diff --git a/examples/refactoring-test.html b/examples/refactoring-test.html
index 2a72c71..7a1148d 100644
--- a/examples/refactoring-test.html
+++ b/examples/refactoring-test.html
@@ -36,13 +36,13 @@ function remove_tmp_directory() {
 };
 
 function create_tmp_file() {
-  return fs.open('/tmp/1', 'CREATE');
+  return fs.open('/tmp/1', 'w+');
 };
 
 fs.promise.then(make_tmp_directory)
           .then(create_tmp_file)
           .then(function() { console.log('done'); })
-          .otherwise(function(error) { console.error(error, error.stack); });
+          .otherwise(function(error) { console.error(error, error.message); });
 
 });
 </script>
diff --git a/src/constants.js b/src/constants.js
index a2f9512..6946629 100644
--- a/src/constants.js
+++ b/src/constants.js
@@ -1,5 +1,12 @@
 define(function(require) {
 
+  var O_READ = 'READ';
+  var O_WRITE = 'WRITE';
+  var O_CREATE = 'CREATE';
+  var O_EXCLUSIVE = 'EXCLUSIVE';
+  var O_TRUNCATE = 'TRUNCATE';
+  var O_APPEND = 'APPEND';
+
   return {
     METADATA_STORE_NAME: 'metadata',
     FILE_STORE_NAME: 'files',
@@ -17,17 +24,28 @@ define(function(require) {
     ROOT_DIRECTORY_NAME: '/', // basename(normalize(path))
     ROOT_NODE_ID: '8a5edab282632443219e051e4ade2d1d5bbc671c781051bf1437897cbdfea0f1', // sha256(ROOT_DIRECTORY_NAME)
 
-  // FileSystem flags
     FS_FORMAT: 'FORMAT',
 
-  // Open flags
-    O_READONLY: 'READONLY',
-    O_READWRITE: 'READWRITE',
-    O_APPEND: 'APPEND',
-    O_CREATE: 'CREATE',
-    O_TRUNCATE: 'TRUNCATE',
+    O_READ: O_READ,
+    O_WRITE: O_WRITE,
+    O_CREATE: O_CREATE,
+    O_EXCLUSIVE: O_EXCLUSIVE,
+    O_TRUNCATE: O_TRUNCATE,
+    O_APPEND: O_APPEND,
+
+    O_FLAGS: {
+      'r': [O_READ],
+      'r+': [O_READ, O_WRITE],
+      'w': [O_WRITE, O_CREATE, O_TRUNCATE],
+      'w+': [O_WRITE, O_READ, O_CREATE, O_TRUNCATE],
+      'wx': [O_WRITE, O_CREATE, O_EXCLUSIVE, O_TRUNCATE],
+      'wx+': [O_WRITE, O_READ, O_CREATE, O_EXCLUSIVE, O_TRUNCATE],
+      'a': [O_WRITE, O_CREATE, O_APPEND],
+      'a+': [O_WRITE, O_READ, O_CREATE, O_APPEND],
+      'ax': [O_WRITE, O_CREATE, O_EXCLUSIVE, O_APPEND],
+      'ax+': [O_WRITE, O_READ, O_CREATE, O_EXCLUSIVE, O_APPEND],
+    },
 
-  // FileSystem readyState
     FS_READY: 'READY',
     FS_PENDING: 'PENDING',
     FS_ERROR: 'ERROR',
diff --git a/src/descriptor.js b/src/descriptor.js
index e4ee30b..50e8de5 100644
--- a/src/descriptor.js
+++ b/src/descriptor.js
@@ -1,10 +1,16 @@
 define(function(require) {
 
-  function Descriptor() {
+  function Descriptor(db, id, flags, mode, size) {
+    this.db = db;
+    this.id = id;
+    this.flags = flags;
+    this.mode = mode;
+    this.size = size || 0;
+    this.position = 0;
   };
 
   return {
-    Descriptor: Descriptor,
-  };
+    Descriptor: Descriptor
+  }
 
 });
\ No newline at end of file
diff --git a/src/directory.js b/src/directory.js
index 164be9d..5db0b7e 100644
--- a/src/directory.js
+++ b/src/directory.js
@@ -7,7 +7,7 @@ define(function(require) {
 
   var ENoEntry = require('src/error').ENoEntry;
   var ENotDirectory = require('src/error').ENotDirectory;
-  var EPathExists = require('src/error').EPathExists;
+  var EExists = require('src/error').EExists;
 
   var MODE_FILE = require('src/constants').MODE_FILE;
   var MODE_DIRECTORY = require('src/constants').MODE_DIRECTORY;
@@ -28,7 +28,7 @@ define(function(require) {
 
     function write_directory_node(error, existingNode) {
       if(!error && existingNode) {
-        callback(new EPathExists());
+        callback(new EExists());
       } else if(error && !error instanceof ENoEntry) {
         callback(error);
       } else {
@@ -62,7 +62,7 @@ define(function(require) {
 
     function check_if_directory_exists(error, result) {
       if(!error && result) {
-        callback(new EPathExists());
+        callback(new EExists());
       } else if(error && !error instanceof ENoEntry) {
         callback(error);
       } else {
diff --git a/src/error.js b/src/error.js
index 29fec28..26ffaf9 100644
--- a/src/error.js
+++ b/src/error.js
@@ -14,58 +14,85 @@ THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 define(function(require) {
   // 'use strict';
 
-  function EPathExists(){ Error.apply(this, arguments); }
-  EPathExists.prototype = new Error();
-  EPathExists.prototype.name = "EPathExists";
-  EPathExists.prototype.constructor = EPathExists;
+  function EExists(message){
+    this.message = message || '';
+  };
+  EExists.prototype = new Error();
+  EExists.prototype.name = "EExists";
+  EExists.prototype.constructor = EExists;
 
-  function EIsDirectory(){ Error.apply(this, arguments); }
+  function EIsDirectory(message){
+    this.message = message || '';
+  };
   EIsDirectory.prototype = new Error();
   EIsDirectory.prototype.name = "EIsDirectory";
   EIsDirectory.prototype.constructor = EIsDirectory;
 
-  function ENoEntry(){ Error.apply(this, arguments); }
+  function ENoEntry(message){
+    this.message = message || '';
+  };
   ENoEntry.prototype = new Error();
   ENoEntry.prototype.name = "ENoEntry";
   ENoEntry.prototype.constructor = ENoEntry;
 
-  function EBusy(){ Error.apply(this, arguments); }
+  function EBusy(message){
+    this.message = message || '';
+  };
   EBusy.prototype = new Error();
   EBusy.prototype.name = "EBusy";
   EBusy.prototype.constructor = EBusy;
 
-  function ENotEmpty(){ Error.apply(this, arguments); }
+  function ENotEmpty(message){
+    this.message = message || '';
+  };
   ENotEmpty.prototype = new Error();
   ENotEmpty.prototype.name = "ENotEmpty";
   ENotEmpty.prototype.constructor = ENotEmpty;
 
-  function ENotDirectory(){ Error.apply(this, arguments); }
+  function ENotDirectory(message){
+    this.message = message || '';
+  };
   ENotDirectory.prototype = new Error();
-  ENotDirectory.prototype.name = "NotADirectoryError";
+  ENotDirectory.prototype.name = "ENotDirectory";
   ENotDirectory.prototype.constructor = ENotDirectory;
 
-  function EBadFileDescriptor(){ Error.apply(this, arguments); }
+  function EBadFileDescriptor(message){
+    this.message = message || '';
+  };
   EBadFileDescriptor.prototype = new Error();
   EBadFileDescriptor.prototype.name = "EBadFileDescriptor";
   EBadFileDescriptor.prototype.constructor = EBadFileDescriptor;
 
-  function ENotImplemented(){ Error.apply(this, arguments); }
+  function ENotImplemented(message){
+    this.message = message || '';
+  };
   ENotImplemented.prototype = new Error();
   ENotImplemented.prototype.name = "ENotImplemented";
   ENotImplemented.prototype.constructor = ENotImplemented;
 
-  function ENotMounted(){ Error.apply(this, arguments); }
+  function ENotMounted(message){
+    this.message = message || '';
+  };
   ENotMounted.prototype = new Error();
   ENotMounted.prototype.name = "ENotMounted";
   ENotMounted.prototype.constructor = ENotMounted;
 
-  function EFileExists(){ Error.apply(this, arguments); }
-  EFileExists.prototype = new Error();
-  EFileExists.prototype.name = "EFileExists";
-  EFileExists.prototype.constructor = EFileExists;
+  function EInvalid(message){
+    this.message = message || '';
+  };
+  EInvalid.prototype = new Error();
+  EInvalid.prototype.name = "EInvalid";
+  EInvalid.prototype.constructor = EInvalid;
+
+  function EIO(message){
+    this.message = message || '';
+  };
+  EIO.prototype = new Error();
+  EIO.prototype.name = "EIO";
+  EIO.prototype.constructor = EIO;
 
   return {
-    EPathExists: EPathExists,
+    EExists: EExists,
     EIsDirectory: EIsDirectory,
     ENoEntry: ENoEntry,
     EBusy: EBusy,
@@ -74,7 +101,8 @@ define(function(require) {
     EBadFileDescriptor: EBadFileDescriptor,
     ENotImplemented: ENotImplemented,
     ENotMounted: ENotMounted,
-    EFileExists: EFileExists
+    EInvalid: EInvalid,
+    EIO: EIO,
   };
 
 });
\ No newline at end of file
diff --git a/src/file-system.js b/src/file-system.js
index f3cc63b..c3b0943 100644
--- a/src/file-system.js
+++ b/src/file-system.js
@@ -9,7 +9,7 @@ define(function(require) {
   var hash = require('src/shared').hash;
   var nop = require('src/shared').nop;
 
-  var EPathExists = require('src/error').EPathExists;
+  var EExists = require('src/error').EExists;
   var EIsDirectory = require('src/error').EIsDirectory;
   var ENoEntry = require('src/error').ENoEntry;
   var EBusy = require('src/error').EBusy;
@@ -18,7 +18,7 @@ define(function(require) {
   var EBadFileDescriptor = require('src/error').EBadFileDescriptor;
   var ENotImplemented = require('src/error').ENotImplemented;
   var ENotMounted = require('src/error').ENotMounted;
-  var EFileExists = require('src/error').EFileExists;
+  var EInvalid = require('src/error').EInvalid;
 
   var FS_FORMAT = require('src/constants').FS_FORMAT;
 
@@ -100,21 +100,21 @@ define(function(require) {
     this.readyState = FS_PENDING;
     this.db = null;
   };
-  FileSystem.prototype.open = function open(path, flags, mode) {
+  FileSystem.prototype.open = function open(path, flags) {
     var deferred = when.defer();
     var transaction = this.db.transaction([FILE_STORE_NAME], IDB_RW);
     var files = transaction.objectStore(FILE_STORE_NAME);
 
     function check_result(error, fd) {
       if(error) {
-        if(transaction.error) transaction.abort();
+        // if(transaction.error) transaction.abort();
         deferred.reject(error);
       } else {
         deferred.resolve();
       }
     };
 
-    open_file(files, path, flags, mode, check_result);
+    open_file(files, path, flags, check_result);
 
     return deferred.promise;
   };
diff --git a/src/file.js b/src/file.js
index 803027c..bb0df9b 100644
--- a/src/file.js
+++ b/src/file.js
@@ -16,7 +16,15 @@ define(function(require) {
   var delete_object = require('src/object-store').delete_object;
   var find_node = require('src/object-store').find_node;
 
+  var O_READ = require('src/constants').O_READ;
+  var O_WRITE = require('src/constants').O_WRITE;
   var O_CREATE = require('src/constants').O_CREATE;
+  var O_EXCLUSIVE = require('src/constants').O_EXCLUSIVE;
+  var O_TRUNCATE = require('src/constants').O_TRUNCATE;
+  var O_APPEND = require('src/constants').O_APPEND;
+  var O_FLAGS = require('src/constants').O_FLAGS;
+
+  var EInvalid = require('src/error').EInvalid;
 
   function DirectoryEntry(id, type) {
     this.id = id;
@@ -39,15 +47,24 @@ define(function(require) {
     this.data = hash(this.id) // id for data object
   };
 
-  function open_file(objectStore, path, flags, mode, callback) {
+  function open_file(objectStore, path, flags, callback) {
     path = normalize(path);
     var name = basename(path);
+    var parentPath = dirname(path);
 
     var directoryNode;
     var directoryData;
     var fileNode;
     var fileData;
 
+    if(!_(O_FLAGS).has(flags)) {
+      return callback(new EInvalid('flags is not valid'));
+    } else {
+      flags = O_FLAGS[flags];
+    }
+
+    find_node(objectStore, parentPath, read_directory_data);
+
     function read_directory_data(error, result) {
       if(error) {
         callback(error);
@@ -63,12 +80,14 @@ define(function(require) {
       } else {
         directoryData = result;
         if(_(directoryData).has(name)) {
-          // file exists
+          if(_(flags).contains(O_EXCLUSIVE)) {
+            callback(new ENoEntry('O_CREATE and O_EXCLUSIVE are set, and the named file exists'))
+          }
         } else {
-          if(_(flags).contains(O_CREATE)) {
-            write_file_node();
+          if(!_(flags).contains(O_CREATE)) {
+            callback(new ENoEntry('O_CREATE is not set and the named file does not exist'));
           } else {
-            callback(error);
+            write_file_node();
           }
         }
       }
@@ -105,9 +124,6 @@ define(function(require) {
         callback(undefined, '!');
       }
     };
-
-    var parentPath = dirname(path);
-    find_node(objectStore, parentPath, read_directory_data);
   };
 
   return {
diff --git a/src/object-store.js b/src/object-store.js
index 1022377..8e6e944 100644
--- a/src/object-store.js
+++ b/src/object-store.js
@@ -2,7 +2,7 @@ define(function(require) {
 
   var ENoEntry = require('src/error').ENoEntry;
   var ENotDirectory = require('src/error').ENotDirectory;
-  var EPathExists = require('src/error').EPathExists;
+  var EExists = require('src/error').EExists;
 
   var normalize = require('src/path').normalize;
   var dirname = require('src/path').dirname;
@@ -17,6 +17,9 @@ define(function(require) {
   // out: node structure, or error
   function find_node(objectStore, path, callback) {
     path = normalize(path);
+    if(!path) {
+      return callback(new ENoEntry('path is an empty string'));
+    }
     var name = basename(path);
 
     if(ROOT_DIRECTORY_NAME == name) {
@@ -24,7 +27,7 @@ define(function(require) {
         if(error) {
           callback(error);
         } else if(!rootDirectoryNode) {
-          callback(new ENoEntry());
+          callback(new ENoEntry('path does not exist'));
         } else {
           callback(undefined, rootDirectoryNode);
         }
@@ -38,7 +41,7 @@ define(function(require) {
         if(error) {
           callback(error);
         } else if(!_(parentDirectoryNode).has('data') || !parentDirectoryNode.type == MODE_DIRECTORY) {
-          callback(new ENotDirectory());
+          callback(new ENotDirectory('a component of the path prefix is not a directory'));
         } else {
           read_object(objectStore, parentDirectoryNode.data, get_node_id_from_parent_directory_data);
         }
@@ -51,7 +54,7 @@ define(function(require) {
           callback(error);
         } else {
           if(!_(parentDirectoryData).has(name)) {
-            callback(new ENoEntry());
+            callback(new ENoEntry('path does not exist'));
           } else {
             var nodeId = parentDirectoryData[name].id;
             read_object(objectStore, nodeId, callback);