From f50911ea4949cbbfcf596392fadfb24b1df28d2b Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Fri, 2 Aug 2024 02:21:44 -0500
Subject: [PATCH 1/2] FDS: prepare for auto mod

---
 src/engine/dispatch.h       | 2 ++
 src/engine/platform/fds.cpp | 6 ++++++
 src/engine/platform/fds.h   | 3 +++
 src/engine/playback.cpp     | 4 +++-
 src/engine/sysDef.cpp       | 1 +
 5 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/engine/dispatch.h b/src/engine/dispatch.h
index aa355f488..e077d8f4d 100644
--- a/src/engine/dispatch.h
+++ b/src/engine/dispatch.h
@@ -263,6 +263,8 @@ enum DivDispatchCmds {
   DIV_CMD_BIFURCATOR_STATE_LOAD,
   DIV_CMD_BIFURCATOR_PARAMETER,
 
+  DIV_CMD_FDS_MOD_AUTO,
+
   DIV_CMD_MAX
 };
 
diff --git a/src/engine/platform/fds.cpp b/src/engine/platform/fds.cpp
index 6d6a3c768..8cc729038 100644
--- a/src/engine/platform/fds.cpp
+++ b/src/engine/platform/fds.cpp
@@ -342,6 +342,12 @@ int DivPlatformFDS::dispatch(DivCommand c) {
       rWrite(0x4087,chan[c.chan].modFreq>>8);
       break;
     }
+    case DIV_CMD_FDS_MOD_AUTO:
+      chan[c.chan].autoModNum=c.value>>4;
+      chan[c.chan].autoModDen=c.value&15;
+      chan[c.chan].freqChanged=true;
+      chan[c.chan].modOn=(chan[c.chan].autoModNum || chan[c.chan].autoModDen);
+      break;
     case DIV_CMD_NOTE_PORTA: {
       int destFreq=NOTE_FREQUENCY(c.value2);
       bool return2=false;
diff --git a/src/engine/platform/fds.h b/src/engine/platform/fds.h
index 49b52d063..57c8ff0bc 100644
--- a/src/engine/platform/fds.h
+++ b/src/engine/platform/fds.h
@@ -29,6 +29,7 @@ class DivPlatformFDS: public DivDispatch {
   struct Channel: public SharedChannel<signed char> {
     int prevFreq, modFreq;
     unsigned char duty, sweep, modDepth, modPos;
+    unsigned char autoModNum, autoModDen;
     bool sweepChanged, modOn;
     signed short wave;
     signed char modTable[32];
@@ -40,6 +41,8 @@ class DivPlatformFDS: public DivDispatch {
       sweep(8),
       modDepth(0),
       modPos(0),
+      autoModNum(0),
+      autoModDen(0),
       sweepChanged(false),
       modOn(false),
       wave(-1) {
diff --git a/src/engine/playback.cpp b/src/engine/playback.cpp
index b7b8098b6..c61415abd 100644
--- a/src/engine/playback.cpp
+++ b/src/engine/playback.cpp
@@ -261,7 +261,9 @@ const char* cmdName[]={
   "MINMOD_ECHO",
 
   "BIFURCATOR_STATE_LOAD",
-  "BIFURCATOR_PARAMETER"
+  "BIFURCATOR_PARAMETER",
+
+  "FDS_MOD_AUTO"
 };
 
 static_assert((sizeof(cmdName)/sizeof(void*))==DIV_CMD_MAX,"update cmdName!");
diff --git a/src/engine/sysDef.cpp b/src/engine/sysDef.cpp
index 20e8461d5..093369fc2 100644
--- a/src/engine/sysDef.cpp
+++ b/src/engine/sysDef.cpp
@@ -1053,6 +1053,7 @@ void DivEngine::registerSystems() {
       {0x13, {DIV_CMD_FDS_MOD_LOW, _("13xx: Set modulation speed low byte")}},
       {0x14, {DIV_CMD_FDS_MOD_POS, _("14xx: Set modulator position")}},
       {0x15, {DIV_CMD_FDS_MOD_WAVE, _("15xx: Set modulator table to waveform")}},
+      {0x16, {DIV_CMD_FDS_MOD_AUTO, _("16xy: Automatic modulation speed (x: numerator; y: denominator)")}},
     }
   );
 

From 4e7d566791efd7ffbd0771b06414c72053ef5932 Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Fri, 2 Aug 2024 02:47:55 -0500
Subject: [PATCH 2/2] FDS: implement auto-mod

I don't like it. improvements welcome.
---
 src/engine/platform/fds.cpp | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/engine/platform/fds.cpp b/src/engine/platform/fds.cpp
index 8cc729038..1e42738af 100644
--- a/src/engine/platform/fds.cpp
+++ b/src/engine/platform/fds.cpp
@@ -200,6 +200,15 @@ void DivPlatformFDS::tick(bool sysTick) {
       }
       rWrite(0x4082,chan[i].freq&0xff);
       rWrite(0x4083,(chan[i].freq>>8)&15);
+
+      if (chan[i].autoModNum>0 && chan[i].autoModDen>0) {
+        chan[i].modFreq=(chan[i].freq*chan[i].autoModNum)/chan[i].autoModDen;
+        if (chan[i].modFreq>4095) chan[i].modFreq=4095;
+        if (chan[i].modFreq<0) chan[i].modFreq=0;
+        rWrite(0x4086,chan[i].modFreq&0xff);
+        rWrite(0x4087,chan[i].modFreq>>8);
+      }
+
       if (chan[i].keyOn) chan[i].keyOn=false;
       if (chan[i].keyOff) chan[i].keyOff=false;
       chan[i].freqChanged=false;
@@ -346,7 +355,8 @@ int DivPlatformFDS::dispatch(DivCommand c) {
       chan[c.chan].autoModNum=c.value>>4;
       chan[c.chan].autoModDen=c.value&15;
       chan[c.chan].freqChanged=true;
-      chan[c.chan].modOn=(chan[c.chan].autoModNum || chan[c.chan].autoModDen);
+      chan[c.chan].modOn=(chan[c.chan].autoModNum && chan[c.chan].autoModDen);
+      rWrite(0x4084,(chan[c.chan].modOn<<7)|0x40|chan[c.chan].modDepth);
       break;
     case DIV_CMD_NOTE_PORTA: {
       int destFreq=NOTE_FREQUENCY(c.value2);