diff --git a/lib/x11/corereqs.js b/lib/x11/corereqs.js
index e0a08a6..bcdae15 100644
--- a/lib/x11/corereqs.js
+++ b/lib/x11/corereqs.js
@@ -1,24 +1,82 @@
 var valueMask = {
-    backgroundPixmap: 0x00000001,
-    backgroundPixel : 0x00000002,
-    borderPixmap    : 0x00000004,
-    borderPixel     : 0x00000008,
-    bitGrawity      : 0x00000010,
-    winGravity      : 0x00000020,
-    backingStore    : 0x00000040,
-    backingPlanes   : 0x00000080,
-    backingPixel    : 0x00000100,
-    overrideRedirect: 0x00000200,
-    saveUnder       : 0x00000400,
-    eventMask       : 0x00000800,
-  doNotPropagateMask: 0x00001000,
-    colormap        : 0x00002000,
-    cursor          : 0x00004000
+    CreateWindow: {
+        backgroundPixmap: 0x00000001,
+        backgroundPixel : 0x00000002,
+        borderPixmap    : 0x00000004,
+        borderPixel     : 0x00000008,
+        bitGrawity      : 0x00000010,
+        winGravity      : 0x00000020,
+        backingStore    : 0x00000040,
+        backingPlanes   : 0x00000080,
+        backingPixel    : 0x00000100,
+        overrideRedirect: 0x00000200,
+        saveUnder       : 0x00000400,
+        eventMask       : 0x00000800,
+      doNotPropagateMask: 0x00001000,
+        colormap        : 0x00002000,
+        cursor          : 0x00004000
+    },
+    CreateGC: {
+          'function'    : 0x00000001, // TODO: alias? _function?
+           planeMask    : 0x00000002,
+           foreground   : 0x00000004,
+           background   : 0x00000008,
+           lineWidth    : 0x00000010,
+           lineStyle    : 0x00000020,
+           capStyle     : 0x00000040,
+           joinStyle    : 0x00000080,
+           fillStyle    : 0x00000100,
+           fillRule     : 0x00000200,
+           tile         : 0x00000400,
+           stipple      : 0x00000800,
+           tileStippleXOrigin: 0x00001000,
+           tileStippleYOrigin: 0x00002000,
+           font         : 0x00004000,
+           subwindowMode: 0x00008000,
+           graphicsExposures: 0x00010000,
+           clipXOrigin  : 0x00020000,
+           clipYOrigin  : 0x00040000,
+           clipMask     : 0x00080000,
+           dashOffset   : 0x00100000,
+           dashes       : 0x00200000,
+           arcMode      : 0x00400000
+    }
 };
 
-var valueMaskNames = {};
-for (var m in valueMask) {
-    valueMaskNames[valueMask[m]] = m;
+var valueMaskName = {};
+for (var req in valueMask) {
+    var masks = valueMask[req];
+    var names = valueMaskName[req] = {};
+    for (var m in masks) 
+        names[masks[m]] = m;
+}
+
+function packValueMask(reqname, values)
+{
+    var bitmask = 0;
+    var masksList = [];
+    var reqValueMask = valueMask[reqname];
+    var reqValueMaskName = valueMaskName[reqname];
+
+    if (!reqValueMask)
+        throw new Error(reqname + ': no value mask description');
+
+    for (var v in values)
+    {
+        var valueBit = reqValueMask[v];
+        if (!valueBit)
+            throw new Error('CreateWindow: incorrect value param ' + v);
+        masksList.push(valueBit);
+        bitmask |= valueBit;
+    }
+    masksList.sort();
+    var args = [];
+    for (m in masksList)
+    {    
+       valueName = reqValueMaskName[masksList[m]];
+       args.push( values[valueName] );
+    }
+    return [bitmask, args]
 }
 
 /*
@@ -51,7 +109,7 @@ module.exports = {
            // TODO: ??? there is depth field in xproto, but xlib just sets it to zero
            var depth = 0;
 
-           var packetLength = 8 + Object.keys(values).length;
+           var packetLength = 8 + (values ? Object.keys(values).length : 0);
            // TODO: should be CCSLLssSSSSLL - x,y are signed
            var format = 'CCSLLSSSSSSLL';
 
@@ -65,10 +123,12 @@ module.exports = {
            // bitmask (bytes #24 to #31 in the packet) - 32 bit indicating what adittional arguments we supply
            // values list (bytes #32 .. #32+4*num_values) in order of corresponding bits
          
+
+           // TODO: replace with packValueMask
            var masksList = [];
            for (var v in values)
            {
-               var valueBit = valueMask[v];
+               var valueBit = valueMask['CreateWindow'][v];
                if (!valueBit)
                {
                    throw new Error('CreateWindow: incorrect value param ' + v);
@@ -85,7 +145,7 @@ module.exports = {
            // TODO: maybe it's better just to scan all 32 bits anstead of sorting parameters we are actually have?
            for (m in masksList)
            {
-              valueName = valueMaskNames[masksList[m]];
+              valueName = valueMaskName['CreateWindow'][masksList[m]];
               args.push( values[valueName] );
            }
            return [format, args];
@@ -100,5 +160,23 @@ module.exports = {
 
    UnmapWindow: [
        [ 'CxSL', [10, 2] ]
-   ]  
+   ],
+
+   // opcode 55
+   CreateGC: [
+       function(cid, drawable, values) {
+           var format = 'CxSLL';           
+           var packetLength = 8 + (values ? Object.keys(values).length : 0);
+           var args = [55, packetLength, cid, drawable];
+           var vals = packValueMask('CreateGC', values);
+           args.push(vals[0]);     // values bitmask
+           var valArr = vals[1];
+           for (v in valArr)
+           {
+               format += 'L'; // TODO: we know format string length in advance and += inefficient for string
+               args.push(valArr[v]);
+           }           
+           return [format, args];
+        }
+   ]
 }
diff --git a/test/creategc.js b/test/creategc.js
new file mode 100644
index 0000000..c807657
--- /dev/null
+++ b/test/creategc.js
@@ -0,0 +1,16 @@
+var x11 = require('../lib/x11');
+
+var xclient = x11.createClient();
+var mapped = true;
+xclient.on('connect', function(display) {
+    var X = this;
+    var root = display.screen[0].root;
+    var wid = X.AllocID();
+
+    X.CreateWindow(wid, root, 10, 10, 400, 300, 1, 1, 0, { backgroundPixel: 0, eventMask: 0x00000040 });
+    X.MapWindow(wid);
+  
+    var cid = X.AllocID();
+    X.CreateGC(cid, wid);  
+    
+});
\ No newline at end of file