[Core/VDP] improved VDP status read timing accuracy (fixes 'Marvel Land' demo mode)

core/m68k/m68k.h

@@ -356,6 +356,10 @@ extern void s68k_pulse_reset(void);
 extern void m68k_run(unsigned int cycles);
 extern void s68k_run(unsigned int cycles);
 
+/* Get current instruction execution time */
+extern int m68k_cycles(void);
+extern int s68k_cycles(void);
+
 /* Set the IPL0-IPL2 pins on the CPU (IRQ).
  * A transition from < 7 to 7 will cause a non-maskable interrupt (NMI).
  * Setting IRQ to 0 will clear an interrupt request.

core/m68k/m68kcpu.c

@@ -301,6 +301,11 @@ void m68k_run(unsigned int cycles)
   }
 }
 
+int m68k_cycles(void)
+{
+  return CYC_INSTRUCTION[REG_IR];
+}
+
 void m68k_init(void)
 {
 #ifdef BUILD_TABLES

core/m68k/s68kcpu.c

@@ -265,6 +265,12 @@ void s68k_run(unsigned int cycles)
   }
 }
 
+
+int s68k_cycles(void)
+{
+  return CYC_INSTRUCTION[REG_IR];
+}
+
 void s68k_init(void)
 {
 #ifdef BUILD_TABLES

core/vdp_ctrl.c

@@ -1151,8 +1151,8 @@ 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) */
+  /* Cycle-accurate VDP status read (adjust CPU time with current instruction execution time) */
-  cycles += 4 * 7;
+  cycles += m68k_cycles();
 
   /* Update FIFO status flags if not empty */
   if (fifo_write_cnt)
@@ -1186,7 +1186,7 @@ unsigned int vdp_68k_ctrl_r(unsigned int cycles)
     temp |= 0x08;
   }
 
-  /* Cycle-accurate VINT flag (Ex-Mutants, Tyrant / Mega-Lo-Mania) */
+  /* Cycle-accurate VINT flag (Ex-Mutants, Tyrant / Mega-Lo-Mania, Marvel Land) */
   /* this allows VINT flag to be read just before vertical interrupt is being triggered */
   if ((v_counter == bitmap.viewport.h) && (cycles >= (mcycles_vdp + 788)))
   {