8000 ymfm_opn.cpp: correct write latch behaviour by happppp · Pull Request #67 · aaronsgiles/ymfm · GitHub
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ymfm_opn.cpp: correct write latch behaviour #67

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ymfm_opn.cpp: correct write latch behaviour #67

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13 changes: 5 additions & 8 deletions src/ymfm_opn.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -143,26 +143,23 @@ bool opn_registers_base<IsOpnA>::write(uint16_t index, uint8_t data, uint32_t &c
assert(index < REGISTERS);

// writes in the 0xa0-af/0x1a0-af region are handled as latched pairs
// borrow unused registers 0xb8-bf/0x1b8-bf as temporary holding locations
// borrow unused registers 0xb8-bf as temporary holding locations
if ((index & 0xf0) == 0xa0)
{
if (bitfield(index, 0, 2) == 3)
return false;

uint32_t latchindex = 0xb8 | bitfield(index, 3);
if (IsOpnA)
latchindex |= index & 0x100;

// writes to the upper half just latch (only low 6 bits matter)
if (bitfield(index, 2))
m_regdata[latchindex] = data | 0x80;
m_regdata[latchindex] = data & 0x3f;

// writes to the lower half only commit if the latch is there
else if (bitfield(m_regdata[latchindex], 7))
// writes to the lower half also apply said latch
else
{
m_regdata[index] = data;
m_regdata[index | 4] = m_regdata[latchindex] & 0x3f;
m_regdata[latchindex] = 0;
m_regdata[index | 4] = m_regdata[latchindex];
}
return false;
}
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