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[Core/VDP] added some precision in comments about delay on invalid VRAM writes

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This commit is contained in:
EkeEke 2013-10-28 22:06:28 +01:00
parent e45ab67d05
commit 41b25d3580
1 changed files with 11 additions and 5 deletions
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16
core/vdp_ctrl.c
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@ -805,11 +805,17 @@ void vdp_68k_ctrl_w(unsigned int data)
/*
FIFO emulation (Chaos Engine/Soldier of Fortune, Double Clutch, Sol Deace)
--------------------------------------------------------------------------
Each VRAM access is byte wide, so one VRAM write (word) need twice cycles.
Each VRAM access is byte wide, so one VRAM write (word) need two slot access.
Note: Invalid code 0x02 (register write) apparently behaves the same as VRAM
access, although no data is written in this case (fixes Clue menu)
*/
NOTE: Invalid code 0x02 (register write) should not behave the same as VRAM
access, i.e data is ignored and only one access slot is used for each word,
BUT a few games ("Clue", "Microcosm") which accidentally corrupt code value
will have issues when emulating FIFO timings. They likely work fine on real
hardware because of periodical 68k wait-states which have been observed and
would naturaly add some delay between writes. Until those wait-states are
accurately measured and emulated, delay is forced when invalid code value
is being used.
*/
fifo_byte_access = ((code & 0x0F) <= 0x02);
}
@ -2303,7 +2309,7 @@ static void vdp_bus_w(unsigned int data)
default:
{
/* add some delay until 68k periodical wait-states (RAM refresh ?) are accurately emulated (needed by "Clue" & "Microcosm") */
/* add some delay until 68k periodical wait-states are accurately emulated ("Clue", "Microcosm") */
m68k.cycles += 2;
#ifdef LOGERROR
error("[%d(%d)][%d(%d)] Invalid (%d) 0x%x write -> 0x%x (%x)\n", v_counter, m68k.cycles/MCYCLES_PER_LINE-1, m68k.cycles, m68k.cycles%MCYCLES_PER_LINE, code, addr, data, m68k_get_reg(M68K_REG_PC));