1 /*******************************************************************************
3 * Module Name: hwregs - Read/write access functions for the various ACPI
4 * control and status registers.
6 ******************************************************************************/
9 * Copyright (C) 2000 - 2022, Intel Corp.
10 * All rights reserved.
12 * Redistribution and use in source and binary forms, with or without
13 * modification, are permitted provided that the following conditions
15 * 1. Redistributions of source code must retain the above copyright
16 * notice, this list of conditions, and the following disclaimer,
17 * without modification.
18 * 2. Redistributions in binary form must reproduce at minimum a disclaimer
19 * substantially similar to the "NO WARRANTY" disclaimer below
20 * ("Disclaimer") and any redistribution must be conditioned upon
21 * including a substantially similar Disclaimer requirement for further
22 * binary redistribution.
23 * 3. Neither the names of the above-listed copyright holders nor the names
24 * of any contributors may be used to endorse or promote products derived
25 * from this software without specific prior written permission.
27 * Alternatively, this software may be distributed under the terms of the
28 * GNU General Public License ("GPL") version 2 as published by the Free
29 * Software Foundation.
32 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
33 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
34 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
35 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
36 * HOLDERS OR CONTRIBUTORS BE LIABLE FOR SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
37 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
38 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
39 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
40 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
41 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
42 * POSSIBILITY OF SUCH DAMAGES.
49 #define _COMPONENT ACPI_HARDWARE
50 ACPI_MODULE_NAME ("hwregs")
53 #if (!ACPI_REDUCED_HARDWARE)
55 /* Local Prototypes */
58 AcpiHwGetAccessBitWidth (
60 ACPI_GENERIC_ADDRESS
*Reg
,
66 ACPI_GENERIC_ADDRESS
*RegisterA
,
67 ACPI_GENERIC_ADDRESS
*RegisterB
);
72 ACPI_GENERIC_ADDRESS
*RegisterA
,
73 ACPI_GENERIC_ADDRESS
*RegisterB
);
75 #endif /* !ACPI_REDUCED_HARDWARE */
78 /******************************************************************************
80 * FUNCTION: AcpiHwGetAccessBitWidth
82 * PARAMETERS: Address - GAS register address
83 * Reg - GAS register structure
84 * MaxBitWidth - Max BitWidth supported (32 or 64)
88 * DESCRIPTION: Obtain optimal access bit width
90 ******************************************************************************/
93 AcpiHwGetAccessBitWidth (
95 ACPI_GENERIC_ADDRESS
*Reg
,
102 * GAS format "register", used by FADT:
103 * 1. Detected if BitOffset is 0 and BitWidth is 8/16/32/64;
104 * 2. AccessSize field is ignored and BitWidth field is used for
105 * determining the boundary of the IO accesses.
106 * GAS format "region", used by APEI registers:
107 * 1. Detected if BitOffset is not 0 or BitWidth is not 8/16/32/64;
108 * 2. AccessSize field is used for determining the boundary of the
110 * 3. BitOffset/BitWidth fields are used to describe the "region".
112 * Note: This algorithm assumes that the "Address" fields should always
113 * contain aligned values.
115 if (!Reg
->BitOffset
&& Reg
->BitWidth
&&
116 ACPI_IS_POWER_OF_TWO (Reg
->BitWidth
) &&
117 ACPI_IS_ALIGNED (Reg
->BitWidth
, 8))
119 AccessBitWidth
= Reg
->BitWidth
;
121 else if (Reg
->AccessWidth
)
123 AccessBitWidth
= ACPI_ACCESS_BIT_WIDTH (Reg
->AccessWidth
);
127 AccessBitWidth
= ACPI_ROUND_UP_POWER_OF_TWO_8 (
128 Reg
->BitOffset
+ Reg
->BitWidth
);
129 if (AccessBitWidth
<= 8)
135 while (!ACPI_IS_ALIGNED (Address
, AccessBitWidth
>> 3))
137 AccessBitWidth
>>= 1;
142 /* Maximum IO port access bit width is 32 */
144 if (Reg
->SpaceId
== ACPI_ADR_SPACE_SYSTEM_IO
)
150 * Return access width according to the requested maximum access bit width,
151 * as the caller should know the format of the register and may enforce
154 if (AccessBitWidth
< MaxBitWidth
)
156 return (AccessBitWidth
);
158 return (MaxBitWidth
);
162 /******************************************************************************
164 * FUNCTION: AcpiHwValidateRegister
166 * PARAMETERS: Reg - GAS register structure
167 * MaxBitWidth - Max BitWidth supported (32 or 64)
168 * Address - Pointer to where the gas->address
173 * DESCRIPTION: Validate the contents of a GAS register. Checks the GAS
174 * pointer, Address, SpaceId, BitWidth, and BitOffset.
176 ******************************************************************************/
179 AcpiHwValidateRegister (
180 ACPI_GENERIC_ADDRESS
*Reg
,
188 /* Must have a valid pointer to a GAS structure */
192 return (AE_BAD_PARAMETER
);
196 * Copy the target address. This handles possible alignment issues.
197 * Address must not be null. A null address also indicates an optional
198 * ACPI register that is not supported, so no error message.
200 ACPI_MOVE_64_TO_64 (Address
, &Reg
->Address
);
203 return (AE_BAD_ADDRESS
);
206 /* Validate the SpaceID */
208 if ((Reg
->SpaceId
!= ACPI_ADR_SPACE_SYSTEM_MEMORY
) &&
209 (Reg
->SpaceId
!= ACPI_ADR_SPACE_SYSTEM_IO
))
211 ACPI_ERROR ((AE_INFO
,
212 "Unsupported address space: 0x%X", Reg
->SpaceId
));
216 /* Validate the AccessWidth */
218 if (Reg
->AccessWidth
> 4)
220 ACPI_ERROR ((AE_INFO
,
221 "Unsupported register access width: 0x%X", Reg
->AccessWidth
));
225 /* Validate the BitWidth, convert AccessWidth into number of bits */
227 AccessWidth
= AcpiHwGetAccessBitWidth (*Address
, Reg
, MaxBitWidth
);
228 BitWidth
= ACPI_ROUND_UP (Reg
->BitOffset
+ Reg
->BitWidth
, AccessWidth
);
229 if (MaxBitWidth
< BitWidth
)
231 ACPI_WARNING ((AE_INFO
,
232 "Requested bit width 0x%X is smaller than register bit width 0x%X",
233 MaxBitWidth
, BitWidth
));
241 /******************************************************************************
243 * FUNCTION: AcpiHwRead
245 * PARAMETERS: Value - Where the value is returned
246 * Reg - GAS register structure
250 * DESCRIPTION: Read from either memory or IO space. This is a 64-bit max
251 * version of AcpiRead.
253 * LIMITATIONS: <These limitations also apply to AcpiHwWrite>
254 * SpaceID must be SystemMemory or SystemIO.
256 ******************************************************************************/
261 ACPI_GENERIC_ADDRESS
*Reg
)
273 ACPI_FUNCTION_NAME (HwRead
);
276 /* Validate contents of the GAS register */
278 Status
= AcpiHwValidateRegister (Reg
, 64, &Address
);
279 if (ACPI_FAILURE (Status
))
285 * Initialize entire 64-bit return value to zero, convert AccessWidth
286 * into number of bits based
289 AccessWidth
= AcpiHwGetAccessBitWidth (Address
, Reg
, 64);
290 BitWidth
= Reg
->BitOffset
+ Reg
->BitWidth
;
291 BitOffset
= Reg
->BitOffset
;
294 * Two address spaces supported: Memory or IO. PCI_Config is
295 * not supported here because the GAS structure is insufficient
300 if (BitOffset
>= AccessWidth
)
303 BitOffset
-= AccessWidth
;
307 if (Reg
->SpaceId
== ACPI_ADR_SPACE_SYSTEM_MEMORY
)
309 Status
= AcpiOsReadMemory ((ACPI_PHYSICAL_ADDRESS
)
310 Address
+ Index
* ACPI_DIV_8 (AccessWidth
),
311 &Value64
, AccessWidth
);
313 else /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
315 Status
= AcpiHwReadPort ((ACPI_IO_ADDRESS
)
316 Address
+ Index
* ACPI_DIV_8 (AccessWidth
),
317 &Value32
, AccessWidth
);
318 Value64
= (UINT64
) Value32
;
323 * Use offset style bit writes because "Index * AccessWidth" is
324 * ensured to be less than 64-bits by AcpiHwValidateRegister().
326 ACPI_SET_BITS (Value
, Index
* AccessWidth
,
327 ACPI_MASK_BITS_ABOVE_64 (AccessWidth
), Value64
);
329 BitWidth
-= BitWidth
> AccessWidth
? AccessWidth
: BitWidth
;
333 ACPI_DEBUG_PRINT ((ACPI_DB_IO
,
334 "Read: %8.8X%8.8X width %2d from %8.8X%8.8X (%s)\n",
335 ACPI_FORMAT_UINT64 (*Value
), AccessWidth
,
336 ACPI_FORMAT_UINT64 (Address
), AcpiUtGetRegionName (Reg
->SpaceId
)));
342 /******************************************************************************
344 * FUNCTION: AcpiHwWrite
346 * PARAMETERS: Value - Value to be written
347 * Reg - GAS register structure
351 * DESCRIPTION: Write to either memory or IO space. This is a 64-bit max
352 * version of AcpiWrite.
354 ******************************************************************************/
359 ACPI_GENERIC_ADDRESS
*Reg
)
370 ACPI_FUNCTION_NAME (HwWrite
);
373 /* Validate contents of the GAS register */
375 Status
= AcpiHwValidateRegister (Reg
, 64, &Address
);
376 if (ACPI_FAILURE (Status
))
381 /* Convert AccessWidth into number of bits based */
383 AccessWidth
= AcpiHwGetAccessBitWidth (Address
, Reg
, 64);
384 BitWidth
= Reg
->BitOffset
+ Reg
->BitWidth
;
385 BitOffset
= Reg
->BitOffset
;
388 * Two address spaces supported: Memory or IO. PCI_Config is
389 * not supported here because the GAS structure is insufficient
395 * Use offset style bit reads because "Index * AccessWidth" is
396 * ensured to be less than 64-bits by AcpiHwValidateRegister().
398 Value64
= ACPI_GET_BITS (&Value
, Index
* AccessWidth
,
399 ACPI_MASK_BITS_ABOVE_64 (AccessWidth
));
401 if (BitOffset
>= AccessWidth
)
403 BitOffset
-= AccessWidth
;
407 if (Reg
->SpaceId
== ACPI_ADR_SPACE_SYSTEM_MEMORY
)
409 Status
= AcpiOsWriteMemory ((ACPI_PHYSICAL_ADDRESS
)
410 Address
+ Index
* ACPI_DIV_8 (AccessWidth
),
411 Value64
, AccessWidth
);
413 else /* ACPI_ADR_SPACE_SYSTEM_IO, validated earlier */
415 Status
= AcpiHwWritePort ((ACPI_IO_ADDRESS
)
416 Address
+ Index
* ACPI_DIV_8 (AccessWidth
),
417 (UINT32
) Value64
, AccessWidth
);
422 * Index * AccessWidth is ensured to be less than 32-bits by
423 * AcpiHwValidateRegister().
425 BitWidth
-= BitWidth
> AccessWidth
? AccessWidth
: BitWidth
;
429 ACPI_DEBUG_PRINT ((ACPI_DB_IO
,
430 "Wrote: %8.8X%8.8X width %2d to %8.8X%8.8X (%s)\n",
431 ACPI_FORMAT_UINT64 (Value
), AccessWidth
,
432 ACPI_FORMAT_UINT64 (Address
), AcpiUtGetRegionName (Reg
->SpaceId
)));
438 #if (!ACPI_REDUCED_HARDWARE)
439 /*******************************************************************************
441 * FUNCTION: AcpiHwClearAcpiStatus
447 * DESCRIPTION: Clears all fixed and general purpose status bits
449 ******************************************************************************/
452 AcpiHwClearAcpiStatus (
456 ACPI_CPU_FLAGS LockFlags
= 0;
459 ACPI_FUNCTION_TRACE (HwClearAcpiStatus
);
462 ACPI_DEBUG_PRINT ((ACPI_DB_IO
, "About to write %04X to %8.8X%8.8X\n",
463 ACPI_BITMASK_ALL_FIXED_STATUS
,
464 ACPI_FORMAT_UINT64 (AcpiGbl_XPm1aStatus
.Address
)));
466 LockFlags
= AcpiOsAcquireLock (AcpiGbl_HardwareLock
);
468 /* Clear the fixed events in PM1 A/B */
470 Status
= AcpiHwRegisterWrite (ACPI_REGISTER_PM1_STATUS
,
471 ACPI_BITMASK_ALL_FIXED_STATUS
);
473 AcpiOsReleaseLock (AcpiGbl_HardwareLock
, LockFlags
);
475 if (ACPI_FAILURE (Status
))
480 /* Clear the GPE Bits in all GPE registers in all GPE blocks */
482 Status
= AcpiEvWalkGpeList (AcpiHwClearGpeBlock
, NULL
);
485 return_ACPI_STATUS (Status
);
489 /*******************************************************************************
491 * FUNCTION: AcpiHwGetBitRegisterInfo
493 * PARAMETERS: RegisterId - Index of ACPI Register to access
495 * RETURN: The bitmask to be used when accessing the register
497 * DESCRIPTION: Map RegisterId into a register bitmask.
499 ******************************************************************************/
501 ACPI_BIT_REGISTER_INFO
*
502 AcpiHwGetBitRegisterInfo (
505 ACPI_FUNCTION_ENTRY ();
508 if (RegisterId
> ACPI_BITREG_MAX
)
510 ACPI_ERROR ((AE_INFO
, "Invalid BitRegister ID: 0x%X", RegisterId
));
514 return (&AcpiGbl_BitRegisterInfo
[RegisterId
]);
518 /******************************************************************************
520 * FUNCTION: AcpiHwWritePm1Control
522 * PARAMETERS: Pm1aControl - Value to be written to PM1A control
523 * Pm1bControl - Value to be written to PM1B control
527 * DESCRIPTION: Write the PM1 A/B control registers. These registers are
528 * different than the PM1 A/B status and enable registers
529 * in that different values can be written to the A/B registers.
530 * Most notably, the SLP_TYP bits can be different, as per the
531 * values returned from the _Sx predefined methods.
533 ******************************************************************************/
536 AcpiHwWritePm1Control (
543 ACPI_FUNCTION_TRACE (HwWritePm1Control
);
546 Status
= AcpiHwWrite (Pm1aControl
, &AcpiGbl_FADT
.XPm1aControlBlock
);
547 if (ACPI_FAILURE (Status
))
549 return_ACPI_STATUS (Status
);
552 if (AcpiGbl_FADT
.XPm1bControlBlock
.Address
)
554 Status
= AcpiHwWrite (Pm1bControl
, &AcpiGbl_FADT
.XPm1bControlBlock
);
556 return_ACPI_STATUS (Status
);
560 /******************************************************************************
562 * FUNCTION: AcpiHwRegisterRead
564 * PARAMETERS: RegisterId - ACPI Register ID
565 * ReturnValue - Where the register value is returned
567 * RETURN: Status and the value read.
569 * DESCRIPTION: Read from the specified ACPI register
571 ******************************************************************************/
583 ACPI_FUNCTION_TRACE (HwRegisterRead
);
588 case ACPI_REGISTER_PM1_STATUS
: /* PM1 A/B: 16-bit access each */
590 Status
= AcpiHwReadMultiple (&Value
,
591 &AcpiGbl_XPm1aStatus
,
592 &AcpiGbl_XPm1bStatus
);
595 case ACPI_REGISTER_PM1_ENABLE
: /* PM1 A/B: 16-bit access each */
597 Status
= AcpiHwReadMultiple (&Value
,
598 &AcpiGbl_XPm1aEnable
,
599 &AcpiGbl_XPm1bEnable
);
602 case ACPI_REGISTER_PM1_CONTROL
: /* PM1 A/B: 16-bit access each */
604 Status
= AcpiHwReadMultiple (&Value
,
605 &AcpiGbl_FADT
.XPm1aControlBlock
,
606 &AcpiGbl_FADT
.XPm1bControlBlock
);
609 * Zero the write-only bits. From the ACPI specification, "Hardware
610 * Write-Only Bits": "Upon reads to registers with write-only bits,
611 * software masks out all write-only bits."
613 Value
&= ~ACPI_PM1_CONTROL_WRITEONLY_BITS
;
616 case ACPI_REGISTER_PM2_CONTROL
: /* 8-bit access */
618 Status
= AcpiHwRead (&Value64
, &AcpiGbl_FADT
.XPm2ControlBlock
);
619 if (ACPI_SUCCESS (Status
))
621 Value
= (UINT32
) Value64
;
625 case ACPI_REGISTER_PM_TIMER
: /* 32-bit access */
627 Status
= AcpiHwRead (&Value64
, &AcpiGbl_FADT
.XPmTimerBlock
);
628 if (ACPI_SUCCESS (Status
))
630 Value
= (UINT32
) Value64
;
635 case ACPI_REGISTER_SMI_COMMAND_BLOCK
: /* 8-bit access */
637 Status
= AcpiHwReadPort (AcpiGbl_FADT
.SmiCommand
, &Value
, 8);
642 ACPI_ERROR ((AE_INFO
, "Unknown Register ID: 0x%X",
644 Status
= AE_BAD_PARAMETER
;
648 if (ACPI_SUCCESS (Status
))
650 *ReturnValue
= (UINT32
) Value
;
653 return_ACPI_STATUS (Status
);
657 /******************************************************************************
659 * FUNCTION: AcpiHwRegisterWrite
661 * PARAMETERS: RegisterId - ACPI Register ID
662 * Value - The value to write
666 * DESCRIPTION: Write to the specified ACPI register
668 * NOTE: In accordance with the ACPI specification, this function automatically
669 * preserves the value of the following bits, meaning that these bits cannot be
670 * changed via this interface:
672 * PM1_CONTROL[0] = SCI_EN
677 * 1) Hardware Ignored Bits: When software writes to a register with ignored
678 * bit fields, it preserves the ignored bit fields
679 * 2) SCI_EN: OSPM always preserves this bit position
681 ******************************************************************************/
684 AcpiHwRegisterWrite (
693 ACPI_FUNCTION_TRACE (HwRegisterWrite
);
698 case ACPI_REGISTER_PM1_STATUS
: /* PM1 A/B: 16-bit access each */
700 * Handle the "ignored" bit in PM1 Status. According to the ACPI
701 * specification, ignored bits are to be preserved when writing.
702 * Normally, this would mean a read/modify/write sequence. However,
703 * preserving a bit in the status register is different. Writing a
704 * one clears the status, and writing a zero preserves the status.
705 * Therefore, we must always write zero to the ignored bit.
707 * This behavior is clarified in the ACPI 4.0 specification.
709 Value
&= ~ACPI_PM1_STATUS_PRESERVED_BITS
;
711 Status
= AcpiHwWriteMultiple (Value
,
712 &AcpiGbl_XPm1aStatus
,
713 &AcpiGbl_XPm1bStatus
);
716 case ACPI_REGISTER_PM1_ENABLE
: /* PM1 A/B: 16-bit access each */
718 Status
= AcpiHwWriteMultiple (Value
,
719 &AcpiGbl_XPm1aEnable
,
720 &AcpiGbl_XPm1bEnable
);
723 case ACPI_REGISTER_PM1_CONTROL
: /* PM1 A/B: 16-bit access each */
725 * Perform a read first to preserve certain bits (per ACPI spec)
726 * Note: This includes SCI_EN, we never want to change this bit
728 Status
= AcpiHwReadMultiple (&ReadValue
,
729 &AcpiGbl_FADT
.XPm1aControlBlock
,
730 &AcpiGbl_FADT
.XPm1bControlBlock
);
731 if (ACPI_FAILURE (Status
))
736 /* Insert the bits to be preserved */
738 ACPI_INSERT_BITS (Value
, ACPI_PM1_CONTROL_PRESERVED_BITS
, ReadValue
);
740 /* Now we can write the data */
742 Status
= AcpiHwWriteMultiple (Value
,
743 &AcpiGbl_FADT
.XPm1aControlBlock
,
744 &AcpiGbl_FADT
.XPm1bControlBlock
);
747 case ACPI_REGISTER_PM2_CONTROL
: /* 8-bit access */
749 * For control registers, all reserved bits must be preserved,
750 * as per the ACPI spec.
752 Status
= AcpiHwRead (&ReadValue64
, &AcpiGbl_FADT
.XPm2ControlBlock
);
753 if (ACPI_FAILURE (Status
))
757 ReadValue
= (UINT32
) ReadValue64
;
759 /* Insert the bits to be preserved */
761 ACPI_INSERT_BITS (Value
, ACPI_PM2_CONTROL_PRESERVED_BITS
, ReadValue
);
763 Status
= AcpiHwWrite (Value
, &AcpiGbl_FADT
.XPm2ControlBlock
);
766 case ACPI_REGISTER_PM_TIMER
: /* 32-bit access */
768 Status
= AcpiHwWrite (Value
, &AcpiGbl_FADT
.XPmTimerBlock
);
771 case ACPI_REGISTER_SMI_COMMAND_BLOCK
: /* 8-bit access */
773 /* SMI_CMD is currently always in IO space */
775 Status
= AcpiHwWritePort (AcpiGbl_FADT
.SmiCommand
, Value
, 8);
780 ACPI_ERROR ((AE_INFO
, "Unknown Register ID: 0x%X",
782 Status
= AE_BAD_PARAMETER
;
787 return_ACPI_STATUS (Status
);
791 /******************************************************************************
793 * FUNCTION: AcpiHwReadMultiple
795 * PARAMETERS: Value - Where the register value is returned
796 * RegisterA - First ACPI register (required)
797 * RegisterB - Second ACPI register (optional)
801 * DESCRIPTION: Read from the specified two-part ACPI register (such as PM1 A/B)
803 ******************************************************************************/
808 ACPI_GENERIC_ADDRESS
*RegisterA
,
809 ACPI_GENERIC_ADDRESS
*RegisterB
)
817 /* The first register is always required */
819 Status
= AcpiHwRead (&Value64
, RegisterA
);
820 if (ACPI_FAILURE (Status
))
824 ValueA
= (UINT32
) Value64
;
826 /* Second register is optional */
828 if (RegisterB
->Address
)
830 Status
= AcpiHwRead (&Value64
, RegisterB
);
831 if (ACPI_FAILURE (Status
))
835 ValueB
= (UINT32
) Value64
;
839 * OR the two return values together. No shifting or masking is necessary,
840 * because of how the PM1 registers are defined in the ACPI specification:
842 * "Although the bits can be split between the two register blocks (each
843 * register block has a unique pointer within the FADT), the bit positions
844 * are maintained. The register block with unimplemented bits (that is,
845 * those implemented in the other register block) always returns zeros,
846 * and writes have no side effects"
848 *Value
= (ValueA
| ValueB
);
853 /******************************************************************************
855 * FUNCTION: AcpiHwWriteMultiple
857 * PARAMETERS: Value - The value to write
858 * RegisterA - First ACPI register (required)
859 * RegisterB - Second ACPI register (optional)
863 * DESCRIPTION: Write to the specified two-part ACPI register (such as PM1 A/B)
865 ******************************************************************************/
868 AcpiHwWriteMultiple (
870 ACPI_GENERIC_ADDRESS
*RegisterA
,
871 ACPI_GENERIC_ADDRESS
*RegisterB
)
876 /* The first register is always required */
878 Status
= AcpiHwWrite (Value
, RegisterA
);
879 if (ACPI_FAILURE (Status
))
885 * Second register is optional
887 * No bit shifting or clearing is necessary, because of how the PM1
888 * registers are defined in the ACPI specification:
890 * "Although the bits can be split between the two register blocks (each
891 * register block has a unique pointer within the FADT), the bit positions
892 * are maintained. The register block with unimplemented bits (that is,
893 * those implemented in the other register block) always returns zeros,
894 * and writes have no side effects"
896 if (RegisterB
->Address
)
898 Status
= AcpiHwWrite (Value
, RegisterB
);
904 #endif /* !ACPI_REDUCED_HARDWARE */