From 82567c0bd6ab981c92b2134bf426db0fd643315d Mon Sep 17 00:00:00 2001
From: Sjoerd van der Berg <harekiet@users.sourceforge.net>
Date: Mon, 20 Nov 2017 17:27:53 +0000
Subject: [PATCH] Allow for direct changing of 4op chaining without having to
 rewrite the algorithm mode.

Imported-from: https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@4064
---
 src/hardware/dbopl.cpp | 49 +++++++++++++++++++++++++-----------------
 src/hardware/dbopl.h   |  4 +++-
 2 files changed, 32 insertions(+), 21 deletions(-)

diff --git a/src/hardware/dbopl.cpp b/src/hardware/dbopl.cpp
index c7d61ca3..8b6df319 100644
--- a/src/hardware/dbopl.cpp
+++ b/src/hardware/dbopl.cpp
@@ -716,18 +716,23 @@ void Channel::WriteB0( const Chip* chip, Bit8u val ) {
 	}
 }
 
-void Channel::WriteC0( const Chip* chip, Bit8u val ) {
+void Channel::WriteC0(const Chip* chip, Bit8u val) {
 	Bit8u change = val ^ regC0;
-	if ( !change )
+	if (!change)
 		return;
 	regC0 = val;
-	feedback = ( val >> 1 ) & 7;
-	if ( feedback ) {
+	feedback = (regC0 >> 1) & 7;
+	if (feedback) {
 		//We shift the input to the right 10 bit wave index value
 		feedback = 9 - feedback;
-	} else {
+	}
+	else {
 		feedback = 31;
 	}
+	UpdateSynth(chip);
+}
+
+void Channel::UpdateSynth( const Chip* chip ) {
 	//Select the new synth mode
 	if ( chip->opl3Active ) {
 		//4-op mode enabled for this channel
@@ -761,20 +766,20 @@ void Channel::WriteC0( const Chip* chip, Bit8u val ) {
 		} else if ((fourMask & 0x40) && ( chip->regBD & 0x20) ) {
 
 		//Regular dual op, am or fm
-		} else if ( val & 1 ) {
+		} else if (regC0 & 1 ) {
 			synthHandler = &Channel::BlockTemplate< sm3AM >;
 		} else {
 			synthHandler = &Channel::BlockTemplate< sm3FM >;
 		}
-		maskLeft = ( val & 0x10 ) ? -1 : 0;
-		maskRight = ( val & 0x20 ) ? -1 : 0;
+		maskLeft = (regC0 & 0x10 ) ? -1 : 0;
+		maskRight = (regC0 & 0x20 ) ? -1 : 0;
 	//opl2 active
 	} else { 
 		//Disable updating percussion channels
 		if ( (fourMask & 0x40) && ( chip->regBD & 0x20 ) ) {
 
 		//Regular dual op, am or fm
-		} else if ( val & 1 ) {
+		} else if (regC0 & 1 ) {
 			synthHandler = &Channel::BlockTemplate< sm2AM >;
 		} else {
 			synthHandler = &Channel::BlockTemplate< sm2FM >;
@@ -782,12 +787,6 @@ void Channel::WriteC0( const Chip* chip, Bit8u val ) {
 	}
 }
 
-void Channel::ResetC0( const Chip* chip ) {
-	Bit8u val = regC0;
-	regC0 ^= 0xff;
-	WriteC0( chip, val );
-};
-
 template< bool opl3Mode>
 INLINE void Channel::GeneratePercussion( Chip* chip, Bit32s* output ) {
 	Channel* chan = this;
@@ -1071,7 +1070,8 @@ void Chip::WriteBD( Bit8u val ) {
 	//Toggle keyoffs when we turn off the percussion
 	} else if ( change & 0x20 ) {
 		//Trigger a reset to setup the original synth handler
-		chan[6].ResetC0( this );
+		//This makes it call
+		chan[6].UpdateSynth( this );
 		chan[6].op[0].KeyOff( 0x2 );
 		chan[6].op[1].KeyOff( 0x2 );
 		chan[7].op[0].KeyOff( 0x2 );
@@ -1096,6 +1096,14 @@ void Chip::WriteBD( Bit8u val ) {
 		regChan->_FUNC_( this, val );													\
 	}
 
+//Update the 0xc0 register for all channels to signal the switch to mono/stereo handlers
+void Chip::UpdateSynths() {
+	for (int i = 0; i < 18; i++) {
+		chan[i].UpdateSynth(this);
+	}
+}
+
+
 void Chip::WriteReg( Bit32u reg, Bit8u val ) {
 	Bitu index;
 	switch ( (reg & 0xf0) >> 4 ) {
@@ -1108,15 +1116,16 @@ void Chip::WriteReg( Bit32u reg, Bit8u val ) {
 				return;
 			//Always keep the highest bit enabled, for checking > 0x80
 			reg104 = 0x80 | ( val & 0x3f );
+			//Switch synths when changing the 4op combinations
+			UpdateSynths();
 		} else if ( reg == 0x105 ) {
 			//MAME says the real opl3 doesn't reset anything on opl3 disable/enable till the next write in another register
 			if ( !((opl3Active ^ val) & 1 ) )
 				return;
 			opl3Active = ( val & 1 ) ? 0xff : 0;
-			//Update the 0xc0 register for all channels to signal the switch to mono/stereo handlers
-			for ( int i = 0; i < 18;i++ ) {
-				chan[i].ResetC0( this );
-			}
+			//Just tupdate the synths now that opl3 most have been enabled
+			//This isn't how the real card handles it but need to switch to stereo generating handlers
+			UpdateSynths();
 		} else if ( reg == 0x08 ) {
 			reg08 = val;
 		}
diff --git a/src/hardware/dbopl.h b/src/hardware/dbopl.h
index c3c4d428..e33d7e35 100644
--- a/src/hardware/dbopl.h
+++ b/src/hardware/dbopl.h
@@ -176,10 +176,10 @@ struct Channel {
 	void SetChanData( const Chip* chip, Bit32u data );
 	//Change in the chandata, check for new values and if we have to forward to operators
 	void UpdateFrequency( const Chip* chip, Bit8u fourOp );
+	void UpdateSynth(const Chip* chip);
 	void WriteA0( const Chip* chip, Bit8u val );
 	void WriteB0( const Chip* chip, Bit8u val );
 	void WriteC0( const Chip* chip, Bit8u val );
-	void ResetC0( const Chip* chip );
 
 	//call this for the first channel
 	template< bool opl3Mode >
@@ -239,6 +239,8 @@ struct Chip {
 	void GenerateBlock2( Bitu samples, Bit32s* output );
 	void GenerateBlock3( Bitu samples, Bit32s* output );
 
+	//Update the synth handlers in all channels
+	void UpdateSynths();
 	void Generate( Bit32u samples );
 	void Setup( Bit32u r );