From 134092feb7ea612db0a74c547667978aab980595 Mon Sep 17 00:00:00 2001
From: kub <derkub@gmail.com>
Date: Wed, 5 Apr 2023 20:02:47 +0200
Subject: [PATCH] md, implement z80 cycle stealing when accessing 68k bus
this fixes sound issues in Overdrive 2
---
pico/memory.c | 10 ++++++++++
pico/pico_cmn.c | 30 ++++++++++++++++++++++++------
pico/pico_int.h | 4 +++-
3 files changed, 37 insertions(+), 7 deletions(-)
diff --git a/pico/memory.c b/pico/memory.c
index 03450191..b268e85e 100644
--- a/pico/memory.c
+++ b/pico/memory.c
@@ -1294,7 +1294,11 @@ static unsigned char z80_md_bank_read(unsigned short a)
unsigned int addr68k;
unsigned char ret;
+ // account for 68K bus access on both CPUs.
+ // don't use SekCyclesBurn(7) here since the Z80 doesn't run in cycle lock to
+ // the 68K. Count the stolen cycles to be accounted later in the 68k CPU runs
z80_subCLeft(3);
+ Pico.t.z80_buscycles += 7;
addr68k = Pico.m.z80_bank68k << 15;
addr68k |= a & 0x7fff;
@@ -1335,6 +1339,12 @@ static void z80_md_bank_write(unsigned int a, unsigned char data)
{
unsigned int addr68k;
+ // account for 68K bus access on both CPUs.
+ // don't use SekCyclesBurn(7) here since the Z80 doesn't run in cycle lock to
+ // the 68K. Count the stolen cycles to be accounted later in the 68K CPU runs
+ z80_subCLeft(3);
+ Pico.t.z80_buscycles += 7;
+
addr68k = Pico.m.z80_bank68k << 15;
addr68k += a & 0x7fff;
diff --git a/pico/pico_cmn.c b/pico/pico_cmn.c
index 8cad1dd7..4cffc55f 100644
--- a/pico/pico_cmn.c
+++ b/pico/pico_cmn.c
@@ -41,11 +41,22 @@ static void SekExecM68k(int cyc_do)
static void SekSyncM68k(void)
{
int cyc_do;
+
pprof_start(m68k);
pevt_log_m68k_o(EVT_RUN_START);
- while ((cyc_do = Pico.t.m68c_aim - Pico.t.m68c_cnt) > 0)
- SekExecM68k(cyc_do);
+ while ((cyc_do = Pico.t.m68c_aim - Pico.t.m68c_cnt) > 0) {
+ // the Z80 CPU is stealing some bus cycles from the 68K main CPU when
+ // accessing the 68K RAM or ROM. Account for these by shortening the time
+ // the 68K CPU runs.
+ int z80_buscyc = Pico.t.z80_buscycles;
+ if (z80_buscyc <= cyc_do)
+ SekExecM68k(cyc_do - z80_buscyc);
+ else
+ z80_buscyc = cyc_do;
+ Pico.t.m68c_cnt += z80_buscyc;
+ Pico.t.z80_buscycles -= z80_buscyc;
+ }
SekTrace(0);
pevt_log_m68k_o(EVT_RUN_END);
@@ -56,9 +67,7 @@ static __inline void SekRunM68k(int cyc)
{
Pico.t.m68c_aim += cyc;
Pico.t.m68c_cnt += cyc >> 6; // refresh slowdowns
- cyc = Pico.t.m68c_aim - Pico.t.m68c_cnt;
- if (cyc <= 0)
- return;
+
SekSyncM68k();
}
@@ -90,11 +99,20 @@ static void do_hint(struct PicoVideo *pv)
static void do_timing_hacks_end(struct PicoVideo *pv)
{
PicoVideoFIFOSync(CYCLES_M68K_LINE);
+
+ // need rather tight Z80 sync for emulation of main bus cycle stealing
+ if (Pico.m.z80Run && !Pico.m.z80_reset && (PicoIn.opt&POPT_EN_Z80) && (Pico.m.scanline&1))
+ PicoSyncZ80(Pico.t.m68c_aim);
}
static void do_timing_hacks_start(struct PicoVideo *pv)
{
- SekCyclesBurn(PicoVideoFIFOHint()); // prolong cpu HOLD if necessary
+ int cycles = PicoVideoFIFOHint();
+
+ SekCyclesBurn(cycles); // prolong cpu HOLD if necessary
+ // XXX how to handle Z80 bus cycle stealing during DMA correctly?
+ if ((Pico.t.z80_buscycles -= cycles) < 0)
+ Pico.t.z80_buscycles = 0;
}
static int PicoFrameHints(void)
diff --git a/pico/pico_int.h b/pico/pico_int.h
index 6b245c33..57b6c544 100644
--- a/pico/pico_int.h
+++ b/pico/pico_int.h
@@ -204,7 +204,8 @@ extern struct DrZ80 drZ80;
#define z80_cyclesDone() \
(Pico.t.z80c_aim - z80_cyclesLeft)
-#define cycles_68k_to_z80(x) ((x) * 3822 >> 13)
+// one line has 488 68K cycles and 228 Z80 cycles, 228/488*8192=3827
+#define cycles_68k_to_z80(x) ((x) * 3857 >> 13)
// ----------------------- SH2 CPU -----------------------
@@ -447,6 +448,7 @@ struct PicoTiming
unsigned int z80c_aim;
unsigned int z80c_line_start;
int z80_scanline;
+ int z80_buscycles;
int timer_a_next_oflow, timer_a_step; // in z80 cycles
int timer_b_next_oflow, timer_b_step;
--
2.39.5