C64: another tiny fix

also document conversion approach

papers/format.md

@@ -634,7 +634,9 @@ size | description
   1  | osc sync
   1  | to filter
   1  | init filter
-  1  | vol macro is cutoff
+  1  | vol macro is cutoff (<187) or reserved
+     | - from version 187 onwards, volume and cutoff macros are separate.
+     | - if this is on and the version is less than 187, move the volume macro into the ALG one.
   1  | resonance
   1  | low pass
   1  | band pass
@@ -1122,6 +1124,23 @@ size | description
   - `val[14]`: loop
   - `val[15]`: global (not sure how will I implement this)
 
+## C64 compatibility note (>=187)
+
+in Furnace dev187 the volume and cutoff macros have been separated, as noted above.
+however, there's another change as well: a new, improved Special macro.
+
+if version is less than 187, you must convert these:
+1. do not continue if ex4 is not a Sequence type macro!
+2. move bit 0 of ex4 macro data into bit 3.
+3. set bit 0 on all steps of ex4 macro to 1.
+4. if ex3 is not a Sequence type macro, stop here.
+5. if ex3 macro length is 0, stop here.
+6. merge the ex3 macro (former Special) into ex4 (former Test).
+  - use the largest size (between ex3 and ex4).
+  - if the ex3 macro is shorter than the ex4 one, use the last value of ex3, and vice-versa.
+
+don't worry about loop or release...
+
 # wavetable
 
 ```

papers/newIns.md

@@ -295,7 +295,9 @@ size | description
   1  | flags 1
      | - bit 7: dutyIsAbs
      | - bit 6: initFilter
-     | - bit 5: volIsCutoff
+     | - bit 5: volIsCutoff (<187)
+     | - from version 187 onwards, volume and cutoff macros are separate.
+     | - if this is on and the version is less than 187, move the volume macro into the ALG one.
      | - bit 4: toFilter
      | - bit 3: noise on
      | - bit 2: pulse on
@@ -322,6 +324,23 @@ size | description
      | - bit 0-10: cutoff
 ```
 
+## C64 compatibility note (>=187)
+
+in Furnace dev187 the volume and cutoff macros have been separated, as noted above.
+however, there's another change as well: a new, improved Special macro.
+
+if version is less than 187, you must convert these:
+1. do not continue if ex4 is not a Sequence type macro!
+2. move bit 0 of ex4 macro data into bit 3.
+3. set bit 0 on all steps of ex4 macro to 1.
+4. if ex3 is not a Sequence type macro, stop here.
+5. if ex3 macro length is 0, stop here.
+6. merge the ex3 macro (former Special) into ex4 (former Test).
+  - use the largest size (between ex3 and ex4).
+  - if the ex3 macro is shorter than the ex4 one, use the last value of ex3, and vice-versa.
+
+don't worry about loop or release...
+
 # Game Boy data (GB)
 
 ```

src/engine/instrument.cpp

@@ -3477,8 +3477,13 @@ void DivInstrument::convertC64SpecialMacro() {
     std.ex4Macro.val[i]=(std.ex4Macro.val[i]&1)?9:1;
   }
   
-  // merge ex3 into ex4
-  if (std.ex3Macro.len>0) {
+  // merge ex3 into ex4 if viable to
+  if (std.ex3Macro.len>0 && !(std.ex3Macro.open&6)) {
+    if (std.ex4Macro.len>0 && std.ex4Macro.len<maxLen) {
+      for (int i=std.ex4Macro.len; i<maxLen; i++) {
+        std.ex4Macro.val[i]=std.ex3Macro.val[std.ex4Macro.len-1];
+      }
+    }
     for (int i=0; i<maxLen; i++) {
       if (i>=std.ex3Macro.len) {
         std.ex4Macro.val[i]|=(std.ex3Macro.val[std.ex3Macro.len-1]&3)<<1;