From 00496e5b7e7c3444c16daeea28e553e4681b1715 Mon Sep 17 00:00:00 2001
From: kub <derkub@gmail.com>
Date: Sat, 22 Jun 2024 23:12:31 +0200
Subject: [PATCH] core, revisit ym2612 busy flag implementation

---
 pico/memory.c   |  6 +++---
 pico/pico_int.h | 11 +++++++----
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/pico/memory.c b/pico/memory.c
index 168bad919..17139a0d8 100644
--- a/pico/memory.c
+++ b/pico/memory.c
@@ -1174,7 +1174,7 @@ static int ym2612_write_local(u32 a, u32 d, int is_from_z80)
 
       // the busy flag in the YM2612 status is actually a 32 cycle timer
       // (89.6 Z80 cycles), triggered by any write to the data port.
-      Pico.t.ym2612_busy = (cycles + 90) << 8; // Q8 for convenience
+      Pico.t.ym2612_busy = (cycles << 8) + YMBUSY_ZCYCLES; // Q8 for convenience
 
       switch (addr)
       {
@@ -1288,7 +1288,7 @@ void ym2612_pack_state(void)
   tac = 1024 - ym2612.OPN.ST.TA;
   tbc = 256  - ym2612.OPN.ST.TB;
   if (Pico.t.ym2612_busy > 0)
-    busy = (Pico.t.ym2612_busy * 32/90) >> 8;
+    busy = cycles_z80_to_68k(Pico.t.ym2612_busy);
   if (Pico.t.timer_a_next_oflow != TIMER_NO_OFLOW)
     tat = (int)((double)(Pico.t.timer_a_step - Pico.t.timer_a_next_oflow)
           / (double)Pico.t.timer_a_step * tac * 65536);
@@ -1344,7 +1344,7 @@ void ym2612_unpack_state(void)
     return; // no saved timers
   }
 
-  Pico.t.ym2612_busy = (busy << 8) * 90/32;
+  Pico.t.ym2612_busy = cycles_68k_to_z80(busy);
   tac = (1024 - ym2612.OPN.ST.TA) << 16;
   tbc = (256  - ym2612.OPN.ST.TB) << 16;
   if (ym2612.OPN.ST.mode & 1)
diff --git a/pico/pico_int.h b/pico/pico_int.h
index 06f00c2f5..79997d9bf 100644
--- a/pico/pico_int.h
+++ b/pico/pico_int.h
@@ -209,6 +209,7 @@ extern struct DrZ80 drZ80;
 
 // 68k clock = OSC/7, z80 clock = OSC/15, 68k:z80 ratio = 7/15 = 3822.9/8192
 #define cycles_68k_to_z80(x) ((x) * 3823 >> 13)
+#define cycles_z80_to_68k(x) ((x) * 8777 >> 12)
 
 // ----------------------- SH2 CPU -----------------------
 
@@ -898,10 +899,12 @@ void ym2612_unpack_state(void);
 
 #define TIMER_NO_OFLOW 0x70000000
 
-// tA =    72 * (1024 - TA) / M, with M = mclock/2
-#define TIMER_A_TICK_ZCYCLES cycles_68k_to_z80(256LL*   72*2) // Q8
-// tB = 16*72 * ( 256 - TB) / M
-#define TIMER_B_TICK_ZCYCLES cycles_68k_to_z80(256LL*16*72*2) // Q8
+// tA =    24*3 * (1024 - TA) / M, with M = mclock/2
+#define TIMER_A_TICK_ZCYCLES cycles_68k_to_z80(256LL*   24*3*2) // Q8
+// tB = 16*24*3 * ( 256 - TB) / M
+#define TIMER_B_TICK_ZCYCLES cycles_68k_to_z80(256LL*16*24*3*2) // Q8
+// busy =  32*3 / M
+#define YMBUSY_ZCYCLES       cycles_68k_to_z80(256LL*   32*3*2) // Q8
 
 #define timers_cycle(ticks) \
   if (Pico.t.ym2612_busy > 0) \