diff --git a/src/main/java/dk/camelot64/kickc/fragment/AsmFragmentManager.java b/src/main/java/dk/camelot64/kickc/fragment/AsmFragmentManager.java
index 9ecdcf26a..d78c46e87 100644
--- a/src/main/java/dk/camelot64/kickc/fragment/AsmFragmentManager.java
+++ b/src/main/java/dk/camelot64/kickc/fragment/AsmFragmentManager.java
@@ -1,5 +1,6 @@
 package dk.camelot64.kickc.fragment;
 
+import dk.camelot64.kickc.CompileLog;
 import dk.camelot64.kickc.parser.KickCLexer;
 import dk.camelot64.kickc.parser.KickCParser;
 import org.antlr.v4.runtime.*;
@@ -19,6 +20,8 @@ import java.util.regex.Pattern;
  */
 public class AsmFragmentManager {
 
+   static boolean verboseFragmentLog = false;
+
    /**
     * Cache for fragment files. Maps signature to the parsed file.
     */
@@ -26,8 +29,8 @@ public class AsmFragmentManager {
 
    private static KickCParser.AsmFileContext UNKNOWN = new KickCParser.AsmFileContext(null, 0);
 
-   public static AsmFragment getFragment(AsmFragmentSignature signature) {
-      KickCParser.AsmFileContext fragmentFile = getFragmentFile(signature.getSignature());
+   public static AsmFragment getFragment(AsmFragmentSignature signature, CompileLog log) {
+      KickCParser.AsmFileContext fragmentFile = getFragmentFile(signature.getSignature(), log);
       AsmFragment fragment = new AsmFragment(
             signature.getProgram(),
             signature.getSignature(),
@@ -37,14 +40,20 @@ public class AsmFragmentManager {
       return fragment;
    }
 
-   private static KickCParser.AsmFileContext getFragmentFile(String signature) {
+   private static KickCParser.AsmFileContext getFragmentFile(String signature, CompileLog log) {
       KickCParser.AsmFileContext fragment = fragmentFileCache.get(signature);
       if (fragment == UNKNOWN) {
+         if (verboseFragmentLog) {
+            log.append("Unknown fragment " + signature);
+         }
          throw new UnknownFragmentException(signature);
       }
       if (fragment == null) {
-         CharStream fragmentCharStream = loadOrSynthesizeFragment(signature);
+         CharStream fragmentCharStream = loadOrSynthesizeFragment(signature, log);
          if (fragmentCharStream == null) {
+            if (verboseFragmentLog) {
+               log.append("Unknown fragment " + signature);
+            }
             fragmentFileCache.put(signature, UNKNOWN);
             throw new UnknownFragmentException(signature);
          }
@@ -54,10 +63,17 @@ public class AsmFragmentManager {
       return fragment;
    }
 
-   private static CharStream loadOrSynthesizeFragment(String signature) {
+   private static CharStream loadOrSynthesizeFragment(String signature, CompileLog log) {
       CharStream fragmentCharStream = loadFragment(signature);
       if (fragmentCharStream == null) {
-         fragmentCharStream = synthesizeFragment(signature);
+         if (verboseFragmentLog) {
+            log.append("Attempting fragment synthesis " + signature);
+         }
+         fragmentCharStream = synthesizeFragment(signature, log);
+      } else {
+         if (verboseFragmentLog) {
+            log.append("Succesfully loaded fragment " + signature);
+         }
       }
       return fragmentCharStream;
    }
@@ -68,7 +84,7 @@ public class AsmFragmentManager {
     * @param signature The signature of the fragment to synthesize
     * @return The synthesized fragment file contents. Null if the fragment could not be synthesized.
     */
-   private static CharStream synthesizeFragment(String signature) {
+   private static CharStream synthesizeFragment(String signature, CompileLog log) {
 
       Map<String, String> mapZpsby = new LinkedHashMap<>();
       mapZpsby.put("zpsby2", "zpsby1");
@@ -105,12 +121,12 @@ public class AsmFragmentManager {
       synths.add(new FragmentSynthesis("(.*)=zpby1(.*)", ".*=.*as?by.*|zpby1=.*", "lda {zpby1}\n", "$1=aby$2", null, mapZpby));
       synths.add(new FragmentSynthesis("(.*)=zpsby1(.*)", ".*=.*as?by.*|zpsby1=.*", "lda {zpsby1}\n", "$1=aby$2", null, mapZpsby));
       synths.add(new FragmentSynthesis("(.*)=_deref_cowo1(.*)", ".*=.*as?by.*", "lda {cowo1}\n", "$1=aby$2", null, mapConst));
-      synths.add(new FragmentSynthesis("(.*)=_deref_zpptrby1(.*)", ".*=.*as?by.*|.*=.*ys?by.*", "ldy #0\n"+"lda ({zpptrby1}),y\n", "$1=aby$2", null, mapZpptrby));
+      synths.add(new FragmentSynthesis("(.*)=_deref_zpptrby1(.*)", ".*=.*as?by.*|.*=.*ys?by.*", "ldy #0\n" + "lda ({zpptrby1}),y\n", "$1=aby$2", null, mapZpptrby));
 
-      synths.add(new FragmentSynthesis("(.*)=(.*)_xby", ".*=[ax]s?by.*xs?by", "txa\n", "$1=$2_aby", null, null));
-      synths.add(new FragmentSynthesis("(.*)=(.*)_xsby", ".*=[ax]s?by.*xs?by", "txa\n", "$1=$2_asby", null, null));
-      synths.add(new FragmentSynthesis("(.*)=(.*)_yby", ".*=[ay]s?by.*ys?by", "tya\n", "$1=$2_aby", null, null));
-      synths.add(new FragmentSynthesis("(.*)=(.*)_ysby", ".*=[ay]s?by.*ys?by", "tya\n", "$1=$2_asby", null, null));
+      synths.add(new FragmentSynthesis("(.*)=(.*)_xby", ".*=[ax]s?by.*xs?by|.*derefidx_xs?by", "txa\n", "$1=$2_aby", null, null));
+      synths.add(new FragmentSynthesis("(.*)=(.*)_xsby", ".*=[ax]s?by.*xs?by|.*derefidx_xs?by", "txa\n", "$1=$2_asby", null, null));
+      synths.add(new FragmentSynthesis("(.*)=(.*)_yby", ".*=[ay]s?by.*ys?by|.*derefidx_ys?by", "tya\n", "$1=$2_aby", null, null));
+      synths.add(new FragmentSynthesis("(.*)=(.*)_ysby", ".*=[ay]s?by.*ys?by|.*derefidx_ys?by", "tya\n", "$1=$2_asby", null, null));
       synths.add(new FragmentSynthesis("(.*)=(.*)_zpby1", ".*=.*as?by.*", "lda {zpby1}\n", "$1=$2_aby", null, mapZpby));
       synths.add(new FragmentSynthesis("(.*)=(.*)_zpsby1", ".*=.*as?by.*", "lda {zpsby1}\n", "$1=$2_asby", null, mapZpsby));
 
@@ -125,7 +141,7 @@ public class AsmFragmentManager {
       synths.add(new FragmentSynthesis("zpby1_(lt|gt|le|ge|eq|neq)_(.*)", ".*as?by.*", "lda {zpby1}\n", "aby_$1_$2", null, mapZpby));
       synths.add(new FragmentSynthesis("zpsby1_(lt|gt|le|ge|eq|neq)_(.*)", ".*as?by.*", "lda {zpsby1}\n", "asby_$1_$2", null, mapZpsby));
       synths.add(new FragmentSynthesis("_deref_cowo1_(lt|gt|le|ge|eq|neq)_(.*)", ".*as?by.*", "lda {cowo1}\n", "aby_$1_$2", null, mapConst));
-      synths.add(new FragmentSynthesis("_deref_zpptrby1_(lt|gt|le|ge|eq|neq)_(.*)", ".*=.*as?by.*|.*=.*ys?by.*", "ldy #0\n"+"lda ({zpptrby1}),y\n", "aby_$1_$2", null, mapZpptrby));
+      synths.add(new FragmentSynthesis("_deref_zpptrby1_(lt|gt|le|ge|eq|neq)_(.*)", ".*=.*as?by.*|.*=.*ys?by.*", "ldy #0\n" + "lda ({zpptrby1}),y\n", "aby_$1_$2", null, mapZpptrby));
       synths.add(new FragmentSynthesis("(.*)_ge_(as?by)_then_(.*)", ".*[a]s?by.*_ge.*", null, "$2_lt_$1_then_$3", null, null));
       synths.add(new FragmentSynthesis("(.*)_ge_(xs?by)_then_(.*)", ".*[ax]s?by.*_ge.*", null, "$2_lt_$1_then_$3", null, null));
       synths.add(new FragmentSynthesis("(.*)_ge_(ys?by)_then_(.*)", ".*[axy]s?by.*_ge.*", null, "$2_lt_$1_then_$3", null, null));
@@ -146,8 +162,11 @@ public class AsmFragmentManager {
       synths.add(new FragmentSynthesis("(.*)_eq_(ys?by)_then_(.*)", ".*[axy]s?by.*_eq.*", null, "$2_eq_$1_then_$3", null, null));
 
       for (FragmentSynthesis synth : synths) {
-         CharStream synthesized = synth.synthesize(signature);
-         if(synthesized!=null) {
+         CharStream synthesized = synth.synthesize(signature, log);
+         if (synthesized != null) {
+            if (verboseFragmentLog) {
+               log.append("Succesfully synthesized fragment " + signature + " (from " + synth.getSubSignature() + ")");
+            }
             return synthesized;
          }
       }
@@ -156,7 +175,9 @@ public class AsmFragmentManager {
 
    }
 
-   /** AsmFragment synthesis based on matching fragment signature and reusing another fragment with added prefix/postfix and some bind-mappings*/
+   /**
+    * AsmFragment synthesis based on matching fragment signature and reusing another fragment with added prefix/postfix and some bind-mappings
+    */
    private static class FragmentSynthesis {
 
       private String sigMatch;
@@ -165,6 +186,7 @@ public class AsmFragmentManager {
       private String sigReplace;
       private String asmPostfix;
       private Map<String, String> bindMappings;
+      private String subSignature;
 
       public FragmentSynthesis(String sigMatch, String sigAvoid, String asmPrefix, String sigReplace, String asmPostfix, Map<String, String> bindMappings) {
          this.sigMatch = sigMatch;
@@ -175,23 +197,23 @@ public class AsmFragmentManager {
          this.bindMappings = bindMappings;
       }
 
-      public CharStream synthesize(String signature) {
-         if (signature.matches(sigMatch) ) {
+      public CharStream synthesize(String signature, CompileLog log) {
+         if (signature.matches(sigMatch)) {
             if (sigAvoid == null || !signature.matches(sigAvoid)) {
-               String subSignature = regexpRewriteSignature(signature, sigMatch, sigReplace);
-               if(bindMappings!=null) {
+               subSignature = regexpRewriteSignature(signature, sigMatch, sigReplace);
+               if (bindMappings != null) {
                   for (String bound : bindMappings.keySet()) {
                      subSignature = subSignature.replace(bound, bindMappings.get(bound));
                   }
                }
-               CharStream subCharStream = loadOrSynthesizeFragment(subSignature);
+               CharStream subCharStream = loadOrSynthesizeFragment(subSignature, log);
                if (subCharStream != null) {
                   StringBuilder newFragment = new StringBuilder();
-                  if(asmPrefix!=null) {
+                  if (asmPrefix != null) {
                      newFragment.append(asmPrefix);
                   }
                   String subFragment = subCharStream.toString();
-                  if(bindMappings!=null) {
+                  if (bindMappings != null) {
                      List<String> reverse = new ArrayList<>(bindMappings.keySet());
                      Collections.reverse(reverse);
                      for (String bound : reverse) {
@@ -199,7 +221,7 @@ public class AsmFragmentManager {
                      }
                   }
                   newFragment.append(subFragment);
-                  if(asmPostfix!=null) {
+                  if (asmPostfix != null) {
                      newFragment.append("\n");
                      newFragment.append(asmPostfix);
                   }
@@ -210,6 +232,9 @@ public class AsmFragmentManager {
          return null;
       }
 
+      public String getSubSignature() {
+         return subSignature;
+      }
    }
 
 
diff --git a/src/main/java/dk/camelot64/kickc/passes/Pass4CodeGeneration.java b/src/main/java/dk/camelot64/kickc/passes/Pass4CodeGeneration.java
index 456d2cf38..603b03ceb 100644
--- a/src/main/java/dk/camelot64/kickc/passes/Pass4CodeGeneration.java
+++ b/src/main/java/dk/camelot64/kickc/passes/Pass4CodeGeneration.java
@@ -205,7 +205,7 @@ public class Pass4CodeGeneration {
          }
          StatementAssignment assignment = (StatementAssignment) statement;
          AsmFragmentSignature signature = new AsmFragmentSignature(assignment, assignmentAlu, program);
-         AsmFragment asmFragment = AsmFragmentManager.getFragment(signature);
+         AsmFragment asmFragment = AsmFragmentManager.getFragment(signature, program.getLog());
          asm.getCurrentSegment().setFragment(asmFragment.getName());
          asmFragment.generate(asm);
          aluState.clear();
@@ -232,14 +232,14 @@ public class Pass4CodeGeneration {
                   asm.addComment(lValue.toString(program) + " = " + assignment.getrValue2().toString(program) + "  // register copy " + getRegister(lValue));
                } else {
                   AsmFragmentSignature asmFragmentSignature = new AsmFragmentSignature(assignment, program);
-                  AsmFragment asmFragment = AsmFragmentManager.getFragment(asmFragmentSignature);
+                  AsmFragment asmFragment = AsmFragmentManager.getFragment(asmFragmentSignature, program.getLog());
                   asm.getCurrentSegment().setFragment(asmFragment.getName());
                   asmFragment.generate(asm);
                }
             }
          } else if (statement instanceof StatementConditionalJump) {
             AsmFragmentSignature asmSignature = new AsmFragmentSignature((StatementConditionalJump) statement, block, program, getGraph());
-            AsmFragment asmFragment = AsmFragmentManager.getFragment(asmSignature);
+            AsmFragment asmFragment = AsmFragmentManager.getFragment(asmSignature, program.getLog());
             asm.getCurrentSegment().setFragment(asmFragment.getName());
             asmFragment.generate(asm);
          } else if (statement instanceof StatementCall) {
@@ -349,7 +349,7 @@ public class Pass4CodeGeneration {
                asm.getCurrentSegment().setFragment("register_copy");
             } else {
                AsmFragmentSignature asmSignature = new AsmFragmentSignature(lValue, rValue, program, scope);
-               AsmFragment asmFragment = AsmFragmentManager.getFragment(asmSignature);
+               AsmFragment asmFragment = AsmFragmentManager.getFragment(asmSignature, program.getLog());
                asm.getCurrentSegment().setFragment(asmFragment.getName());
                asmFragment.generate(asm);
             }
diff --git a/src/main/java/dk/camelot64/kickc/test/casting.kc b/src/main/java/dk/camelot64/kickc/test/casting.kc
index c6e6e7c33..007e3336e 100644
--- a/src/main/java/dk/camelot64/kickc/test/casting.kc
+++ b/src/main/java/dk/camelot64/kickc/test/casting.kc
@@ -3,7 +3,7 @@ byte* SCREEN2 = $0600;
 
 void main() {
     for( byte b: 0..100) {
-        byte b2 = 100-b;
+        byte b2 = 100-b; // Subtract byte from byte (not signed)
         SCREEN2[b] = b2;
         signed byte sb = - (signed byte)b;
         SCREEN[b] = (byte)sb;