IntelFrameworkModulePkg/Csm/LegacyBiosDxe/Thunk.c - third_party/edk2 - Git at Google

 /** @file
   Call into 16-bit BIOS code, Use AsmThunk16 function of BaseLib.

 Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>

 This program and the accompanying materials
 are licensed and made available under the terms and conditions
 of the BSD License which accompanies this distribution.  The
 full text of the license may be found at
 http://opensource.org/licenses/bsd-license.php

 THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
 WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

 **/

 #include "LegacyBiosInterface.h"

 THUNK_CONTEXT      mThunkContext;

 /**
   Sets the counter value for Timer #0 in a legacy 8254 timer.

   @param  Count - The 16-bit counter value to program into Timer #0 of the legacy 8254 timer.

 **/
 VOID
 SetPitCount (
   IN UINT16  Count
   )
 {
   IoWrite8 (TIMER_CONTROL_PORT, TIMER0_CONTROL_WORD);
   IoWrite8 (TIMER0_COUNT_PORT, (UINT8) (Count & 0xFF));
   IoWrite8 (TIMER0_COUNT_PORT, (UINT8) ((Count>>8) & 0xFF));
 }

 /**
   Thunk to 16-bit real mode and execute a software interrupt with a vector
   of BiosInt. Regs will contain the 16-bit register context on entry and
   exit.

   @param  This    Protocol instance pointer.
   @param  BiosInt Processor interrupt vector to invoke
   @param  Regs    Register contexted passed into (and returned) from thunk to
                   16-bit mode

   @retval FALSE   Thunk completed, and there were no BIOS errors in the target code.
                   See Regs for status.
   @retval TRUE    There was a BIOS erro in the target code.

 **/
 BOOLEAN
 EFIAPI
 LegacyBiosInt86 (
   IN  EFI_LEGACY_BIOS_PROTOCOL      *This,
   IN  UINT8                         BiosInt,
   IN  EFI_IA32_REGISTER_SET         *Regs
   )
 {
   UINT16                Segment;
   UINT16                Offset;

   Regs->X.Flags.Reserved1 = 1;
   Regs->X.Flags.Reserved2 = 0;
   Regs->X.Flags.Reserved3 = 0;
   Regs->X.Flags.Reserved4 = 0;
   Regs->X.Flags.IOPL      = 3;
   Regs->X.Flags.NT        = 0;
   Regs->X.Flags.IF        = 0;
   Regs->X.Flags.TF        = 0;
   Regs->X.Flags.CF        = 0;
   //
   // The base address of legacy interrupt vector table is 0.
   // We use this base address to get the legacy interrupt handler.
   //
   ACCESS_PAGE0_CODE (
     Segment               = (UINT16)(((UINT32 *)0)[BiosInt] >> 16);
     Offset                = (UINT16)((UINT32 *)0)[BiosInt];
   );

   return InternalLegacyBiosFarCall (
            This,
            Segment,
            Offset,
            Regs,
            &Regs->X.Flags,
            sizeof (Regs->X.Flags)
            );
 }

 /**
   Thunk to 16-bit real mode and call Segment:Offset. Regs will contain the
   16-bit register context on entry and exit. Arguments can be passed on
   the Stack argument

   @param  This                   Protocol instance pointer.
   @param  Segment                Segemnt of 16-bit mode call
   @param  Offset                 Offset of 16-bit mdoe call
   @param  Regs                   Register contexted passed into (and returned) from
                                  thunk to  16-bit mode
   @param  Stack                  Caller allocated stack used to pass arguments
   @param  StackSize              Size of Stack in bytes

   @retval FALSE                  Thunk completed, and there were no BIOS errors in
                                  the target code. See Regs for status.
   @retval TRUE                   There was a BIOS erro in the target code.

 **/
 BOOLEAN
 EFIAPI
 LegacyBiosFarCall86 (
   IN  EFI_LEGACY_BIOS_PROTOCOL        *This,
   IN  UINT16                          Segment,
   IN  UINT16                          Offset,
   IN  EFI_IA32_REGISTER_SET           *Regs,
   IN  VOID                            *Stack,
   IN  UINTN                           StackSize
   )
 {
   Regs->X.Flags.Reserved1 = 1;
   Regs->X.Flags.Reserved2 = 0;
   Regs->X.Flags.Reserved3 = 0;
   Regs->X.Flags.Reserved4 = 0;
   Regs->X.Flags.IOPL      = 3;
   Regs->X.Flags.NT        = 0;
   Regs->X.Flags.IF        = 1;
   Regs->X.Flags.TF        = 0;
   Regs->X.Flags.CF        = 0;

   return InternalLegacyBiosFarCall (This, Segment, Offset, Regs, Stack, StackSize);
 }

 /**
   Provide NULL interrupt handler which is used to check
   if there is more than one HW interrupt registers with the CPU AP.

   @param  InterruptType - The type of interrupt that occured
   @param  SystemContext - A pointer to the system context when the interrupt occured

 **/
 VOID
 EFIAPI
 LegacyBiosNullInterruptHandler (
   IN EFI_EXCEPTION_TYPE   InterruptType,
   IN EFI_SYSTEM_CONTEXT   SystemContext
   )
 {
 }

 /**
   Thunk to 16-bit real mode and call Segment:Offset. Regs will contain the
   16-bit register context on entry and exit. Arguments can be passed on
   the Stack argument

   @param  This       Protocol instance pointer.
   @param  Segment    Segemnt of 16-bit mode call
   @param  Offset     Offset of 16-bit mdoe call
   @param  Regs       Register contexted passed into (and returned) from thunk to
                      16-bit mode
   @param  Stack      Caller allocated stack used to pass arguments
   @param  StackSize  Size of Stack in bytes

   @retval FALSE      Thunk completed, and there were no BIOS errors in the target code.
                      See Regs for status.
   @retval TRUE       There was a BIOS erro in the target code.

 **/
 BOOLEAN
 EFIAPI
 InternalLegacyBiosFarCall (
   IN  EFI_LEGACY_BIOS_PROTOCOL        *This,
   IN  UINT16                          Segment,
   IN  UINT16                          Offset,
   IN  EFI_IA32_REGISTER_SET           *Regs,
   IN  VOID                            *Stack,
   IN  UINTN                           StackSize
   )
 {
   UINTN                 Status;
   LEGACY_BIOS_INSTANCE  *Private;
   UINT16                *Stack16;
   EFI_TPL               OriginalTpl;
   IA32_REGISTER_SET     ThunkRegSet;
   BOOLEAN               InterruptState;
   UINT64                TimerPeriod;

   Private = LEGACY_BIOS_INSTANCE_FROM_THIS (This);

   ZeroMem (&ThunkRegSet, sizeof (ThunkRegSet));
   ThunkRegSet.X.DI   = Regs->X.DI;
   ThunkRegSet.X.SI   = Regs->X.SI;
   ThunkRegSet.X.BP   = Regs->X.BP;
   ThunkRegSet.X.BX   = Regs->X.BX;
   ThunkRegSet.X.DX   = Regs->X.DX;
   //
   // Sometimes, ECX is used to pass in 32 bit data. For example, INT 1Ah, AX = B10Dh is
   // "PCI BIOS v2.0c + Write Configuration DWORD" and ECX has the dword to write.
   //
   ThunkRegSet.E.ECX   = Regs->E.ECX;
   ThunkRegSet.X.AX   = Regs->X.AX;
   ThunkRegSet.E.DS   = Regs->X.DS;
   ThunkRegSet.E.ES   = Regs->X.ES;

   CopyMem (&(ThunkRegSet.E.EFLAGS.UintN), &(Regs->X.Flags), sizeof (Regs->X.Flags));

   //
   // Clear the error flag; thunk code may set it. Stack16 should be the high address
   // Make Statk16 address the low 16 bit must be not zero.
   //
   Stack16 = (UINT16 *)((UINT8 *) mThunkContext.RealModeBuffer + mThunkContext.RealModeBufferSize - sizeof (UINT16));

   //
   // Save current rate of DXE Timer
   //
   Private->Timer->GetTimerPeriod (Private->Timer, &TimerPeriod);

   //
   // Disable DXE Timer while executing in real mode
   //
   Private->Timer->SetTimerPeriod (Private->Timer, 0);

   //
   // Save and disable interrupt of debug timer
   //
   InterruptState = SaveAndSetDebugTimerInterrupt (FALSE);

   //
   // The call to Legacy16 is a critical section to EFI
   //
   OriginalTpl = gBS->RaiseTPL (TPL_HIGH_LEVEL);

   //
   // Check to see if there is more than one HW interrupt registers with the CPU AP.
   // If there is, then ASSERT() since that is not compatible with the CSM because
   // interupts other than the Timer interrupt that was disabled above can not be
   // handled properly from real mode.
   //
   DEBUG_CODE (
     UINTN  Vector;
     UINTN  Count;

     for (Vector = 0x20, Count = 0; Vector < 0x100; Vector++) {
       Status = Private->Cpu->RegisterInterruptHandler (Private->Cpu, Vector, LegacyBiosNullInterruptHandler);
       if (Status == EFI_ALREADY_STARTED) {
         Count++;
       }
       if (Status == EFI_SUCCESS) {
         Private->Cpu->RegisterInterruptHandler (Private->Cpu, Vector, NULL);
       }
     }
     if (Count >= 2) {
       DEBUG ((EFI_D_ERROR, "ERROR: More than one HW interrupt active with CSM enabled\n"));
     }
     ASSERT (Count < 2);
   );

   //
   // If the Timer AP has enabled the 8254 timer IRQ and the current 8254 timer
   // period is less than the CSM required rate of 54.9254, then force the 8254
   // PIT counter to 0, which is the CSM required rate of 54.9254 ms
   //
   if (Private->TimerUses8254 && TimerPeriod < 549254) {
     SetPitCount (0);
   }

   if (Stack != NULL && StackSize != 0) {
     //
     // Copy Stack to low memory stack
     //
     Stack16 -= StackSize / sizeof (UINT16);
     CopyMem (Stack16, Stack, StackSize);
   }

   ThunkRegSet.E.SS   = (UINT16) (((UINTN) Stack16 >> 16) << 12);
   ThunkRegSet.E.ESP  = (UINT16) (UINTN) Stack16;
   ThunkRegSet.E.CS   = Segment;
   ThunkRegSet.E.Eip  = Offset;

   mThunkContext.RealModeState      = &ThunkRegSet;

   //
   // Set Legacy16 state. 0x08, 0x70 is legacy 8259 vector bases.
   //
   Status = Private->Legacy8259->SetMode (Private->Legacy8259, Efi8259LegacyMode, NULL, NULL);
   ASSERT_EFI_ERROR (Status);

   AsmThunk16 (&mThunkContext);

   if (Stack != NULL && StackSize != 0) {
     //
     // Copy low memory stack to Stack
     //
     CopyMem (Stack, Stack16, StackSize);
   }

   //
   // Restore protected mode interrupt state
   //
   Status = Private->Legacy8259->SetMode (Private->Legacy8259, Efi8259ProtectedMode, NULL, NULL);
   ASSERT_EFI_ERROR (Status);

   mThunkContext.RealModeState = NULL;

   //
   // Enable and restore rate of DXE Timer
   //
   Private->Timer->SetTimerPeriod (Private->Timer, TimerPeriod);

   //
   // End critical section
   //
   gBS->RestoreTPL (OriginalTpl);

   //
   // OPROM may allocate EBDA range by itself and change EBDA base and EBDA size.
   // Get the current EBDA base address, and compared with pre-allocate minimum
   // EBDA base address, if the current EBDA base address is smaller, it indicates
   // PcdEbdaReservedMemorySize should be adjusted to larger for more OPROMs.
   //
   DEBUG_CODE (
     {
       UINTN                 EbdaBaseAddress;
       UINTN                 ReservedEbdaBaseAddress;

       ACCESS_PAGE0_CODE (
         EbdaBaseAddress = (*(UINT16 *) (UINTN) 0x40E) << 4;
         ReservedEbdaBaseAddress = CONVENTIONAL_MEMORY_TOP
                                   - PcdGet32 (PcdEbdaReservedMemorySize);
         ASSERT (ReservedEbdaBaseAddress <= EbdaBaseAddress);
       );
     }
   );

   //
   // Restore interrupt of debug timer
   //
   SaveAndSetDebugTimerInterrupt (InterruptState);

   Regs->E.EDI      = ThunkRegSet.E.EDI;
   Regs->E.ESI      = ThunkRegSet.E.ESI;
   Regs->E.EBP      = ThunkRegSet.E.EBP;
   Regs->E.EBX      = ThunkRegSet.E.EBX;
   Regs->E.EDX      = ThunkRegSet.E.EDX;
   Regs->E.ECX      = ThunkRegSet.E.ECX;
   Regs->E.EAX      = ThunkRegSet.E.EAX;
   Regs->X.SS       = ThunkRegSet.E.SS;
   Regs->X.CS       = ThunkRegSet.E.CS;
   Regs->X.DS       = ThunkRegSet.E.DS;
   Regs->X.ES       = ThunkRegSet.E.ES;

   CopyMem (&(Regs->X.Flags), &(ThunkRegSet.E.EFLAGS.UintN), sizeof (Regs->X.Flags));

   return (BOOLEAN) (Regs->X.Flags.CF == 1);
 }

 /**
   Allocate memory < 1 MB and copy the thunker code into low memory. Se up
   all the descriptors.

   @param  Private                Private context for Legacy BIOS

   @retval EFI_SUCCESS            Should only pass.

 **/
 EFI_STATUS
 LegacyBiosInitializeThunk (
   IN  LEGACY_BIOS_INSTANCE    *Private
   )
 {
   EFI_STATUS              Status;
   EFI_PHYSICAL_ADDRESS    MemoryAddress;
   UINT8                   TimerVector;

   MemoryAddress   = (EFI_PHYSICAL_ADDRESS) (UINTN) Private->IntThunk;

   mThunkContext.RealModeBuffer     = (VOID *) (UINTN) (MemoryAddress + ((sizeof (LOW_MEMORY_THUNK) / EFI_PAGE_SIZE) + 1) * EFI_PAGE_SIZE);
   mThunkContext.RealModeBufferSize = EFI_PAGE_SIZE;
   mThunkContext.ThunkAttributes    = THUNK_ATTRIBUTE_BIG_REAL_MODE | THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15;

   AsmPrepareThunk16 (&mThunkContext);

   //
   // Get the interrupt vector number corresponding to IRQ0 from the 8259 driver
   //
   TimerVector = 0;
   Status = Private->Legacy8259->GetVector (Private->Legacy8259, Efi8259Irq0, &TimerVector);
   ASSERT_EFI_ERROR (Status);

   //
   // Check to see if the Timer AP has hooked the IRQ0 from the 8254 PIT
   //
   Status = Private->Cpu->RegisterInterruptHandler (
                            Private->Cpu,
                            TimerVector,
                            LegacyBiosNullInterruptHandler
                            );
   if (Status == EFI_SUCCESS) {
     //
     // If the Timer AP has not enabled the 8254 timer IRQ, then force the 8254 PIT
     // counter to 0, which is the CSM required rate of 54.9254 ms
     //
     Private->Cpu->RegisterInterruptHandler (
                     Private->Cpu,
                     TimerVector,
                     NULL
                     );
     SetPitCount (0);

     //
     // Save status that the Timer AP is not using the 8254 PIT
     //
     Private->TimerUses8254 = FALSE;
   } else if (Status == EFI_ALREADY_STARTED) {
     //
     // Save status that the Timer AP is using the 8254 PIT
     //
     Private->TimerUses8254 = TRUE;
   } else {
     //
     // Unexpected status from CPU AP RegisterInterruptHandler()
     //
     ASSERT (FALSE);
   }

   return EFI_SUCCESS;
 }
	/** @file
	Call into 16-bit BIOS code, Use AsmThunk16 function of BaseLib.

	Copyright (c) 2006 - 2011, Intel Corporation. All rights reserved.<BR>

	This program and the accompanying materials
	are licensed and made available under the terms and conditions
	of the BSD License which accompanies this distribution. The
	full text of the license may be found at
	http://opensource.org/licenses/bsd-license.php

	THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
	WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.

	**/

	#include "LegacyBiosInterface.h"

	THUNK_CONTEXT mThunkContext;

	/**
	Sets the counter value for Timer #0 in a legacy 8254 timer.

	@param Count - The 16-bit counter value to program into Timer #0 of the legacy 8254 timer.

	**/
	VOID
	SetPitCount (
	IN UINT16 Count
	)
	{
	IoWrite8 (TIMER_CONTROL_PORT, TIMER0_CONTROL_WORD);
	IoWrite8 (TIMER0_COUNT_PORT, (UINT8) (Count & 0xFF));
	IoWrite8 (TIMER0_COUNT_PORT, (UINT8) ((Count>>8) & 0xFF));
	}

	/**
	Thunk to 16-bit real mode and execute a software interrupt with a vector
	of BiosInt. Regs will contain the 16-bit register context on entry and
	exit.

	@param This Protocol instance pointer.
	@param BiosInt Processor interrupt vector to invoke
	@param Regs Register contexted passed into (and returned) from thunk to
	16-bit mode

	@retval FALSE Thunk completed, and there were no BIOS errors in the target code.
	See Regs for status.
	@retval TRUE There was a BIOS erro in the target code.

	**/
	BOOLEAN
	EFIAPI
	LegacyBiosInt86 (
	IN EFI_LEGACY_BIOS_PROTOCOL *This,
	IN UINT8 BiosInt,
	IN EFI_IA32_REGISTER_SET *Regs
	)
	{
	UINT16 Segment;
	UINT16 Offset;

	Regs->X.Flags.Reserved1 = 1;
	Regs->X.Flags.Reserved2 = 0;
	Regs->X.Flags.Reserved3 = 0;
	Regs->X.Flags.Reserved4 = 0;
	Regs->X.Flags.IOPL = 3;
	Regs->X.Flags.NT = 0;
	Regs->X.Flags.IF = 0;
	Regs->X.Flags.TF = 0;
	Regs->X.Flags.CF = 0;
	//
	// The base address of legacy interrupt vector table is 0.
	// We use this base address to get the legacy interrupt handler.
	//
	ACCESS_PAGE0_CODE (
	Segment = (UINT16)(((UINT32 *)0)[BiosInt] >> 16);
	Offset = (UINT16)((UINT32 *)0)[BiosInt];
	);

	return InternalLegacyBiosFarCall (
	This,
	Segment,
	Offset,
	Regs,
	&Regs->X.Flags,
	sizeof (Regs->X.Flags)
	);
	}

	/**
	Thunk to 16-bit real mode and call Segment:Offset. Regs will contain the
	16-bit register context on entry and exit. Arguments can be passed on
	the Stack argument

	@param This Protocol instance pointer.
	@param Segment Segemnt of 16-bit mode call
	@param Offset Offset of 16-bit mdoe call
	@param Regs Register contexted passed into (and returned) from
	thunk to 16-bit mode
	@param Stack Caller allocated stack used to pass arguments
	@param StackSize Size of Stack in bytes

	@retval FALSE Thunk completed, and there were no BIOS errors in
	the target code. See Regs for status.
	@retval TRUE There was a BIOS erro in the target code.

	**/
	BOOLEAN
	EFIAPI
	LegacyBiosFarCall86 (
	IN EFI_LEGACY_BIOS_PROTOCOL *This,
	IN UINT16 Segment,
	IN UINT16 Offset,
	IN EFI_IA32_REGISTER_SET *Regs,
	IN VOID *Stack,
	IN UINTN StackSize
	)
	{
	Regs->X.Flags.Reserved1 = 1;
	Regs->X.Flags.Reserved2 = 0;
	Regs->X.Flags.Reserved3 = 0;
	Regs->X.Flags.Reserved4 = 0;
	Regs->X.Flags.IOPL = 3;
	Regs->X.Flags.NT = 0;
	Regs->X.Flags.IF = 1;
	Regs->X.Flags.TF = 0;
	Regs->X.Flags.CF = 0;

	return InternalLegacyBiosFarCall (This, Segment, Offset, Regs, Stack, StackSize);
	}

	/**
	Provide NULL interrupt handler which is used to check
	if there is more than one HW interrupt registers with the CPU AP.

	@param InterruptType - The type of interrupt that occured
	@param SystemContext - A pointer to the system context when the interrupt occured

	**/
	VOID
	EFIAPI
	LegacyBiosNullInterruptHandler (
	IN EFI_EXCEPTION_TYPE InterruptType,
	IN EFI_SYSTEM_CONTEXT SystemContext
	)
	{
	}

	/**
	Thunk to 16-bit real mode and call Segment:Offset. Regs will contain the
	16-bit register context on entry and exit. Arguments can be passed on
	the Stack argument

	@param This Protocol instance pointer.
	@param Segment Segemnt of 16-bit mode call
	@param Offset Offset of 16-bit mdoe call
	@param Regs Register contexted passed into (and returned) from thunk to
	16-bit mode
	@param Stack Caller allocated stack used to pass arguments
	@param StackSize Size of Stack in bytes

	@retval FALSE Thunk completed, and there were no BIOS errors in the target code.
	See Regs for status.
	@retval TRUE There was a BIOS erro in the target code.

	**/
	BOOLEAN
	EFIAPI
	InternalLegacyBiosFarCall (
	IN EFI_LEGACY_BIOS_PROTOCOL *This,
	IN UINT16 Segment,
	IN UINT16 Offset,
	IN EFI_IA32_REGISTER_SET *Regs,
	IN VOID *Stack,
	IN UINTN StackSize
	)
	{
	UINTN Status;
	LEGACY_BIOS_INSTANCE *Private;
	UINT16 *Stack16;
	EFI_TPL OriginalTpl;
	IA32_REGISTER_SET ThunkRegSet;
	BOOLEAN InterruptState;
	UINT64 TimerPeriod;

	Private = LEGACY_BIOS_INSTANCE_FROM_THIS (This);

	ZeroMem (&ThunkRegSet, sizeof (ThunkRegSet));
	ThunkRegSet.X.DI = Regs->X.DI;
	ThunkRegSet.X.SI = Regs->X.SI;
	ThunkRegSet.X.BP = Regs->X.BP;
	ThunkRegSet.X.BX = Regs->X.BX;
	ThunkRegSet.X.DX = Regs->X.DX;
	//
	// Sometimes, ECX is used to pass in 32 bit data. For example, INT 1Ah, AX = B10Dh is
	// "PCI BIOS v2.0c + Write Configuration DWORD" and ECX has the dword to write.
	//
	ThunkRegSet.E.ECX = Regs->E.ECX;
	ThunkRegSet.X.AX = Regs->X.AX;
	ThunkRegSet.E.DS = Regs->X.DS;
	ThunkRegSet.E.ES = Regs->X.ES;

	CopyMem (&(ThunkRegSet.E.EFLAGS.UintN), &(Regs->X.Flags), sizeof (Regs->X.Flags));

	//
	// Clear the error flag; thunk code may set it. Stack16 should be the high address
	// Make Statk16 address the low 16 bit must be not zero.
	//
	Stack16 = (UINT16 )((UINT8 ) mThunkContext.RealModeBuffer + mThunkContext.RealModeBufferSize - sizeof (UINT16));

	//
	// Save current rate of DXE Timer
	//
	Private->Timer->GetTimerPeriod (Private->Timer, &TimerPeriod);

	//
	// Disable DXE Timer while executing in real mode
	//
	Private->Timer->SetTimerPeriod (Private->Timer, 0);

	//
	// Save and disable interrupt of debug timer
	//
	InterruptState = SaveAndSetDebugTimerInterrupt (FALSE);

	//
	// The call to Legacy16 is a critical section to EFI
	//
	OriginalTpl = gBS->RaiseTPL (TPL_HIGH_LEVEL);

	//
	// Check to see if there is more than one HW interrupt registers with the CPU AP.
	// If there is, then ASSERT() since that is not compatible with the CSM because
	// interupts other than the Timer interrupt that was disabled above can not be
	// handled properly from real mode.
	//
	DEBUG_CODE (
	UINTN Vector;
	UINTN Count;

	for (Vector = 0x20, Count = 0; Vector < 0x100; Vector++) {
	Status = Private->Cpu->RegisterInterruptHandler (Private->Cpu, Vector, LegacyBiosNullInterruptHandler);
	if (Status == EFI_ALREADY_STARTED) {
	Count++;
	}
	if (Status == EFI_SUCCESS) {
	Private->Cpu->RegisterInterruptHandler (Private->Cpu, Vector, NULL);
	}
	}
	if (Count >= 2) {
	DEBUG ((EFI_D_ERROR, "ERROR: More than one HW interrupt active with CSM enabled\n"));
	}
	ASSERT (Count < 2);
	);

	//
	// If the Timer AP has enabled the 8254 timer IRQ and the current 8254 timer
	// period is less than the CSM required rate of 54.9254, then force the 8254
	// PIT counter to 0, which is the CSM required rate of 54.9254 ms
	//
	if (Private->TimerUses8254 && TimerPeriod < 549254) {
	SetPitCount (0);
	}

	if (Stack != NULL && StackSize != 0) {
	//
	// Copy Stack to low memory stack
	//
	Stack16 -= StackSize / sizeof (UINT16);
	CopyMem (Stack16, Stack, StackSize);
	}

	ThunkRegSet.E.SS = (UINT16) (((UINTN) Stack16 >> 16) << 12);
	ThunkRegSet.E.ESP = (UINT16) (UINTN) Stack16;
	ThunkRegSet.E.CS = Segment;
	ThunkRegSet.E.Eip = Offset;

	mThunkContext.RealModeState = &ThunkRegSet;

	//
	// Set Legacy16 state. 0x08, 0x70 is legacy 8259 vector bases.
	//
	Status = Private->Legacy8259->SetMode (Private->Legacy8259, Efi8259LegacyMode, NULL, NULL);
	ASSERT_EFI_ERROR (Status);

	AsmThunk16 (&mThunkContext);

	if (Stack != NULL && StackSize != 0) {
	//
	// Copy low memory stack to Stack
	//
	CopyMem (Stack, Stack16, StackSize);
	}

	//
	// Restore protected mode interrupt state
	//
	Status = Private->Legacy8259->SetMode (Private->Legacy8259, Efi8259ProtectedMode, NULL, NULL);
	ASSERT_EFI_ERROR (Status);

	mThunkContext.RealModeState = NULL;

	//
	// Enable and restore rate of DXE Timer
	//
	Private->Timer->SetTimerPeriod (Private->Timer, TimerPeriod);

	//
	// End critical section
	//
	gBS->RestoreTPL (OriginalTpl);

	//
	// OPROM may allocate EBDA range by itself and change EBDA base and EBDA size.
	// Get the current EBDA base address, and compared with pre-allocate minimum
	// EBDA base address, if the current EBDA base address is smaller, it indicates
	// PcdEbdaReservedMemorySize should be adjusted to larger for more OPROMs.
	//
	DEBUG_CODE (
	{
	UINTN EbdaBaseAddress;
	UINTN ReservedEbdaBaseAddress;

	ACCESS_PAGE0_CODE (
	EbdaBaseAddress = ((UINT16 ) (UINTN) 0x40E) << 4;
	ReservedEbdaBaseAddress = CONVENTIONAL_MEMORY_TOP
	- PcdGet32 (PcdEbdaReservedMemorySize);
	ASSERT (ReservedEbdaBaseAddress <= EbdaBaseAddress);
	);
	}
	);

	//
	// Restore interrupt of debug timer
	//
	SaveAndSetDebugTimerInterrupt (InterruptState);

	Regs->E.EDI = ThunkRegSet.E.EDI;
	Regs->E.ESI = ThunkRegSet.E.ESI;
	Regs->E.EBP = ThunkRegSet.E.EBP;
	Regs->E.EBX = ThunkRegSet.E.EBX;
	Regs->E.EDX = ThunkRegSet.E.EDX;
	Regs->E.ECX = ThunkRegSet.E.ECX;
	Regs->E.EAX = ThunkRegSet.E.EAX;
	Regs->X.SS = ThunkRegSet.E.SS;
	Regs->X.CS = ThunkRegSet.E.CS;
	Regs->X.DS = ThunkRegSet.E.DS;
	Regs->X.ES = ThunkRegSet.E.ES;

	CopyMem (&(Regs->X.Flags), &(ThunkRegSet.E.EFLAGS.UintN), sizeof (Regs->X.Flags));

	return (BOOLEAN) (Regs->X.Flags.CF == 1);
	}

	/**
	Allocate memory < 1 MB and copy the thunker code into low memory. Se up
	all the descriptors.

	@param Private Private context for Legacy BIOS

	@retval EFI_SUCCESS Should only pass.

	**/
	EFI_STATUS
	LegacyBiosInitializeThunk (
	IN LEGACY_BIOS_INSTANCE *Private
	)
	{
	EFI_STATUS Status;
	EFI_PHYSICAL_ADDRESS MemoryAddress;
	UINT8 TimerVector;

	MemoryAddress = (EFI_PHYSICAL_ADDRESS) (UINTN) Private->IntThunk;

	mThunkContext.RealModeBuffer = (VOID ) (UINTN) (MemoryAddress + ((sizeof (LOW_MEMORY_THUNK) / EFI_PAGE_SIZE) + 1) EFI_PAGE_SIZE);
	mThunkContext.RealModeBufferSize = EFI_PAGE_SIZE;
	mThunkContext.ThunkAttributes = THUNK_ATTRIBUTE_BIG_REAL_MODE \| THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15;

	AsmPrepareThunk16 (&mThunkContext);

	//
	// Get the interrupt vector number corresponding to IRQ0 from the 8259 driver
	//
	TimerVector = 0;
	Status = Private->Legacy8259->GetVector (Private->Legacy8259, Efi8259Irq0, &TimerVector);
	ASSERT_EFI_ERROR (Status);

	//
	// Check to see if the Timer AP has hooked the IRQ0 from the 8254 PIT
	//
	Status = Private->Cpu->RegisterInterruptHandler (
	Private->Cpu,
	TimerVector,
	LegacyBiosNullInterruptHandler
	);
	if (Status == EFI_SUCCESS) {
	//
	// If the Timer AP has not enabled the 8254 timer IRQ, then force the 8254 PIT
	// counter to 0, which is the CSM required rate of 54.9254 ms
	//
	Private->Cpu->RegisterInterruptHandler (
	Private->Cpu,
	TimerVector,
	NULL
	);
	SetPitCount (0);

	//
	// Save status that the Timer AP is not using the 8254 PIT
	//
	Private->TimerUses8254 = FALSE;
	} else if (Status == EFI_ALREADY_STARTED) {
	//
	// Save status that the Timer AP is using the 8254 PIT
	//
	Private->TimerUses8254 = TRUE;
	} else {
	//
	// Unexpected status from CPU AP RegisterInterruptHandler()
	//
	ASSERT (FALSE);
	}

	return EFI_SUCCESS;
	}