diff --git a/_cache_service_worker.js b/_cache_service_worker.js
index 1cf24ec..b16a7a4 100644
--- a/_cache_service_worker.js
+++ b/_cache_service_worker.js
@@ -6,7 +6,7 @@
 */
 
 var PRECACHE_ID = 'rom-patcher-js';
-var PRECACHE_VERSION = 'v30finalfix2';
+var PRECACHE_VERSION = 'v31';
 var PRECACHE_URLS = [
 	'/RomPatcher.js/', '/RomPatcher.js/index.html',
 	'/RomPatcher.js/manifest.json',
diff --git a/index.html b/index.html
index 1715c9c..b615605 100644
--- a/index.html
+++ b/index.html
@@ -151,7 +151,7 @@
 		<button id="button-settings" class="btn-transparent"><img src="./webapp/icon_settings.svg" loading="lazy" class="icon settings" /> <span data-localize="yes">Settings</span></button>
 	</div>
 
-	Rom Patcher JS <small><a href="legacy/" rel="nofollow">v3.0</a></small> by <a href="/">Marc Robledo</a>
+	Rom Patcher JS <small><a href="legacy/" rel="nofollow">v3.1</a></small> by <a href="/">Marc Robledo</a>
 	<br />
 	<img src="./webapp/icon_github.svg" loading="lazy" class="icon github" /> <a href="https://github.com/marcrobledo/RomPatcher.js/" target="_blank">See on GitHub</a>
 	<img src="./webapp/icon_heart.svg" loading="lazy" class="icon heart" /> <a href="https://www.paypal.me/marcrobledo/5" target="_blank" rel="nofollow">Donate</a>
diff --git a/rom-patcher-js/RomPatcher.js b/rom-patcher-js/RomPatcher.js
index f28f95a..3e312f1 100644
--- a/rom-patcher-js/RomPatcher.js
+++ b/rom-patcher-js/RomPatcher.js
@@ -219,6 +219,8 @@ const RomPatcher = (function () {
 
 			if (options.outputSuffix) {
 				patchedRom.fileName = romFile.fileName.replace(/\.([^\.]*?)$/, ' (patched).$1');
+				if(patchedRom.unpatched)
+					patchedRom.fileName = patchedRom.fileName.replace(' (patched)', ' (unpatched)');
 			} else if (patch._originalPatchFile) {
 				patchedRom.fileName = patch._originalPatchFile.fileName.replace(/\.\w+$/i, (/\.\w+$/i.test(romFile.fileName) ? romFile.fileName.match(/\.\w+$/i)[0] : ''));
 			} else {
diff --git a/rom-patcher-js/modules/RomPatcher.format.rup.js b/rom-patcher-js/modules/RomPatcher.format.rup.js
index 5b0ed2b..322c841 100644
--- a/rom-patcher-js/modules/RomPatcher.format.rup.js
+++ b/rom-patcher-js/modules/RomPatcher.format.rup.js
@@ -1,4 +1,4 @@
-/* RUP module for Rom Patcher JS v20240721 - Marc Robledo 2018-2024 - http://www.marcrobledo.com/license */
+/* RUP module for Rom Patcher JS v20241102 - Marc Robledo 2018-2024 - http://www.marcrobledo.com/license */
 /* File format specification: http://www.romhacking.net/documents/288/ */
 
 const RUP_MAGIC='NINJA2';
@@ -40,7 +40,11 @@ RUP.prototype.toString=function(){
 		s+='\nTarget file size: '+file.targetFileSize;
 		s+='\nSource MD5: '+file.sourceMD5;
 		s+='\nTarget MD5: '+file.targetMD5;
-		s+='\nOverflow text: '+file.overflowText;
+		if(file.overflowMode==='A'){
+			s+='\nOverflow mode: Append ' + file.overflowData.length + ' bytes';
+		}else if(file.overflowMode==='M'){
+			s+='\nOverflow mode: Minify ' + file.overflowData.length + ' bytes';
+		}
 		s+='\n#records: '+file.records.length;
 	}
 	return s
@@ -50,8 +54,11 @@ RUP.prototype.toString=function(){
 RUP.prototype.validateSource=function(romFile,headerSize){
 	var md5string=romFile.hashMD5(headerSize);
 	for(var i=0; i<this.files.length; i++){
-		if(this.files[i].sourceMD5===md5string){
-			return this.files[i];
+		if(this.files[i].sourceMD5===md5string || this.files[i].targetMD5===md5string){
+			return {
+				file:this.files[i],
+				undo:this.files[i].targetMD5===md5string
+			};
 		}
 	}
 	return false;
@@ -77,32 +84,54 @@ RUP.prototype.apply=function(romFile, validate){
 		if(!validFile)
 			throw new Error('Source ROM checksum mismatch');
 	}else{
-		validFile=this.files[0];
+		validFile={
+			file:this.files[0],
+			undo:this.files[0].targetMD5===romFile.hashMD5()
+		};
 	}
 
+	var undo=validFile.undo;
+	var patch=validFile.file;
 
-
-	tempFile=new BinFile(validFile.targetFileSize);
+	tempFile=new BinFile(!undo? patch.targetFileSize : patch.sourceFileSize);
 	/* copy original file */
 	romFile.copyTo(tempFile, 0);
 
 
-	for(var i=0; i<validFile.records.length; i++){
-		var offset=validFile.records[i].offset;
+	for(var i=0; i<patch.records.length; i++){
+		var offset=patch.records[i].offset;
 		romFile.seek(offset);
 		tempFile.seek(offset);
-		for(var j=0; j<validFile.records[i].xor.length; j++){
+		for(var j=0; j<patch.records[i].xor.length; j++){
 			tempFile.writeU8(
-				(romFile.isEOF()?0x00:romFile.readU8()) ^ validFile.records[i].xor[j]
+				(romFile.isEOF()?0x00:romFile.readU8()) ^ patch.records[i].xor[j]
 			);
 		}
 	}
 
+	/* add overflow data if needed */
+	if(patch.overflowMode==='A' && !undo){ /* append */
+		tempFile.seek(patch.sourceFileSize);
+		tempFile.writeBytes(patch.overflowData.map((byte) => byte ^ 0xff));
+	}else if(patch.overflowMode==='M' && undo){ /* minify */
+		tempFile.seek(patch.targetFileSize);
+		tempFile.writeBytes(patch.overflowData.map((byte) => byte ^ 0xff));
+	}
 
-	if(validate && tempFile.hashMD5()!==validFile.targetMD5){
+
+	if(
+		validate && 
+		(
+			(!undo && tempFile.hashMD5()!==patch.targetMD5) ||
+			(undo && tempFile.hashMD5()!==patch.sourceMD5)
+		)
+	){
 		throw new Error('Target ROM checksum mismatch');
 	}
 
+	if(undo)
+		tempFile.unpatched=true;
+
 	return tempFile
 }
 
@@ -161,8 +190,10 @@ RUP.fromFile=function(file){
 
 
 			if(nextFile.sourceFileSize!==nextFile.targetFileSize){
-				file.skip(1); //skip 'M' (source>target) or 'A' (source<target)
-				nextFile.overflowText=file.readString(file.readVLV());
+				nextFile.overflowMode=file.readString(1); // 'M' (source>target) or 'A' (source<target)
+				if(nextFile.overflowMode!=='M' && nextFile.overflowMode!=='A')
+					throw new Error('RUP: invalid overflow mode');
+				nextFile.overflowData=file.readBytes(file.readVLV());
 			}
 
 		}else if(command===RUP_COMMAND_XOR_RECORD){
@@ -229,8 +260,8 @@ RUP.prototype.export=function(fileName){
 
 		if(file.sourceFileSize!==file.targetFileSize){
 			patchFileSize++; // M or A
-			patchFileSize+=RUP_getVLVLen(file.overflowText);
-			patchFileSize+=file.overflowText;
+			patchFileSize+=RUP_getVLVLen(file.overflowData.length);
+			patchFileSize+=file.overflowData.length;
 		}
 		for(var j=0; j<file.records.length; j++){
 			patchFileSize++; //command 0x01
@@ -279,8 +310,8 @@ RUP.prototype.export=function(fileName){
 
 		if(file.sourceFileSize!==file.targetFileSize){
 			patchFile.writeString(file.sourceFileSize>file.targetFileSize?'M':'A');
-			patchFile.writeVLV(file.overflowText.length);
-			patchFile.writeString(file.overflowText);
+			patchFile.writeVLV(file.overflowData.length);
+			patchFile.writeBytes(file.overflowData);
 		}
 
 		for(var j=0; j<file.records.length; j++){
@@ -314,10 +345,23 @@ RUP.buildFromRoms=function(original, modified){
 		targetFileSize:modified.fileSize,
 		sourceMD5:original.hashMD5(),
 		targetMD5:modified.hashMD5(),
-		overflowText:'',
+		overflowMode:null,
+		overflowData:[],
 		records:[]
 	};
 
+	if(file.sourceFileSize<file.targetFileSize){
+		modified.seek(file.sourceFileSize);
+		file.overflowMode='A';
+		file.overflowData=modified.readBytes(file.targetFileSize-file.sourceFileSize).map((byte) => byte ^ 0xff);
+		modified=modified.slice(0, file.sourceFileSize);
+	}else if(file.sourceFileSize>file.targetFileSize){
+		original.seek(file.targetFileSize);
+		file.overflowMode='M';
+		file.overflowData=original.readBytes(file.sourceFileSize-file.targetFileSize).map((byte) => byte ^ 0xff);
+		original=original.slice(0, file.targetFileSize);
+	}
+
 
 	original.seek(0);
 	modified.seek(0);