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add OG's note about why Gain Ground drops into Free Play mode, and added a kludge to work around it for now.

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not ideal, but less of a problem than having to tell people to use versions that are 8 years old and frankly worse in every single other possible way.
This commit is contained in:
David Haywood 2016-04-12 15:17:10 +01:00
parent 79bfc4b153
commit fbc8dbf193
2 changed files with 54 additions and 0 deletions
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47
src/mame/drivers/segas24.cpp
View File

@ -680,6 +680,46 @@ WRITE16_MEMBER( segas24_state::iod_w )
logerror("IO daughterboard write %02x, %04x & %04x (%x)\n", offset, data, mem_mask, space.device().safe_pc());
}
/* HACK for Gain Ground to avoid 'forced free play' issue
Notes from Olivier
The encrypted CPU does:
849c: moveq #-1, d1
849e: move.w 0xa00000, d0
84a4: cmp.w 0xa00000, d0
84aa: beq.s 84a4
84ac: add.w #0x200, d0
84b0: andi.w #0xfff, d0
84b4: cmp.w 0xa00000, d0 // 16 cycles
84ba: dbeq d1, 84b4 // 10 cycles
84be: addi.w #0x1b5f, d1
84c2: bpl 84c8
84c4: st 0x404 // Force freeplay
84c8: ...
a00000 is the timer 12bit counter. It is configured to be clocked by
the hsync pulse. That code counts how many loops it takes for the
counter to count 512 times. The force freeplay happens if the count
is more than 7007.
Pixel clock is 16MHz, hsync is every 656 pixels, cpu clock is 10MHz.
So that's 656*10/16=410 cpu clocks per hsync, or 410*512=209902 total.
With 26 cycles per loop, that's 8073 loops. Freeplay it is.
So, now what? Maybe the cpu is 8Mhz, maybe there's a wait time on the
a00000 access.
*/
TIMER_CALLBACK_MEMBER(segas24_state::gground_hack_timer_callback)
{
m_subcpu->set_clock_scale(1.0f);
}
// Cpu #1 reset control
@ -693,6 +733,11 @@ void segas24_state::reset_reset()
m_subcpu->set_input_line(INPUT_LINE_RESET, PULSE_LINE);
// osd_printf_debug("enable 2nd cpu!\n");
// debugger_break(machine);
if (m_gground_hack_timer)
{
m_subcpu->set_clock_scale(0.7f); // reduce clock speed temporarily so a check passes, see notes above
m_gground_hack_timer->adjust(attotime::from_seconds(2));
}
} else
m_subcpu->set_input_line(INPUT_LINE_HALT, ASSERT_LINE);
@ -2450,6 +2495,8 @@ DRIVER_INIT_MEMBER(segas24_state,gground)
io_w = &segas24_state::hotrod_io_w;
mlatch_table = nullptr;
track_size = 0x2d00;
m_gground_hack_timer = machine().scheduler().timer_alloc(timer_expired_delegate(FUNC(segas24_state::gground_hack_timer_callback), this));
}
DRIVER_INIT_MEMBER(segas24_state,crkdown)

7
src/mame/includes/segas24.h
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@ -20,6 +20,7 @@ public:
, m_palette(*this, "palette")
, m_generic_paletteram_16(*this, "paletteram")
, m_romboard(*this, "romboard")
, m_gground_hack_timer(nullptr)
{
}
@ -142,4 +143,10 @@ public:
TIMER_DEVICE_CALLBACK_MEMBER(irq_timer_clear_cb);
TIMER_DEVICE_CALLBACK_MEMBER(irq_frc_cb);
TIMER_DEVICE_CALLBACK_MEMBER(irq_vbl);
// game specific
TIMER_CALLBACK_MEMBER(gground_hack_timer_callback);
emu_timer *m_gground_hack_timer;
};