added proper HV Counter latch support, as verified on real hardware (fixes Sunset Riders intro)

HISTORY.txt

@@ -25,6 +25,7 @@ of samples per frame and keeping PSG & FM chips in sync.
 ---------------
 * added support for CRAM writes during horizontal blanking (Striker, Zero the Kamikaze Squirrel,...)
 * added support for 2-Cell vertical scrolling in Interlaced 2 mode
+* added proper HV Counter latch support (fixes Sunset Riders intro)
 * fixed left-most column vertical scrolling when horizontally scrolled (Gynoug, F1) 
 * fixed VBLANK transition line checks
 * improved VDP FIFO timings accuracy: fixes Sol Deace intro
@@ -32,9 +33,10 @@ of samples per frame and keeping PSG & FM chips in sync.
 * improved HBLANK flag timing accuracy: fixes Mega Turrican (Sky level)
 * improved sprites processing accuracy: fixes (un)masked sprites in Mickey Mania (3D level), Sonic 2 (VS mode).
 * improved horizontal blanking & HINT/VINT occurence timing accuracy, as measured on real hardware.
-* improved H-Counter accuracy in 40-cell mode, as measured on real hardware.
+* improved HCounter accuracy in 40-cell mode, as measured on real hardware.
 * improved color accuracy in VDP highlight mode to match results observed on real hardware.
 
+
 [Core/CPU]
 ---------------
 * updated Z80 core to last version (fixes interrupt Mode 0 timing and some BIT instructions).

source/gen_input.c

@@ -90,16 +90,17 @@ static inline void lightgun_update(int num)
       if (reg[11] & 0x08) 
         irq_status = (irq_status & ~0x40) | 0x12;
 
-      /* Horizontal Counter Latch:
-        1) some games does not set HVC latch but instead use bigger X offset 
-        2) for games using H40 mode, the gun routine scales up the Hcounter value,
-           H-Counter range is approx. 292 pixel clocks
+      /* HV Counter Latch:
+        1) some games does not enable HVC latch but instead use bigger X offset 
+            --> we force the HV counter value read by the gun routine 
+        2) for games using H40 mode, the gun routine scales up the Hcounter value
+            --> H-Counter range is approx. 290 dot clocks
       */
-      hc_latch = 0x100;
+      hvc_latch = 0x10000 | (vctab[v_counter] << 8);
       if (reg[12] & 1) 
-        hc_latch |= hc_320[((input.analog[num][0] * 290) / (2 * 320) + input.x_offset) % 210];
+        hvc_latch |= hc_320[((input.analog[num][0] * 290) / (2 * 320) + input.x_offset) % 210];
       else
-        hc_latch |= hc_256[(input.analog[num][0] / 2 + input.x_offset)%171];
+        hvc_latch |= hc_256[(input.analog[num][0] / 2 + input.x_offset)%171];
     }
   }
 }

source/vdp.c

@@ -66,8 +66,8 @@ uint8 bg_pattern_cache[0x80000];  /* Cached and flipped patterns */
 uint8 playfield_shift;            /* Width of planes A, B (in bits) */
 uint8 playfield_col_mask;         /* Vertical scroll mask */
 uint16 playfield_row_mask;        /* Horizontal scroll mask */
-uint16 hc_latch;                  /* latched HCounter (INT2) */
 uint16 v_counter;                 /* VDP scanline counter */
+uint32 hvc_latch;                 /* latched HVCounter (INT2) */
 uint32 dma_length;                /* Current DMA remaining bytes */
 int32 fifo_write_cnt;             /* VDP writes fifo count */
 uint32 fifo_lastwrite;            /* last VDP write cycle */
@@ -161,7 +161,7 @@ void vdp_reset(void)
   hint_pending    = 0;
   vint_pending    = 0;
   irq_status      = 0;
-  hc_latch        = 0;
+  hvc_latch       = 0;
   v_counter       = 0;
   dmafill         = 0;
   dma_length      = 0;
@@ -455,8 +455,12 @@ unsigned int vdp_ctrl_r(void)
 
 unsigned int vdp_hvc_r(void)
 {
+  /* HVC is frozen (Lightgun games + Sunset Riders) */
+  if (hvc_latch)
+    return (hvc_latch & 0xffff);
+
   /* Horizontal Counter (Striker, Mickey Mania, Skitchin, Road Rash I,II,III, ...) */
-  uint8 hc = (hc_latch & 0x100) ? (hc_latch & 0xFF) : hctab[mcycles_68k%MCYCLES_PER_LINE];
+  uint8 hc = hctab[mcycles_68k%MCYCLES_PER_LINE];
 
   /* Vertical Counter */
   uint8 vc = vctab[v_counter];
@@ -780,6 +784,16 @@ static void reg_w(unsigned int r, unsigned int d)
         for (i = 1; i < 0x40; i += 1)
           color_update (i, *(uint16 *) & cram[i << 1]);
       }
+
+      /* HVC latch bit */
+      if (r & 0x02)
+      {
+        if (reg[0] & 2) /* latch current HVC */
+          hvc_latch = 0x10000 | (vctab[v_counter] << 8) | hctab[mcycles_68k%MCYCLES_PER_LINE];
+        else            /* free-running HVC */
+          hvc_latch = 0;
+      }
+
       break;
 
     case 1: /* CTRL #2 */

source/vdp.h

@@ -54,8 +54,8 @@ extern uint8 bg_pattern_cache[0x80000];
 extern uint8 playfield_shift;
 extern uint8 playfield_col_mask;
 extern uint16 playfield_row_mask;
-extern uint16 hc_latch;
 extern uint16 v_counter;
+extern uint32 hvc_latch;
 extern uint32 dma_length;
 extern int32 fifo_write_cnt;
 extern uint32 fifo_lastwrite;