Index: int10.c
===================================================================
--- win32ss/drivers/videoprt/int10.c	(revision 74578)
+++ win32ss/drivers/videoprt/int10.c	(working copy)
@@ -296,7 +296,9 @@
     BiosContext.SegEs = BiosArguments->SegEs;
 
     /* Do the ROM BIOS call */
+    (void)KeWaitForMutexObject(&VideoPortInt10Mutex, Executive, KernelMode, FALSE, NULL);
     Status = Ke386CallBios(0x10, &BiosContext);
+    KeReleaseMutex(&VideoPortInt10Mutex, FALSE);
 
     /* Return the arguments */
     BiosArguments->Eax = BiosContext.Eax;
@@ -354,7 +356,9 @@
     BiosContext.Ebp = BiosArguments->Ebp;
 
     /* Do the ROM BIOS call */
+    (void)KeWaitForMutexObject(&VideoPortInt10Mutex, Executive, KernelMode, FALSE, NULL);
     Status = Ke386CallBios(0x10, &BiosContext);
+    KeReleaseMutex(&VideoPortInt10Mutex, FALSE);
 
     /* Return the arguments */
     BiosArguments->Eax = BiosContext.Eax;
Index: videoprt.c
===================================================================
--- win32ss/drivers/videoprt/videoprt.c	(revision 74578)
+++ win32ss/drivers/videoprt/videoprt.c	(working copy)
@@ -33,6 +33,7 @@
 ULONG CsrssInitialized = FALSE;
 PKPROCESS Csrss = NULL;
 ULONG VideoPortDeviceNumber = 0;
+KMUTEX VideoPortInt10Mutex;
 
 /* PRIVATE FUNCTIONS **********************************************************/
 
@@ -464,9 +465,16 @@
     NTSTATUS Status;
     PVIDEO_PORT_DRIVER_EXTENSION DriverExtension;
     BOOLEAN PnpDriver = FALSE, LegacyDetection = FALSE;
+    static BOOLEAN Int10MutexInitialized;
 
     TRACE_(VIDEOPRT, "VideoPortInitialize\n");
 
+    if (!Int10MutexInitialized)
+    {
+        KeInitializeMutex(&VideoPortInt10Mutex, 0);
+        Int10MutexInitialized = TRUE;
+    }
+
     /* As a first thing do parameter checks. */
     if (HwInitializationData->HwInitDataSize > sizeof(VIDEO_HW_INITIALIZATION_DATA))
     {
Index: videoprt.h
===================================================================
--- win32ss/drivers/videoprt/videoprt.h	(revision 74578)
+++ win32ss/drivers/videoprt/videoprt.h	(working copy)
@@ -247,6 +247,7 @@
 extern ULONG CsrssInitialized;
 extern PKPROCESS Csrss;
 extern ULONG VideoPortDeviceNumber;
+extern KMUTEX VideoPortInt10Mutex;
 
 VOID FASTCALL
 IntAttachToCSRSS(PKPROCESS *CallingProcess, PKAPC_STATE ApcState);