diff --git a/lib/pure/asyncio.nim b/lib/pure/asyncio.nim
index a1ce75af3..4ff6e0ced 100644
--- a/lib/pure/asyncio.nim
+++ b/lib/pure/asyncio.nim
@@ -167,7 +167,7 @@ proc AsyncSocket*(domain: TDomain = AF_INET, typ: TType = SOCK_STREAM,
   result = newAsyncSocket()
   result.socket = socket(domain, typ, protocol, buffered)
   result.proto = protocol
-  if result.socket == InvalidSocket: OSError()
+  if result.socket == InvalidSocket: OSError(OSLastError())
   result.socket.setBlocking(false)
 
 proc toAsyncSocket*(sock: TSocket, state: TInfo = SockConnected): PAsyncSocket =
@@ -349,7 +349,7 @@ proc acceptAddr*(server: PAsyncSocket, client: var PAsyncSocket,
     client.sslNeedAccept = false
     client.info = SockConnected
 
-  if c == InvalidSocket: OSError()
+  if c == InvalidSocket: SocketError(server.socket)
   c.setBlocking(false) # TODO: Needs to be tested.
   
   # deleg.open is set in ``toDelegate``.
@@ -642,8 +642,7 @@ when isMainModule:
   proc testRead(s: PAsyncSocket, no: int) =
     echo("Reading! " & $no)
     var data = ""
-    if not s.readLine(data):
-      OSError()
+    if not s.readLine(data): return
     if data == "":
       echo("Closing connection. " & $no)
       s.close()
diff --git a/lib/pure/os.nim b/lib/pure/os.nim
index de4893f8c..eaa22d351 100644
--- a/lib/pure/os.nim
+++ b/lib/pure/os.nim
@@ -39,6 +39,8 @@ type
   FWriteDir* = object of FWriteIO ## effect that denotes a write operation to
                                   ## the directory structure
 
+  TOSErrorCode* = distinct int32 ## Specifies an OS Error Code.
+
 const
   doslike = defined(windows) or defined(OS2) or defined(DOS)
     # DOS-like filesystem
@@ -171,10 +173,13 @@ const
     ## The character which separates the base filename from the extension;
     ## for example, the '.' in ``os.nim``.
 
-proc OSErrorMsg*(): string {.rtl, extern: "nos$1".} =
+proc OSErrorMsg*(): string {.rtl, extern: "nos$1", deprecated.} =
   ## Retrieves the operating system's error flag, ``errno``.
   ## On Windows ``GetLastError`` is checked before ``errno``.
   ## Returns "" if no error occured.
+  ##
+  ## **Deprecated since version 0.9.4**: use the other ``OSErrorMsg`` proc.
+  
   result = ""
   when defined(Windows):
     var err = GetLastError()
@@ -194,17 +199,89 @@ proc OSErrorMsg*(): string {.rtl, extern: "nos$1".} =
   if errno != 0'i32:
     result = $os.strerror(errno)
 
-proc OSError*(msg: string = "") {.noinline, rtl, extern: "nos$1".} =
+{.push warning[deprecated]: off.}
+proc OSError*(msg: string = "") {.noinline, rtl, extern: "nos$1", deprecated.} =
   ## raises an EOS exception with the given message ``msg``.
   ## If ``msg == ""``, the operating system's error flag
   ## (``errno``) is converted to a readable error message. On Windows
   ## ``GetLastError`` is checked before ``errno``.
   ## If no error flag is set, the message ``unknown OS error`` is used.
+  ##
+  ## **Deprecated since version 0.9.4**: use the other ``OSError`` proc.
   if len(msg) == 0:
     var m = OSErrorMsg()
     raise newException(EOS, if m.len > 0: m else: "unknown OS error")
   else:
     raise newException(EOS, msg)
+{.pop.}
+
+proc `==`*(err1, err2: TOSErrorCode): bool {.borrow.}
+proc `$`*(err: TOSErrorCode): string {.borrow.}
+
+proc OSErrorMsg*(errorCode: TOSErrorCode): string =
+  ## Converts an OS error code into a human readable string.
+  ##
+  ## The error code can be retrieved using the ``OSLastError`` proc.
+  ##
+  ## If conversion fails, or ``errorCode`` is ``0`` then ``""`` will be
+  ## returned.
+  ##
+  ## On Windows, the ``-d:useWinAnsi`` compilation flag can be used to
+  ## make this procedure use the non-unicode Win API calls to retrieve the
+  ## message.
+  result = ""
+  when defined(Windows):
+    if errorCode != TOSErrorCode(0'i32):
+      when useWinUnicode:
+        var msgbuf: widecstring
+        if FormatMessageW(0x00000100 or 0x00001000 or 0x00000200,
+                        nil, errorCode.int32, 0, addr(msgbuf), 0, nil) != 0'i32:
+          result = $msgbuf
+          if msgbuf != nil: LocalFree(cast[pointer](msgbuf))
+      else:
+        var msgbuf: cstring
+        if FormatMessageA(0x00000100 or 0x00001000 or 0x00000200,
+                        nil, errorCode.int32, 0, addr(msgbuf), 0, nil) != 0'i32:
+          result = $msgbuf
+          if msgbuf != nil: LocalFree(msgbuf)
+  else:
+    if errorCode != TOSErrorCode(0'i32):
+      result = $os.strerror(errorCode.int32)
+
+proc OSError*(errorCode: TOSErrorCode) =
+  ## Raises an ``EOS`` exception. The ``errorCode`` will determine the
+  ## message, ``OSErrorMsg`` will be used to get this message.
+  ##
+  ## The error code can be retrieved using the ``OSLastError`` proc.
+  ##
+  ## If the error code is ``0`` or an error message could not be retrieved,
+  ## the message ``unknown OS error`` will be used.
+  let msg = OSErrorMsg(errorCode)
+  if msg == "":
+    raise newException(EOS, "unknown OS error")
+  else:
+    raise newException(EOS, msg)
+
+{.push stackTrace:off.}
+proc OSLastError*(): TOSErrorCode =
+  ## Retrieves the last operating system error code.
+  ##
+  ## This procedure is useful in the event when an OS call fails. In that case
+  ## this procedure will return the error code describing the reason why the
+  ## OS call failed. The ``OSErrorMsg`` procedure can then be used to convert
+  ## this code into a string.
+  ##
+  ## **Warning**:
+  ## The behaviour of this procedure varies between Windows and POSIX systems.
+  ## On Windows some OS calls can reset the error code to ``0`` causing this
+  ## procedure to return ``0``. It is therefore advised to call this procedure
+  ## immediately after an OS call fails. On POSIX systems this is not a problem.
+  
+  when defined(windows):
+    result = TOSErrorCode(GetLastError())
+  else:
+    result = TOSErrorCode(errno)
+{.pop.}
 
 proc UnixToNativePath*(path: string): string {.
   noSideEffect, rtl, extern: "nos$1".} =
@@ -311,12 +388,12 @@ proc getLastModificationTime*(file: string): TTime {.rtl, extern: "nos$1".} =
   ## Returns the `file`'s last modification time.
   when defined(posix):
     var res: TStat
-    if stat(file, res) < 0'i32: OSError()
+    if stat(file, res) < 0'i32: OSError(OSLastError())
     return res.st_mtime
   else:
     var f: TWIN32_Find_Data
     var h = findfirstFile(file, f)
-    if h == -1'i32: OSError()
+    if h == -1'i32: OSError(OSLastError())
     result = winTimeToUnixTime(rdFileTime(f.ftLastWriteTime))
     findclose(h)
 
@@ -324,12 +401,12 @@ proc getLastAccessTime*(file: string): TTime {.rtl, extern: "nos$1".} =
   ## Returns the `file`'s last read or write access time.
   when defined(posix):
     var res: TStat
-    if stat(file, res) < 0'i32: OSError()
+    if stat(file, res) < 0'i32: OSError(OSLastError())
     return res.st_atime
   else:
     var f: TWIN32_Find_Data
     var h = findfirstFile(file, f)
-    if h == -1'i32: OSError()
+    if h == -1'i32: OSError(OSLastError())
     result = winTimeToUnixTime(rdFileTime(f.ftLastAccessTime))
     findclose(h)
 
@@ -337,12 +414,12 @@ proc getCreationTime*(file: string): TTime {.rtl, extern: "nos$1".} =
   ## Returns the `file`'s creation time.
   when defined(posix):
     var res: TStat
-    if stat(file, res) < 0'i32: OSError()
+    if stat(file, res) < 0'i32: OSError(OSLastError())
     return res.st_ctime
   else:
     var f: TWIN32_Find_Data
     var h = findfirstFile(file, f)
-    if h == -1'i32: OSError()
+    if h == -1'i32: OSError(OSLastError())
     result = winTimeToUnixTime(rdFileTime(f.ftCreationTime))
     findclose(h)
 
@@ -358,30 +435,30 @@ proc getCurrentDir*(): string {.rtl, extern: "nos$1", tags: [].} =
     when useWinUnicode:
       var res = newWideCString("", bufsize)
       var L = GetCurrentDirectoryW(bufsize, res)
-      if L == 0'i32: OSError()
+      if L == 0'i32: OSError(OSLastError())
       result = res$L
     else:
       result = newString(bufsize)
       var L = GetCurrentDirectoryA(bufsize, result)
-      if L == 0'i32: OSError()
+      if L == 0'i32: OSError(OSLastError())
       setLen(result, L)
   else:
     result = newString(bufsize)
     if getcwd(result, bufsize) != nil:
       setlen(result, c_strlen(result))
     else:
-      OSError()
+      OSError(OSLastError())
 
 proc setCurrentDir*(newDir: string) {.inline, tags: [].} =
   ## Sets the `current working directory`:idx:; `EOS` is raised if
   ## `newDir` cannot been set.
   when defined(Windows):
     when useWinUnicode:
-      if SetCurrentDirectoryW(newWideCString(newDir)) == 0'i32: OSError()
+      if SetCurrentDirectoryW(newWideCString(newDir)) == 0'i32: OSError(OSLastError())
     else:
-      if SetCurrentDirectoryA(newDir) == 0'i32: OSError()
+      if SetCurrentDirectoryA(newDir) == 0'i32: OSError(OSLastError())
   else:
-    if chdir(newDir) != 0'i32: OSError()
+    if chdir(newDir) != 0'i32: OSError(OSLastError())
 
 proc JoinPath*(head, tail: string): string {.
   noSideEffect, rtl, extern: "nos$1".} =
@@ -571,23 +648,23 @@ proc expandFilename*(filename: string): string {.rtl, extern: "nos$1",
       var res = newWideCString("", bufsize div 2)
       var L = GetFullPathNameW(newWideCString(filename), bufsize, res, unused)
       if L <= 0'i32 or L >= bufsize: 
-        OSError()
+        OSError(OSLastError())
       result = res$L
     else:
       var unused: cstring
       result = newString(bufsize)
       var L = GetFullPathNameA(filename, bufsize, result, unused)
-      if L <= 0'i32 or L >= bufsize: OSError()
+      if L <= 0'i32 or L >= bufsize: OSError(OSLastError())
       setLen(result, L)
   elif defined(macosx) or defined(bsd):
     # On Mac OS X 10.5, realpath does not allocate the buffer on its own
     var pathBuffer: cstring = newString(pathMax)
     var resultBuffer = realpath(filename, pathBuffer)
-    if resultBuffer == nil: OSError()
+    if resultBuffer == nil: OSError(OSLastError())
     result = $resultBuffer
   else:
     var res = realpath(filename, nil)
-    if res == nil: OSError()
+    if res == nil: OSError(OSLastError())
     result = $res
     c_free(res)
  
@@ -677,6 +754,7 @@ proc sameFile*(path1, path2: string): bool {.rtl, extern: "nos$1",
       var f1 = OpenHandle(path1)
       var f2 = OpenHandle(path2)
 
+    var lastErr: TOSErrorCode
     if f1 != INVALID_HANDLE_VALUE and f2 != INVALID_HANDLE_VALUE:
       var fi1, fi2: TBY_HANDLE_FILE_INFORMATION
 
@@ -685,17 +763,21 @@ proc sameFile*(path1, path2: string): bool {.rtl, extern: "nos$1",
         result = fi1.dwVolumeSerialNumber == fi2.dwVolumeSerialNumber and
                  fi1.nFileIndexHigh == fi2.nFileIndexHigh and
                  fi1.nFileIndexLow == fi2.nFileIndexLow
-      else: success = false
-    else: success = false
+      else:
+        lastErr = OSLastError()
+        success = false
+    else:
+      lastErr = OSLastError()
+      success = false
 
     discard CloseHandle(f1)
     discard CloseHandle(f2)
 
-    if not success: OSError()
+    if not success: OSError(lastErr)
   else:
     var a, b: TStat
     if stat(path1, a) < 0'i32 or stat(path2, b) < 0'i32:
-      OSError()
+      OSError(OSLastError())
     else:
       result = a.st_dev == b.st_dev and a.st_ino == b.st_ino
 
@@ -738,17 +820,17 @@ proc copyFile*(source, dest: string) {.rtl, extern: "nos$1",
     when useWinUnicode:
       let s = newWideCString(source)
       let d = newWideCString(dest)
-      if CopyFileW(s, d, 0'i32) == 0'i32: OSError()
+      if CopyFileW(s, d, 0'i32) == 0'i32: OSError(OSLastError())
     else:
-      if CopyFileA(source, dest, 0'i32) == 0'i32: OSError()
+      if CopyFileA(source, dest, 0'i32) == 0'i32: OSError(OSLastError())
   else:
     # generic version of copyFile which works for any platform:
     const bufSize = 8000 # better for memory manager
     var d, s: TFile
-    if not open(s, source): OSError()
+    if not open(s, source): OSError(OSLastError())
     if not open(d, dest, fmWrite):
       close(s)
-      OSError()
+      OSError(OSLastError())
     var buf = alloc(bufsize)
     while True:
       var bytesread = readBuffer(s, buf, bufsize)
@@ -758,7 +840,7 @@ proc copyFile*(source, dest: string) {.rtl, extern: "nos$1",
           dealloc(buf)
           close(s)
           close(d)
-          OSError()
+          OSError(OSLastError())
       if bytesread != bufSize: break
     dealloc(buf)
     close(s)
@@ -767,7 +849,8 @@ proc copyFile*(source, dest: string) {.rtl, extern: "nos$1",
 proc moveFile*(source, dest: string) {.rtl, extern: "nos$1", 
   tags: [FReadIO, FWriteIO].} =
   ## Moves a file from `source` to `dest`. If this fails, `EOS` is raised.
-  if crename(source, dest) != 0'i32: OSError()
+  if crename(source, dest) != 0'i32:
+    raise newException(EOS, $strerror(errno))
 
 when not defined(ENOENT):
   var ENOENT {.importc, header: "<errno.h>".}: cint
@@ -775,7 +858,8 @@ when not defined(ENOENT):
 proc removeFile*(file: string) {.rtl, extern: "nos$1", tags: [FWriteDir].} =
   ## Removes the `file`. If this fails, `EOS` is raised. This does not fail
   ## if the file never existed in the first place.
-  if cremove(file) != 0'i32 and errno != ENOENT: OSError()
+  if cremove(file) != 0'i32 and errno != ENOENT:
+    raise newException(EOS, $strerror(errno))
 
 proc execShellCmd*(command: string): int {.rtl, extern: "nos$1", 
   tags: [FExecIO].} =
@@ -907,14 +991,14 @@ proc putEnv*(key, val: string) {.tags: [FWriteEnv].} =
     indx = high(environment)
   when defined(unix):
     if cputenv(environment[indx]) != 0'i32:
-      OSError()
+      OSError(OSLastError())
   else:
     when useWinUnicode:
       var k = newWideCString(key)
       var v = newWideCString(val)
-      if SetEnvironmentVariableW(k, v) == 0'i32: OSError()
+      if SetEnvironmentVariableW(k, v) == 0'i32: OSError(OSLastError())
     else:
-      if SetEnvironmentVariableA(key, val) == 0'i32: OSError()
+      if SetEnvironmentVariableA(key, val) == 0'i32: OSError(OSLastError())
 
 iterator envPairs*(): tuple[key, value: TaintedString] {.tags: [FReadEnv].} =
   ## Iterate over all `environments variables`:idx:. In the first component 
@@ -1044,10 +1128,12 @@ proc rawRemoveDir(dir: string) =
       wrapUnary(res, RemoveDirectoryW, dir)
     else:
       var res = RemoveDirectoryA(dir)
-    if res == 0'i32 and GetLastError() != 3'i32 and 
-        GetLastError() != 18'i32: OSError()
+    let lastError = OSLastError()
+    if res == 0'i32 and lastError.int32 != 3'i32 and 
+        lastError.int32 != 18'i32 and lastError.int32 != 2'i32:
+      OSError(lastError)
   else:
-    if rmdir(dir) != 0'i32 and errno != ENOENT: OSError()
+    if rmdir(dir) != 0'i32 and errno != ENOENT: OSError(OSLastError())
 
 proc removeDir*(dir: string) {.rtl, extern: "nos$1", tags: [
   FWriteDir, FReadDir].} =
@@ -1065,14 +1151,14 @@ proc removeDir*(dir: string) {.rtl, extern: "nos$1", tags: [
 proc rawCreateDir(dir: string) =
   when defined(unix):
     if mkdir(dir, 0o711) != 0'i32 and errno != EEXIST:
-      OSError()
+      OSError(OSLastError())
   else:
     when useWinUnicode:
       wrapUnary(res, CreateDirectoryW, dir)
     else:
       var res = CreateDirectoryA(dir)
     if res == 0'i32 and GetLastError() != 183'i32:
-      OSError()
+      OSError(OSLastError())
 
 proc createDir*(dir: string) {.rtl, extern: "nos$1", tags: [FWriteDir].} =
   ## Creates the `directory`:idx: `dir`.
@@ -1213,7 +1299,7 @@ proc getFilePermissions*(filename: string): set[TFilePermission] {.
   ## permission is available in any case.
   when defined(posix):
     var a: TStat
-    if stat(filename, a) < 0'i32: OSError()
+    if stat(filename, a) < 0'i32: OSError(OSLastError())
     result = {}
     if (a.st_mode and S_IRUSR) != 0'i32: result.incl(fpUserRead)
     if (a.st_mode and S_IWUSR) != 0'i32: result.incl(fpUserWrite)
@@ -1231,7 +1317,7 @@ proc getFilePermissions*(filename: string): set[TFilePermission] {.
       wrapUnary(res, GetFileAttributesW, filename)
     else:
       var res = GetFileAttributesA(filename)
-    if res == -1'i32: OSError()
+    if res == -1'i32: OSError(OSLastError())
     if (res and FILE_ATTRIBUTE_READONLY) != 0'i32:
       result = {fpUserExec, fpUserRead, fpGroupExec, fpGroupRead, 
                 fpOthersExec, fpOthersRead}
@@ -1257,13 +1343,13 @@ proc setFilePermissions*(filename: string, permissions: set[TFilePermission]) {.
     if fpOthersWrite in permissions: p = p or S_IWOTH
     if fpOthersExec in permissions: p = p or S_IXOTH
     
-    if chmod(filename, p) != 0: OSError()
+    if chmod(filename, p) != 0: OSError(OSLastError())
   else:
     when useWinUnicode:
       wrapUnary(res, GetFileAttributesW, filename)
     else:
       var res = GetFileAttributesA(filename)
-    if res == -1'i32: OSError()
+    if res == -1'i32: OSError(OSLastError())
     if fpUserWrite in permissions: 
       res = res and not FILE_ATTRIBUTE_READONLY
     else:
@@ -1272,7 +1358,7 @@ proc setFilePermissions*(filename: string, permissions: set[TFilePermission]) {.
       wrapBinary(res2, SetFileAttributesW, filename, res)
     else:
       var res2 = SetFileAttributesA(filename, res)
-    if res2 == - 1'i32: OSError()
+    if res2 == - 1'i32: OSError(OSLastError())
   
 proc inclFilePermissions*(filename: string, 
                           permissions: set[TFilePermission]) {.
@@ -1448,7 +1534,7 @@ proc getFileSize*(file: string): biggestInt {.rtl, extern: "nos$1",
   when defined(windows):
     var a: TWin32FindData
     var resA = findfirstFile(file, a)
-    if resA == -1: OSError()
+    if resA == -1: OSError(OSLastError())
     result = rdFileSize(a)
     findclose(resA)
   else:
@@ -1456,7 +1542,7 @@ proc getFileSize*(file: string): biggestInt {.rtl, extern: "nos$1",
     if open(f, file): 
       result = getFileSize(f)
       close(f)
-    else: OSError()
+    else: OSError(OSLastError())
 
 proc findExe*(exe: string): string {.tags: [FReadDir, FReadEnv].} = 
   ## Searches for `exe` in the current working directory and then
diff --git a/lib/pure/osproc.nim b/lib/pure/osproc.nim
index aa4fbe32d..48a559c4e 100644
--- a/lib/pure/osproc.nim
+++ b/lib/pure/osproc.nim
@@ -370,6 +370,7 @@ when defined(Windows) and not defined(useNimRtl):
     else:
       success = winlean.CreateProcessA(nil,
         cmdl, nil, nil, 1, NORMAL_PRIORITY_CLASS, e, wd, SI, ProcInfo)
+    let lastError = OSLastError()
 
     if poParentStreams notin options:
       FileClose(si.hStdInput)
@@ -379,7 +380,7 @@ when defined(Windows) and not defined(useNimRtl):
 
     if e != nil: dealloc(e)
     dealloc(cmdl)
-    if success == 0: OSError()
+    if success == 0: OSError(lastError)
     # Close the handle now so anyone waiting is woken:
     discard closeHandle(procInfo.hThread)
     result.FProcessHandle = procInfo.hProcess
@@ -450,7 +451,7 @@ when defined(Windows) and not defined(useNimRtl):
       var res = winlean.CreateProcessA(nil, command, nil, nil, 0,
         NORMAL_PRIORITY_CLASS, nil, nil, SI, ProcInfo)
     if res == 0:
-      OSError()
+      OSError(OSLastError())
     else:
       Process = ProcInfo.hProcess
       discard CloseHandle(ProcInfo.hThread)
diff --git a/lib/pure/sockets.nim b/lib/pure/sockets.nim
index 603fd612b..33df72d38 100644
--- a/lib/pure/sockets.nim
+++ b/lib/pure/sockets.nim
@@ -231,7 +231,7 @@ when defined(ssl):
     if err == 0:
       raise newException(ESSL, "No error reported.")
     if err == -1:
-      OSError()
+      OSError(OSLastError())
     var errStr = ErrErrorString(err, nil)
     raise newException(ESSL, $errStr)
 
@@ -347,24 +347,23 @@ proc SocketError*(socket: TSocket, err: int = -1, async = false) =
         else: SSLError("Unknown Error")
   
   if err == -1 and not (when defined(ssl): socket.isSSL else: false):
+    let lastError = OSLastError()
     if async:
       when defined(windows):
-        # TODO: Test on Windows
-        var err = WSAGetLastError()
-        if err == WSAEWOULDBLOCK:
+        if lastError.int32 == WSAEWOULDBLOCK:
           return
-        else: OSError()
+        else: OSError(lastError)
       else:
-        if errno == EAGAIN or errno == EWOULDBLOCK:
+        if lastError.int32 == EAGAIN or lastError.int32 == EWOULDBLOCK:
           return
-        else: OSError()
-    else: OSError()
+        else: OSError(lastError)
+    else: OSError(lastError)
 
 proc listen*(socket: TSocket, backlog = SOMAXCONN) {.tags: [FReadIO].} =
   ## Marks ``socket`` as accepting connections. 
   ## ``Backlog`` specifies the maximum length of the 
   ## queue of pending connections.
-  if listen(socket.fd, cint(backlog)) < 0'i32: OSError()
+  if listen(socket.fd, cint(backlog)) < 0'i32: OSError(OSLastError())
 
 proc invalidIp4(s: string) {.noreturn, noinline.} =
   raise newException(EInvalidValue, "invalid ip4 address: " & s)
@@ -403,7 +402,7 @@ template gaiNim(a, p, h, list: expr): stmt =
     var gaiResult = getAddrInfo(a, $p, addr(h), list)
     if gaiResult != 0'i32:
       when defined(windows):
-        OSError()
+        OSError(OSLastError())
       else:
         OSError($gai_strerror(gaiResult))
 
@@ -423,7 +422,7 @@ proc bindAddr*(socket: TSocket, port = TPort(0), address = "") {.
     name.sin_addr.s_addr = sockets.htonl(INADDR_ANY)
     if bindSocket(socket.fd, cast[ptr TSockAddr](addr(name)),
                   sizeof(name).TSockLen) < 0'i32:
-      OSError()
+      OSError(OSLastError())
   else:
     var hints: TAddrInfo
     var aiList: ptr TAddrInfo = nil
@@ -432,21 +431,7 @@ proc bindAddr*(socket: TSocket, port = TPort(0), address = "") {.
     hints.ai_protocol = toInt(IPPROTO_TCP)
     gaiNim(address, port, hints, aiList)
     if bindSocket(socket.fd, aiList.ai_addr, aiList.ai_addrLen.TSockLen) < 0'i32:
-      OSError()
-
-when false:
-  proc bindAddr*(socket: TSocket, port = TPort(0)) =
-    ## binds a port number to a socket.
-    var name: Tsockaddr_in
-    when defined(Windows):
-      name.sin_family = int16(ord(AF_INET))
-    else:
-      name.sin_family = posix.AF_INET
-    name.sin_port = sockets.htons(int16(port))
-    name.sin_addr.s_addr = sockets.htonl(INADDR_ANY)
-    if bindSocket(cint(socket), cast[ptr TSockAddr](addr(name)),
-                  sizeof(name).TSockLen) < 0'i32:
-      OSError()
+      OSError(OSLastError())
   
 proc getSockName*(socket: TSocket): TPort = 
   ## returns the socket's associated port number.
@@ -460,7 +445,7 @@ proc getSockName*(socket: TSocket): TPort =
   var namelen = sizeof(name).TSockLen
   if getsockname(socket.fd, cast[ptr TSockAddr](addr(name)),
                  addr(namelen)) == -1'i32:
-    OSError()
+    OSError(OSLastError())
   result = TPort(sockets.ntohs(name.sin_port))
 
 template acceptAddrPlain(noClientRet, successRet: expr, 
@@ -472,26 +457,25 @@ template acceptAddrPlain(noClientRet, successRet: expr,
                     addr(addrLen))
   
   if sock < 0:
-    # TODO: Test on Windows.
+    let err = OSLastError()
     when defined(windows):
-      var err = WSAGetLastError()
-      if err == WSAEINPROGRESS:
+      if err.int32 == WSAEINPROGRESS:
         client = InvalidSocket
         address = ""
         when noClientRet.int == -1:
           return
         else:
           return noClientRet
-      else: OSError()
+      else: OSError(err)
     else:
-      if errno == EAGAIN or errno == EWOULDBLOCK:
+      if err.int32 == EAGAIN or err.int32 == EWOULDBLOCK:
         client = InvalidSocket
         address = ""
         when noClientRet.int == -1:
           return
         else:
           return noClientRet
-      else: OSError()
+      else: OSError(err)
   else:
     client.fd = sock
     client.isBuffered = server.isBuffered
@@ -644,7 +628,7 @@ proc getServByName*(name, proto: string): TServent {.tags: [FReadIO].} =
     var s = winlean.getservbyname(name, proto)
   else:
     var s = posix.getservbyname(name, proto)
-  if s == nil: OSError()
+  if s == nil: OSError(OSLastError())
   result.name = $s.s_name
   result.aliases = cstringArrayToSeq(s.s_aliases)
   result.port = TPort(s.s_port)
@@ -656,7 +640,7 @@ proc getServByPort*(port: TPort, proto: string): TServent {.tags: [FReadIO].} =
     var s = winlean.getservbyport(ze(int16(port)).cint, proto)
   else:
     var s = posix.getservbyport(ze(int16(port)).cint, proto)
-  if s == nil: OSError()
+  if s == nil: OSError(OSLastError())
   result.name = $s.s_name
   result.aliases = cstringArrayToSeq(s.s_aliases)
   result.port = TPort(s.s_port)
@@ -670,7 +654,7 @@ proc getHostByAddr*(ip: string): THostEnt {.tags: [FReadIO].} =
   when defined(windows):
     var s = winlean.gethostbyaddr(addr(myaddr), sizeof(myaddr).cuint,
                                   cint(sockets.AF_INET))
-    if s == nil: OSError()
+    if s == nil: OSError(OSLastError())
   else:
     var s = posix.gethostbyaddr(addr(myaddr), sizeof(myaddr).TSockLen, 
                                 cint(posix.AF_INET))
@@ -687,7 +671,7 @@ proc getHostByAddr*(ip: string): THostEnt {.tags: [FReadIO].} =
     elif s.h_addrtype == posix.AF_INET6:
       result.addrType = AF_INET6
     else:
-      OSError("unknown h_addrtype")
+      raise newException(EOS, "unknown h_addrtype")
   result.addrList = cstringArrayToSeq(s.h_addr_list)
   result.length = int(s.h_length)
 
@@ -697,7 +681,7 @@ proc getHostByName*(name: string): THostEnt {.tags: [FReadIO].} =
     var s = winlean.gethostbyname(name)
   else:
     var s = posix.gethostbyname(name)
-  if s == nil: OSError()
+  if s == nil: OSError(OSLastError())
   result.name = $s.h_name
   result.aliases = cstringArrayToSeq(s.h_aliases)
   when defined(windows): 
@@ -708,7 +692,7 @@ proc getHostByName*(name: string): THostEnt {.tags: [FReadIO].} =
     elif s.h_addrtype == posix.AF_INET6:
       result.addrType = AF_INET6
     else:
-      OSError("unknown h_addrtype")
+      raise newException(EOS, "unknown h_addrtype")
   result.addrList = cstringArrayToSeq(s.h_addr_list)
   result.length = int(s.h_length)
 
@@ -719,7 +703,7 @@ proc getSockOptInt*(socket: TSocket, level, optname: int): int {.
   var size = sizeof(res).TSockLen
   if getsockopt(socket.fd, cint(level), cint(optname), 
                 addr(res), addr(size)) < 0'i32:
-    OSError()
+    OSError(OSLastError())
   result = int(res)
 
 proc setSockOptInt*(socket: TSocket, level, optname, optval: int) {.
@@ -728,7 +712,7 @@ proc setSockOptInt*(socket: TSocket, level, optname, optval: int) {.
   var value = cint(optval)
   if setsockopt(socket.fd, cint(level), cint(optname), addr(value),  
                 sizeof(value).TSockLen) < 0'i32:
-    OSError()
+    OSError(OSLastError())
 
 proc connect*(socket: TSocket, address: string, port = TPort(0), 
               af: TDomain = AF_INET) {.tags: [FReadIO].} =
@@ -746,15 +730,17 @@ proc connect*(socket: TSocket, address: string, port = TPort(0),
   gaiNim(address, port, hints, aiList)
   # try all possibilities:
   var success = false
+  var lastError: TOSErrorCode
   var it = aiList
   while it != nil:
     if connect(socket.fd, it.ai_addr, it.ai_addrlen.TSockLen) == 0'i32:
       success = true
       break
+    else: lastError = OSLastError()
     it = it.ai_next
 
   freeaddrinfo(aiList)
-  if not success: OSError()
+  if not success: OSError(lastError)
   
   when defined(ssl):
     if socket.isSSL:
@@ -807,6 +793,7 @@ proc connectAsync*(socket: TSocket, name: string, port = TPort(0),
   gaiNim(name, port, hints, aiList)
   # try all possibilities:
   var success = false
+  var lastError: TOSErrorCode
   var it = aiList
   while it != nil:
     var ret = connect(socket.fd, it.ai_addr, it.ai_addrlen.TSockLen)
@@ -814,22 +801,21 @@ proc connectAsync*(socket: TSocket, name: string, port = TPort(0),
       success = true
       break
     else:
-      # TODO: Test on Windows.
+      lastError = OSLastError()
       when defined(windows):
-        var err = WSAGetLastError()
         # Windows EINTR doesn't behave same as POSIX.
-        if err == WSAEWOULDBLOCK:
+        if lastError.int32 == WSAEWOULDBLOCK:
           success = true
           break
       else:
-        if errno == EINTR or errno == EINPROGRESS:
+        if lastError.int32 == EINTR or lastError.int32 == EINPROGRESS:
           success = true
           break
         
     it = it.ai_next
 
   freeaddrinfo(aiList)
-  if not success: OSError()
+  if not success: OSError(lastError)
   when defined(ssl):
     if socket.isSSL:
       socket.sslNoHandshake = true
@@ -1110,7 +1096,7 @@ proc waitFor(socket: TSocket, waited: var float, timeout, size: int,
     var s = @[socket]
     var startTime = epochTime()
     let selRet = select(s, timeout - int(waited * 1000.0))
-    if selRet < 0: OSError()
+    if selRet < 0: OSError(OSLastError())
     if selRet != 1:
       raise newException(ETimeout, "Call to '" & funcName & "' timed out.")
     waited += (epochTime() - startTime)
@@ -1259,14 +1245,14 @@ proc readLine*(socket: TSocket, line: var TaintedString, timeout = -1) {.
     var c: char
     discard waitFor(socket, waited, timeout, 1, "readLine")
     var n = recv(socket, addr(c), 1)
-    if n < 0: OSError()
+    if n < 0: OSError(OSLastError())
     elif n == 0: return
     if c == '\r':
       discard waitFor(socket, waited, timeout, 1, "readLine")
       n = peekChar(socket, c)
       if n > 0 and c == '\L':
         discard recv(socket, addr(c), 1)
-      elif n <= 0: OSError()
+      elif n <= 0: OSError(OSLastError())
       addNlIfEmpty()
       return
     elif c == '\L': 
@@ -1326,6 +1312,7 @@ proc readLineAsync*(socket: TSocket,
   while true:
     var c: char
     var n = recv(socket, addr(c), 1)
+    #echo(n)
     if n < 0:
       if line.len == 0: errorOrNone else: return ReadPartialLine
     elif n == 0: 
@@ -1352,7 +1339,7 @@ proc recv*(socket: TSocket): TaintedString {.tags: [FReadIO], deprecated.} =
   var pos = 0
   while true:
     var bytesRead = recv(socket, addr(string(result)[pos]), bufSize-1)
-    if bytesRead == -1: OSError()
+    if bytesRead == -1: OSError(OSLastError())
     setLen(result.string, pos + bytesRead)
     if bytesRead != bufSize-1: break
     # increase capacity:
@@ -1364,7 +1351,7 @@ proc recv*(socket: TSocket): TaintedString {.tags: [FReadIO], deprecated.} =
     while true:
       var bytesRead = recv(socket, cstring(buf), bufSize-1)
       # Error
-      if bytesRead == -1: OSError()
+      if bytesRead == -1: OSError(OSLastError())
       
       buf[bytesRead] = '\0' # might not be necessary
       setLen(buf, bytesRead)
@@ -1421,16 +1408,15 @@ proc recvAsync*(socket: TSocket, s: var TaintedString): bool {.
           else: SSLError("Unknown Error")
           
     if bytesRead == -1 and not (when defined(ssl): socket.isSSL else: false):
+      let err = OSLastError()
       when defined(windows):
-        # TODO: Test on Windows
-        var err = WSAGetLastError()
-        if err == WSAEWOULDBLOCK:
+        if err.int32 == WSAEWOULDBLOCK:
           return False
-        else: OSError()
+        else: OSError(err)
       else:
-        if errno == EAGAIN or errno == EWOULDBLOCK:
+        if err.int32 == EAGAIN or err.int32 == EWOULDBLOCK:
           return False
-        else: OSError()
+        else: OSError(err)
 
     setLen(s.string, pos + bytesRead)
     if bytesRead != bufSize-1: break
@@ -1475,16 +1461,15 @@ proc recvFromAsync*(socket: TSocket, data: var String, length: int,
   result = true
   var callRes = recvFrom(socket, data, length, address, port, flags)
   if callRes < 0:
+    let err = OSLastError()
     when defined(windows):
-      # TODO: Test on Windows
-      var err = WSAGetLastError()
-      if err == WSAEWOULDBLOCK:
+      if err.int32 == WSAEWOULDBLOCK:
         return False
-      else: OSError()
+      else: OSError(err)
     else:
-      if errno == EAGAIN or errno == EWOULDBLOCK:
+      if err.int32 == EAGAIN or err.int32 == EWOULDBLOCK:
         return False
-      else: OSError()
+      else: OSError(err)
 
 proc skip*(socket: TSocket) {.tags: [FReadIO], deprecated.} =
   ## skips all the data that is pending for the socket
@@ -1531,7 +1516,7 @@ proc send*(socket: TSocket, data: string) {.tags: [FWriteIO].} =
       if socket.isSSL:
         SSLError()
     
-    OSError()
+    OSError(OSLastError())
 
 proc sendAsync*(socket: TSocket, data: string): int {.tags: [FWriteIO].} =
   ## sends data to a non-blocking socket.
@@ -1561,16 +1546,15 @@ proc sendAsync*(socket: TSocket, data: string): int {.tags: [FWriteIO].} =
       else:
         return
   if result == -1:
+    let err = OSLastError()
     when defined(windows):
-      var err = WSAGetLastError()
-      # TODO: Test on windows.
-      if err == WSAEINPROGRESS:
+      if err.int32 == WSAEINPROGRESS:
         return 0
-      else: OSError()
+      else: OSError(err)
     else:
-      if errno == EAGAIN or errno == EWOULDBLOCK:
+      if err.int32 == EAGAIN or err.int32 == EWOULDBLOCK:
         return 0
-      else: OSError()
+      else: OSError(err)
   
 
 proc trySend*(socket: TSocket, data: string): bool {.tags: [FWriteIO].} =
@@ -1626,15 +1610,15 @@ proc setBlocking(s: TSocket, blocking: bool) =
   when defined(Windows):
     var mode = clong(ord(not blocking)) # 1 for non-blocking, 0 for blocking
     if ioctlsocket(TWinSocket(s.fd), FIONBIO, addr(mode)) == -1:
-      OSError()
+      OSError(OSLastError())
   else: # BSD sockets
     var x: int = fcntl(s.fd, F_GETFL, 0)
     if x == -1:
-      OSError()
+      OSError(OSLastError())
     else:
       var mode = if blocking: x and not O_NONBLOCK else: x or O_NONBLOCK
       if fcntl(s.fd, F_SETFL, mode) == -1:
-        OSError()
+        OSError(OSLastError())
   s.nonblocking = not blocking
   
 proc connect*(socket: TSocket, address: string, port = TPort(0), timeout: int,
@@ -1665,6 +1649,6 @@ proc getFD*(socket: TSocket): cint = return socket.fd
 
 when defined(Windows):
   var wsa: TWSADATA
-  if WSAStartup(0x0101'i16, wsa) != 0: OSError()
+  if WSAStartup(0x0101'i16, addr wsa) != 0: OSError(OSLastError())
 
 
diff --git a/lib/windows/winlean.nim b/lib/windows/winlean.nim
index fa4925ee6..d448d2b10 100644
--- a/lib/windows/winlean.nim
+++ b/lib/windows/winlean.nim
@@ -489,7 +489,7 @@ proc FD_SET*(Socket: TWinSocket, FDSet: var TFDSet) =
 proc FD_ZERO*(FDSet: var TFDSet) =
   FDSet.fd_count = 0
 
-proc WSAStartup*(wVersionRequired: int16, WSData: var TWSAData): cint {.
+proc WSAStartup*(wVersionRequired: int16, WSData: ptr TWSAData): cint {.
   stdcall, importc: "WSAStartup", dynlib: ws2dll.}
 
 proc getaddrinfo*(nodename, servname: cstring, hints: ptr TAddrInfo,