ b/core/input_hw/lightgun.h
@@ -2,7 +2,7 @@
  *  Genesis Plus
  *  Sega Light Phaser, Menacer & Konami Justifiers support
  *
- *  Copyright (C) 2007-2011  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2013  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/vdp_ctrl.c b/core/vdp_ctrl.c
index 96c9f42..e2fe42e 100644
--- a/core/vdp_ctrl.c
+++ b/core/vdp_ctrl.c
@@ -1370,17 +1370,26 @@ unsigned int vdp_hvc_r(unsigned int cycles)
   }
   else
   {
+    /* Mode 5: HV counters are frozen (cf. lightgun games & Sunset Riders) */
     if (reg[1] & 4)
     {
-      /* Mode 5: both counters are frozen (Lightgun games, Sunset Riders) */
+      /* check if HVC is really locked or is read within INT2 callback (HVC latch is forced for lightgun emulation, cf. lightgun.c) */
+      if ((reg[0] & 0x02) || (m68k.int_mask == 0x0200))
+      {
 #ifdef LOGVDP
-      error("[%d(%d)][%d(%d)] HVC read -> 0x%x (%x)\n", v_counter, (cycles/MCYCLES_PER_LINE-1)%lines_per_frame, cycles, cycles%MCYCLES_PER_LINE, hvc_latch & 0xffff, m68k_get_reg(M68K_REG_PC));
+        error("[%d(%d)][%d(%d)] HVC read -> 0x%x (%x)\n", v_counter, (cycles/MCYCLES_PER_LINE-1)%lines_per_frame, cycles, cycles%MCYCLES_PER_LINE, data & 0xffff, m68k_get_reg(M68K_REG_PC));
 #endif
-      return (data & 0xffff);
+        /* return latched HVC value */
+        return (data & 0xffff);
+      }
+
+      /* HV counters should be free-running (fixes "Gunfight - 3 in 1" randomization when Justifiers are enabled) */
+      hvc_latch = 0;
+      data = hctab[cycles % MCYCLES_PER_LINE];
     }
     else
     {
-      /* Mode 4: VCounter runs normally, HCounter is frozen */
+      /* Mode 4: V counter runs normally, H counter is frozen */
       data &= 0xff;
     }
   }