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diff --git a/src/VBox/Runtime/win/amd64/ASMAtomicBitTestAndToggle.asm b/src/VBox/Runtime/win/amd64/ASMAtomicBitTestAndToggle.asm
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+++ b/src/VBox/Runtime/win/amd64/ASMAtomicBitTestAndToggle.asm
@@ -0,0 +1,46 @@
+;; @file
+; IPRT - ASMAtomicBitTestAndToggle().
+;
+
+;
+; Copyright (C) 2006-2019 Oracle Corporation
+;
+; This file is part of VirtualBox Open Source Edition (OSE), as
+; available from http://www.virtualbox.org. This file is free software;
+; you can redistribute it and/or modify it under the terms of the GNU
+; General Public License (GPL) as published by the Free Software
+; Foundation, in version 2 as it comes in the "COPYING" file of the
+; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
+; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
+;
+; The contents of this file may alternatively be used under the terms
+; of the Common Development and Distribution License Version 1.0
+; (CDDL) only, as it comes in the "COPYING.CDDL" file of the
+; VirtualBox OSE distribution, in which case the provisions of the
+; CDDL are applicable instead of those of the GPL.
+;
+; You may elect to license modified versions of this file under the
+; terms and conditions of either the GPL or the CDDL or both.
+;
+
+
+;*******************************************************************************
+;* Header Files                                                                *
+;*******************************************************************************
+%include "iprt/asmdefs.mac"
+
+BEGINCODE
+
+;;
+; Atomically tests and toggles a bit in a bitmap.
+;
+; @returns al(rax)      true if the bit was set.
+; @returns al(rax)      false if the bit was clear.
+; @param   rcx          pvBitmap    Pointer to the bitmap.
+; @param   edx          iBit        The bit to test and toggle.
+BEGINPROC_EXPORTED ASMAtomicBitTestAndToggle
+        lock btc [rcx], edx
+        sbb     eax, eax
+        ret
+ENDPROC ASMAtomicBitTestAndToggle
+