[Core/MD] added basic emulation of 68k bus access refresh delays (fixes Super Airwolf graphical glitch during intro & some Krikzz's mcd-verificator timing tests)

HISTORY.txt

@@ -73,6 +73,7 @@ Genesis Plus GX 1.7.5 (xx/xx/xxxx) (Eke-Eke)
 * added support for some new unlicensed games with copy protection (Thunderbolt II, Tom Clown, Chaoji Puke / Super Poker, Rock Heaven, Rock World)
 * added support for Everdrive extended SSF mapper
 * added support for MegaSD CD hardware overlay (MD+ hacks) and extended SSF2 / ROM write mappers
+* added basic emulation of 68k bus access refresh delays (fixes Super Airwolf graphical glitch during intro & some Krikzz's mcd-verificator timing tests)
 * added (very basic) emulation of Flashkit MD hardware
 * added emulation of Micro Machines USA on-board TMSS bypass logic hardware
 * added SRAM support for games larger than 8MB

core/m68k/m68k.h

@@ -238,7 +238,8 @@ typedef struct
 
   cpu_idle_t poll;      /* polling detection */
 
-  uint cycles;          /* current master cycle count */ 
+  sint cycles;          /* current master cycle count */ 
+  sint refresh_cycles;  /* external bus refresh cycle */ 
   uint cycle_end;       /* aimed master cycle count for current execution frame */
 
   uint dar[16];         /* Data and Address Registers */

core/m68k/m68kcpu.c

@@ -299,6 +299,13 @@ void m68k_run(unsigned int cycles)
     /* Decode next instruction */
     REG_IR = m68ki_read_imm_16();
 
+    /* 68K bus access refresh delay (Mega Drive / Genesis specific) */
+    if (m68k.cycles >= (m68k.refresh_cycles + (128*7)))
+    {
+      m68k.refresh_cycles = (m68k.cycles / (128*7)) * (128*7);
+      m68k.cycles += (2*7);
+    }
+
     /* Execute instruction */
     m68ki_instruction_jump_table[REG_IR]();
     USE_CYCLES(CYC_INSTRUCTION[REG_IR]);
@@ -382,6 +389,9 @@ void m68k_pulse_reset(void)
 #endif
 
   USE_CYCLES(CYC_EXCEPTION[EXCEPTION_RESET]);
+
+  /* Synchronize 68k bus refresh mechanism (Mega Drive / Genesis specific) */
+  m68k.refresh_cycles = (m68k.cycles / (128*7)) * (128*7);
 }
 
 void m68k_pulse_halt(void)

core/m68k/m68kops.h

@@ -8,7 +8,7 @@ INLINE void UseDivuCycles(uint32 dst, uint32 src)
   int i;
 
   /* minimum cycle time */
-  uint mcycles = 38 * MUL;
+  uint mcycles = 76 * MUL;
 
   /* 16-bit divisor */
   src <<= 16;
@@ -27,27 +27,33 @@ INLINE void UseDivuCycles(uint32 dst, uint32 src)
     {
       /* shift dividend and add two cycles */
       dst <<= 1;
-      mcycles += (2 * MUL);
+      mcycles += (4 * MUL);
 
       if (dst >= src)
       {
         /* apply divisor and remove one cycle */
         dst -= src;
-        mcycles -= 1 * MUL;
+        mcycles -= 2 * MUL;
       }
     }
   }
 
-  USE_CYCLES(mcycles << 1);
+  USE_CYCLES(mcycles);
+
+  /* one 68K bus refresh cycle should be skipped if instruction processing time is longer than refresh period (128 CPU cycles on Mega Drive / Genesis) */
+  if (mcycles >= (128*MUL))
+  {
+    m68ki_cpu.refresh_cycles += (128*MUL);
+  }
 }
 
 INLINE void UseDivsCycles(sint32 dst, sint16 src)
 {
   /* minimum cycle time */
-  uint mcycles = 6 * MUL;
+  uint mcycles = 12 * MUL;
 
   /* negative dividend */
-  if (dst < 0) mcycles += 1 * MUL;
+  if (dst < 0) mcycles += 2 * MUL;
 
   if ((abs(dst) >> 16) < abs(src))
   {
@@ -57,30 +63,36 @@ INLINE void UseDivsCycles(sint32 dst, sint16 src)
     uint32 quotient = abs(dst) / abs(src);
 
     /* add default cycle time */
-    mcycles += (55 * MUL);
+    mcycles += (110 * MUL);
 
     /* positive divisor */
     if (src >= 0)
     {
       /* check dividend sign */
-      if (dst >= 0) mcycles -= 1 * MUL;
-      else mcycles += 1 * MUL;
+      if (dst >= 0) mcycles -= 2 * MUL;
+      else mcycles += 2 * MUL;
     }
 
     /* check higher 15-bits of quotient */
     for (i=0; i<15; i++)
     {
       quotient >>= 1;
-      if (!(quotient & 1)) mcycles += 1 * MUL;
+      if (!(quotient & 1)) mcycles += 2 * MUL;
     }
   }
   else
   {
     /* absolute overflow */
-    mcycles += (2 * MUL);
+    mcycles += (4 * MUL);
   }
 
-  USE_CYCLES(mcycles << 1);
+  USE_CYCLES(mcycles);
+  
+  /* one 68K bus refresh cycle should be skipped if instruction processing time is longer than refresh period (128 CPU cycles on Mega Drive / Genesis) */
+  if (mcycles >= (128*MUL))
+  {
+    m68ki_cpu.refresh_cycles += (128*MUL);
+  }
 }
 
 INLINE void UseMuluCycles(uint16 src)
@@ -18692,6 +18704,7 @@ static void m68k_op_reset(void)
   {
     m68ki_output_reset()       /* auto-disable (see m68kcpu.h) */
     USE_CYCLES(CYC_RESET);
+    m68ki_cpu.refresh_cycles += (128*MUL); /* skip one 68K bus refresh cycle as instruction processing time is longer than refresh period (128 CPU cycles on Mega Drive / Genesis) */
     return;
   }
   m68ki_exception_privilege_violation();

core/system.c

@@ -5,7 +5,7 @@
  *  Support for 16-bit & 8-bit hardware modes
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2021  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2024  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -659,6 +659,7 @@ void system_frame_gen(int do_skip)
 
   /* adjust timings for next frame */
   input_end_frame(mcycles_vdp);
+  m68k.refresh_cycles -= mcycles_vdp;
   m68k.cycles -= mcycles_vdp;
   Z80.cycles -= mcycles_vdp;
 }
@@ -983,6 +984,7 @@ void system_frame_scd(int do_skip)
   /* adjust timings for next frame */
   scd_end_frame(scd.cycles);
   input_end_frame(mcycles_vdp);
+  m68k.refresh_cycles -= mcycles_vdp;
   m68k.cycles -= mcycles_vdp;
   Z80.cycles -= mcycles_vdp;
 }

core/system.h

@@ -5,7 +5,7 @@
  *  Support for 16-bit & 8-bit hardware modes
  *
  *  Copyright (C) 1998-2003  Charles Mac Donald (original code)
- *  Copyright (C) 2007-2022  Eke-Eke (Genesis Plus GX)
+ *  Copyright (C) 2007-2024  Eke-Eke (Genesis Plus GX)
  *
  *  Redistribution and use of this code or any derivative works are permitted
  *  provided that the following conditions are met:
@@ -95,7 +95,6 @@ typedef struct
   blip_t* blips[3];     /* Blip Buffer resampling (stereo) */
 } t_snd;
 
-
 /* Global variables */
 extern t_bitmap bitmap;
 extern t_snd snd;
@@ -119,4 +118,3 @@ extern void system_frame_scd(int do_skip);
 extern void system_frame_sms(int do_skip);
 
 #endif /* _SYSTEM_H_ */
-