diff --git a/dlls/wow64cpu/Makefile.in b/dlls/wow64cpu/Makefile.in
index 261363fa72a..d574b68aee5 100644
--- a/dlls/wow64cpu/Makefile.in
+++ b/dlls/wow64cpu/Makefile.in
@@ -1 +1,6 @@
 MODULE    = wow64cpu.dll
+IMPORTS   = wow64 ntdll winecrt0
+
+EXTRADLLFLAGS = -nodefaultlibs -mno-cygwin -Wl,--image-base,0x6f100000
+
+C_SRCS = cpu.c
diff --git a/dlls/wow64cpu/cpu.c b/dlls/wow64cpu/cpu.c
new file mode 100644
index 00000000000..df292221117
--- /dev/null
+++ b/dlls/wow64cpu/cpu.c
@@ -0,0 +1,139 @@
+/*
+ * WoW64 CPU support
+ *
+ * Copyright 2021 Alexandre Julliard
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
+ */
+
+#include <stdarg.h>
+
+#include "ntstatus.h"
+#define WIN32_NO_STATUS
+#include "windef.h"
+#include "winnt.h"
+#include "winternl.h"
+#include "wine/asm.h"
+#include "wine/exception.h"
+#include "wine/debug.h"
+
+WINE_DEFAULT_DEBUG_CHANNEL(wow);
+
+#include "pshpack1.h"
+struct thunk_32to64
+{
+    BYTE  ljmp;   /* ljmp %cs:1f */
+    DWORD addr;
+    WORD  cs;
+};
+#include "poppack.h"
+
+static BYTE DECLSPEC_ALIGN(4096) code_buffer[0x1000];
+
+
+BOOL WINAPI DllMain( HINSTANCE inst, DWORD reason, void *reserved )
+{
+    if (reason == DLL_PROCESS_ATTACH) LdrDisableThreadCalloutsForDll( inst );
+    return TRUE;
+}
+
+
+/**********************************************************************
+ *           syscall_32to64
+ *
+ * Execute a 64-bit syscall from 32-bit code, then return to 32-bit.
+ */
+extern void WINAPI syscall_32to64(void) DECLSPEC_HIDDEN;
+__ASM_GLOBAL_FUNC( syscall_32to64,
+                   /* cf. BTCpuSimulate prolog */
+                   __ASM_SEH(".seh_stackalloc 0x28\n\t")
+                   __ASM_SEH(".seh_endprologue\n\t")
+                   __ASM_CFI(".cfi_adjust_cfa_offset 0x28\n\t")
+                   "xchgq %r14,%rsp\n\t"
+                   "movl %edi,0x9c(%r13)\n\t"   /* context->Edi */
+                   "movl %esi,0xa0(%r13)\n\t"   /* context->Esi */
+                   "movl %ebx,0xa4(%r13)\n\t"   /* context->Ebx */
+                   "movl %ebp,0xb4(%r13)\n\t"   /* context->Ebp */
+                   "movl (%r14),%edx\n\t"
+                   "movl %edx,0xb8(%r13)\n\t"   /* context->Eip */
+                   "pushfq\n\t"
+                   "popq %rdx\n\t"
+                   "movl %edx,0xc0(%r13)\n\t"   /* context->EFlags */
+                   "leaq 4(%r14),%rdx\n\t"
+                   "movl %edx,0xc4(%r13)\n\t"   /* context->Esp */
+                   "movq %rax,%rcx\n\t"         /* syscall number */
+                   "leaq 8(%r14),%rdx\n\t"      /* parameters */
+                   "call " __ASM_NAME("Wow64SystemServiceEx") "\n\t"
+
+                   "syscall_32to64_return:\n\t"
+                   "movl 0x9c(%r13),%edi\n\t"   /* context->Edi */
+                   "movl 0xa0(%r13),%esi\n\t"   /* context->Esi */
+                   "movl 0xa4(%r13),%ebx\n\t"   /* context->Ebx */
+                   "movl 0xb4(%r13),%ebp\n\t"   /* context->Ebp */
+                   "movl 0xb8(%r13),%edx\n\t"   /* context->Eip */
+                   "movl %edx,(%rsp)\n\t"
+                   "movl 0xbc(%r13),%edx\n\t"   /* context->SegCs */
+                   "movl %edx,4(%rsp)\n\t"
+                   "movl 0xc4(%r13),%r14d\n\t"  /* context->Esp */
+                   "xchgq %r14,%rsp\n\t"
+                   "ljmp *(%r14)" )
+
+
+/**********************************************************************
+ *           BTCpuSimulate  (wow64cpu.@)
+ */
+__ASM_STDCALL_FUNC( BTCpuSimulate, 0,
+                    "subq $0x28,%rsp\n"
+                   __ASM_SEH(".seh_stackalloc 0x28\n\t")
+                   __ASM_SEH(".seh_endprologue\n\t")
+                   __ASM_CFI(".cfi_adjust_cfa_offset 0x28\n\t")
+                    "movq %gs:0x30,%r12\n\t"
+                    "movq 0x1488(%r12),%rcx\n\t" /* NtCurrentTeb()->TlsSlots[WOW64_TLS_CPURESERVED] */
+                    "leaq 4(%rcx),%r13\n"        /* cpu->Context */
+                    "jmp syscall_32to64_return\n" )
+
+
+/**********************************************************************
+ *           BTCpuProcessInit  (wow64cpu.@)
+ */
+NTSTATUS WINAPI BTCpuProcessInit(void)
+{
+    struct thunk_32to64 *thunk = (struct thunk_32to64 *)code_buffer;
+    SIZE_T size = sizeof(*thunk);
+    ULONG old_prot;
+    CONTEXT context;
+
+    if ((ULONG_PTR)syscall_32to64 >> 32)
+    {
+        ERR( "wow64cpu loaded above 4G, disabling\n" );
+        return STATUS_INVALID_ADDRESS;
+    }
+
+    RtlCaptureContext( &context );
+    thunk->ljmp = 0xea;
+    thunk->addr = PtrToUlong( syscall_32to64 );
+    thunk->cs   = context.SegCs;
+    NtProtectVirtualMemory( GetCurrentProcess(), (void **)&thunk, &size, PAGE_EXECUTE_READ, &old_prot );
+    return STATUS_SUCCESS;
+}
+
+
+/**********************************************************************
+ *           BTCpuGetBopCode  (wow64cpu.@)
+ */
+void * WINAPI BTCpuGetBopCode(void)
+{
+    return code_buffer;
+}
diff --git a/dlls/wow64cpu/wow64cpu.spec b/dlls/wow64cpu/wow64cpu.spec
index 768b6627a1d..ba0abb7a8ed 100644
--- a/dlls/wow64cpu/wow64cpu.spec
+++ b/dlls/wow64cpu/wow64cpu.spec
@@ -1,9 +1,9 @@
-@ stub BTCpuGetBopCode
-@ stub BTCpuGetContext
-@ stub BTCpuProcessInit
-@ stub BTCpuResetToConsistentState
-@ stub BTCpuSetContext
-@ stub BTCpuSimulate
-@ stub BTCpuTurboThunkControl
-@ stub TurboDispatchJumpAddressEnd
-@ stub TurboDispatchJumpAddressStart
+@ stdcall BTCpuGetBopCode()
+#@ stub BTCpuGetContext
+@ stdcall BTCpuProcessInit()
+#@ stub BTCpuResetToConsistentState
+#@ stub BTCpuSetContext
+@ stdcall BTCpuSimulate()
+#@ stub BTCpuTurboThunkControl
+#@ stub TurboDispatchJumpAddressEnd
+#@ stub TurboDispatchJumpAddressStart