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Diffstat (limited to 'src/mame/drivers/mac128.cpp')
-rw-r--r--src/mame/drivers/mac128.cpp33
1 files changed, 27 insertions, 6 deletions
diff --git a/src/mame/drivers/mac128.cpp b/src/mame/drivers/mac128.cpp
index 11cb0cc75c0..884f29500bb 100644
--- a/src/mame/drivers/mac128.cpp
+++ b/src/mame/drivers/mac128.cpp
@@ -193,6 +193,7 @@ private:
void ram_w_se(offs_t offset, uint16_t data, uint16_t mem_mask = ~0);
uint16_t ram_600000_r(offs_t offset);
void ram_600000_w(offs_t offset, uint16_t data, uint16_t mem_mask = ~ 0);
+ void via_sync();
uint16_t mac_via_r(offs_t offset);
void mac_via_w(offs_t offset, uint16_t data);
uint16_t mac_autovector_r(offs_t offset);
@@ -253,7 +254,6 @@ private:
int m_scsi_drq;
// wait states for accessing the VIA
- int m_via_cycles;
bool m_snd_enable;
bool m_main_buffer;
int m_snd_vol;
@@ -304,7 +304,6 @@ void mac128_state::machine_start()
void mac128_state::machine_reset()
{
- m_via_cycles = -10;
m_last_taken_interrupt = -1;
m_overlay = 1;
m_screen_buffer = 1;
@@ -680,6 +679,28 @@ WRITE_LINE_MEMBER(mac128_state::mac_via_irq)
set_via_interrupt(state);
}
+void mac128_state::via_sync()
+{
+ // The VIA runs from the E clock of the 68k and uses VPA.
+
+ // That means:
+ // - The 68000 starts the access cycle, with AS and the address bus. It's validated at cycle+1.
+
+ // - The glue chip sets VPA. The 68000 sees it and acts on it at cycle+2.
+
+ // - Between cycle+2 and cycle+11, the E clock goes up. The VIA
+ // is synced on that clock, so that's at a multiple of 10 in
+ // absolute time
+
+ // - 4 cycles later E goes down and that's the end of the access
+
+ uint64_t cur_cycle = m_maincpu->total_cycles();
+ uint64_t vpa_cycle = cur_cycle+2;
+ uint64_t via_start_cycle = (vpa_cycle + 9) / 10;
+ uint64_t end_cycle = via_start_cycle * 10 + 4;
+ m_maincpu->adjust_icount(cur_cycle - end_cycle);
+}
+
uint16_t mac128_state::mac_via_r(offs_t offset)
{
uint16_t data;
@@ -687,9 +708,9 @@ uint16_t mac128_state::mac_via_r(offs_t offset)
offset >>= 8;
offset &= 0x0f;
- data = m_via->read(offset);
+ via_sync();
- m_maincpu->adjust_icount(m_via_cycles);
+ data = m_via->read(offset);
return (data & 0xff) | (data << 8);
}
@@ -699,9 +720,9 @@ void mac128_state::mac_via_w(offs_t offset, uint16_t data)
offset >>= 8;
offset &= 0x0f;
- m_via->write(offset, (data >> 8) & 0xff);
+ via_sync();
- m_maincpu->adjust_icount(m_via_cycles);
+ m_via->write(offset, (data >> 8) & 0xff);
}
void mac128_state::mac_autovector_w(offs_t offset, uint16_t data)