diff --git a/misc/yosysjs/demo02.html b/misc/yosysjs/demo02.html
index d36e223c..a6c4eb96 100644
--- a/misc/yosysjs/demo02.html
+++ b/misc/yosysjs/demo02.html
@@ -7,7 +7,6 @@
 		padding:15px; text-align:center;"><span id="popupmsg">Loading...</span></div>
 	</div>
 	<h1>YosysJS Example Application #02</h1>
-	<iframe id="ys" style="width: 800px; height: 100px;"></iframe><p/>
 	<textarea id="code" style="width: 800px; height: 300px;">
 // borrowed with some modifications from
 // http://www.ee.ed.ac.uk/~gerard/Teach/Verilog/manual/Example/lrgeEx2/cooley.html
@@ -56,47 +55,41 @@ endmodule
 			document.getElementById('popupmsg').textContent = 'Please wait..';
 		}
 		function synth1() {
-			function work() {
-				ys.write_file("input.v", document.getElementById('code').value);
-				ys.run('design -reset; read_verilog input.v; show -stretch');
-				YosysJS.dot_into_svg(ys.read_file('show.dot'), 'svg');
-				document.getElementById('popup').style.visibility = 'hidden';
-			}
 			document.getElementById('popup').style.visibility = 'visible';
-			window.setTimeout(work, 100);
+			ys.write_file("input.v", document.getElementById('code').value);
+			ys.run('design -reset; read_verilog input.v; show -stretch');
+			ys.read_file('show.dot', (function(text){
+				YosysJS.dot_into_svg(text, 'svg');
+				document.getElementById('popup').style.visibility = 'hidden';
+			}));
 		}
 		function synth2() {
-			function work() {
-				ys.write_file("input.v", document.getElementById('code').value);
-				ys.run('design -reset; read_verilog input.v; proc; opt_clean; show -stretch');
-				YosysJS.dot_into_svg(ys.read_file('show.dot'), 'svg');
-				document.getElementById('popup').style.visibility = 'hidden';
-			}
 			document.getElementById('popup').style.visibility = 'visible';
-			window.setTimeout(work, 100);
+			ys.write_file("input.v", document.getElementById('code').value);
+			ys.run('design -reset; read_verilog input.v; proc; opt_clean; show -stretch');
+			ys.read_file('show.dot', (function(text){
+				YosysJS.dot_into_svg(text, 'svg');
+				document.getElementById('popup').style.visibility = 'hidden';
+			}));
 		}
 		function synth3() {
-			function work() {
-				ys.write_file("input.v", document.getElementById('code').value);
-				ys.run('design -reset; read_verilog input.v; synth -run coarse; show -stretch');
-				YosysJS.dot_into_svg(ys.read_file('show.dot'), 'svg');
-				document.getElementById('popup').style.visibility = 'hidden';
-			}
 			document.getElementById('popup').style.visibility = 'visible';
-			window.setTimeout(work, 100);
+			ys.write_file("input.v", document.getElementById('code').value);
+			ys.run('design -reset; read_verilog input.v; synth -run coarse; show -stretch');
+			ys.read_file('show.dot', (function(text){
+				YosysJS.dot_into_svg(text, 'svg');
+				document.getElementById('popup').style.visibility = 'hidden';
+			}));
 		}
 		function synth4() {
-			function work() {
-				ys.write_file("input.v", document.getElementById('code').value);
-				ys.run('design -reset; read_verilog input.v; synth -run coarse; synth -run fine; show -stretch');
-				YosysJS.dot_into_svg(ys.read_file('show.dot'), 'svg');
-				document.getElementById('popup').style.visibility = 'hidden';
-			}
 			document.getElementById('popup').style.visibility = 'visible';
-			window.setTimeout(work, 100);
+			ys.write_file("input.v", document.getElementById('code').value);
+			ys.run('design -reset; read_verilog input.v; synth -run coarse; synth -run fine; show -stretch');
+			ys.read_file('show.dot', (function(text){
+				YosysJS.dot_into_svg(text, 'svg');
+				document.getElementById('popup').style.visibility = 'hidden';
+			}));
 		}
-		var ys = YosysJS.create('ys', on_ys_ready);
-		ys.verbose = true;
-		ys.echo = true;
+		var ys = YosysJS.create_worker(on_ys_ready);
 	</script>
 </body></html>
diff --git a/misc/yosysjs/yosysjs.js b/misc/yosysjs/yosysjs.js
index 2c78451d..65ed3049 100644
--- a/misc/yosysjs/yosysjs.js
+++ b/misc/yosysjs/yosysjs.js
@@ -172,7 +172,7 @@ var YosysJS = new function() {
 			try {
 				mod.ccall('run', '', ['string'], [cmd]);
 			} catch (e) {
-				ys.errmsg = mod.ccall('errmsg', 'string', [], []);;
+				ys.errmsg = mod.ccall('errmsg', 'string', [], []);
 			}
 			return ys.print_buffer;
 		}
@@ -220,4 +220,80 @@ var YosysJS = new function() {
 
 		return ys;
 	}
+
+	this.create_worker = function(on_ready) {
+		var ys = new Object();
+		ys.YosysJS = this;
+		ys.worker = new Worker('yosyswrk.js');
+		ys.callback_idx = 1;
+		ys.callback_cache = {};
+
+		ys.callback_cache[0] = on_ready;
+		on_ready = null;
+
+		ys.worker.onmessage = function(e) {
+			var response = e.data[0];
+			var callback = ys.callback_cache[response.idx];
+			delete ys.callback_cache[response.idx];
+			if (callback) callback.apply(null, response.args);
+		}
+
+		ys.run = function(cmd, callback) {
+			var request = {
+				"idx": ys.callback_idx,
+				"mode": "run",
+				"cmd": cmd
+			};
+
+			ys.callback_cache[ys.callback_idx++] = callback;
+			ys.worker.postMessage([request]);
+		}
+
+		ys.read_file = function(filename, callback) {
+			var request = {
+				"idx": ys.callback_idx,
+				"mode": "read_file",
+				"filename": filename
+			};
+
+			ys.callback_cache[ys.callback_idx++] = callback;
+			ys.worker.postMessage([request]);
+		}
+
+		ys.write_file = function(filename, text, callback) {
+			var request = {
+				"idx": ys.callback_idx,
+				"mode": "write_file",
+				"filename": filename,
+				"text": text
+			};
+
+			ys.callback_cache[ys.callback_idx++] = callback;
+			ys.worker.postMessage([request]);
+		}
+
+		ys.read_dir = function(dirname, callback) {
+			var request = {
+				"idx": ys.callback_idx,
+				"mode": "read_dir",
+				"dirname": dirname
+			};
+
+			ys.callback_cache[ys.callback_idx++] = callback;
+			ys.worker.postMessage([request]);
+		}
+
+		ys.remove_file = function(filename, callback) {
+			var request = {
+				"idx": ys.callback_idx,
+				"mode": "remove_file",
+				"filename": filename
+			};
+
+			ys.callback_cache[ys.callback_idx++] = callback;
+			ys.worker.postMessage([request]);
+		}
+
+		return ys;
+	}
 }
diff --git a/misc/yosysjs/yosyswrk.js b/misc/yosysjs/yosyswrk.js
new file mode 100644
index 00000000..1d77b3d2
--- /dev/null
+++ b/misc/yosysjs/yosyswrk.js
@@ -0,0 +1,43 @@
+importScripts('yosys.js');
+
+onmessage = function(e) {
+	var request = e.data[0];
+	var response = { "idx": request.idx, "args": [] };
+
+	if (request.mode == "run") {
+		try {
+			Module.ccall('run', '', ['string'], [request.cmd]);
+			response.args.push("");
+		} catch (e) {
+			response.args.push(mod.ccall('errmsg', 'string', [], []));
+		}
+	}
+
+	if (request.mode == "read_file") {
+		try {
+			response.args.push(FS.readFile(request.filename, {encoding: 'utf8'}));
+		} catch (e) { }
+	}
+
+	if (request.mode == "write_file") {
+		try {
+			FS.writeFile(request.filename, request.text, {encoding: 'utf8'});
+		} catch (e) { }
+	}
+
+	if (request.mode == "read_dir") {
+		try {
+			response.args.push(FS.readdir(request.dirname));
+		} catch (e) { }
+	}
+
+	if (request.mode == "remove_file") {
+		try {
+			FS.unlink(request.filename);
+		} catch (e) { }
+	}
+
+	postMessage([response]);
+}
+
+postMessage([{ "idx": 0, "args": [] }]);