[Core/VDP] improved VDP status timing accuracy

HISTORY.txt

@@ -95,6 +95,7 @@ Genesis Plus GX 1.7.5 (xx/xx/xxxx) (Eke-Eke)
 * improved Mode 5 sprites rendering timings (fixes "Overdrive" demo)
 * improved FIFO timings accuracy (fixes "Overdrive" Demo)
 * improved H-Counter accuracy in H32 mode
+* improved VDP status timing accuracy
 * improved accuracy of Master System color palette brightness range (verified against real hardware)
 * fixed misaligned buffer writes in Mode 4 when -DALIGN_LONG option is used
 * fixed alpha channel for 15-bit (RGB555) and 32-bit (RGB888) color support

core/system.c

@@ -481,12 +481,6 @@ void system_frame_gen(int do_skip)
   /* refresh inputs just before VINT (Warriors of Eternal Sun) */
   osd_input_update();
 
-  /* delay between VINT flag & Vertical Interrupt (Ex-Mutants, Tyrant) */
-  m68k_run(588);
-  
-  /* set VINT flag */
-  status |= 0x80;
-
   /* delay between VBLANK flag & Vertical Interrupt (Dracula, OutRunners, VR Troopers) */
   m68k_run(788);
   if (zstate == 1)
@@ -498,6 +492,9 @@ void system_frame_gen(int do_skip)
     Z80.cycles = 788;
   }
 
+  /* set VINT flag */
+  status |= 0x80;
+
   /* Vertical Interrupt */
   vint_pending = 0x20;
   if (reg[1] & 0x20)
@@ -853,12 +850,6 @@ void system_frame_scd(int do_skip)
   /* refresh inputs just before VINT */
   osd_input_update();
 
-  /* delay between VINT flag & Vertical Interrupt (Ex-Mutants, Tyrant) */
-  m68k_run(588);
-  
-  /* set VINT flag */
-  status |= 0x80;
-
   /* delay between VBLANK flag & Vertical Interrupt (Dracula, OutRunners, VR Troopers) */
   m68k_run(788);
   if (zstate == 1)
@@ -870,6 +861,9 @@ void system_frame_scd(int do_skip)
     Z80.cycles = 788;
   }
 
+  /* set VINT flag */
+  status |= 0x80;
+
   /* Vertical Interrupt */
   vint_pending = 0x20;
   if (reg[1] & 0x20)

core/vdp_ctrl.c

@@ -1134,6 +1134,9 @@ unsigned int vdp_68k_ctrl_r(unsigned int cycles)
 {
   unsigned int temp;
 
+  /* Cycle-accurate VDP status read (68k read cycle takes four CPU cycles i.e 28 Mcycles) */
+  cycles += 4 * 7;
+
   /* Update FIFO status flags if not empty */
   if (fifo_write_cnt)
   {
@@ -1160,14 +1163,25 @@ unsigned int vdp_68k_ctrl_r(unsigned int cycles)
   /* Clear SOVR & SCOL flags */
   status &= 0xFF9F;
 
-  /* Display OFF: VBLANK flag is set */
+  /* VBLANK flag is set when display is disabled */
   if (!(reg[1] & 0x40))
   {
     temp |= 0x08;
   }
 
-  /* HBLANK flag (Sonic 3 and Sonic 2 "VS Modes", Lemmings 2, Mega Turrican, V.R Troopers, Gouketsuji Ichizoku,...) */
+  /* Cycle-accurate VINT flag (Ex-Mutants, Tyrant / Mega-Lo-Mania) */
-  /* NB: this is not 100% accurate and need to be verified on real hardware */
+  /* this allows VINT flag to be read just before vertical interrupt is being triggered */
+  if ((v_counter == bitmap.viewport.h) && (cycles >= (mcycles_vdp + 788)))
+  {
+    /* check Z80 interrupt state to assure VINT has not already been triggered (and flag cleared) */
+    if (Z80.irq_state != ASSERT_LINE)
+    {
+      temp |= 0x80;
+    }
+  }
+
+  /* Cycle-accurate HBLANK flag (Sonic 3 & Sonic 2 "VS Modes", Bugs Bunny Double Trouble, Lemmings 2, Mega Turrican, V.R Troopers, Gouketsuji Ichizoku,...) */
+  /* NB: this is not 100% accurate (see hvc.h for horizontal events timings in H32 and H40 mode) but is close enough to make no noticeable difference for games */
   if ((cycles % MCYCLES_PER_LINE) < 588)
   {
     temp |= 0x04;