Motorola Mobility, Inc. v. Microsoft Corporation

Filing 125

NOTICE by Microsoft Corporation re #124 Defendant's MOTION Claim Construction of Filing Exhibits (Attachments: #1 Appendix (Index to Exhibits), #2 Exhibit 101, #3 Exhibit 102, #4 Exhibit 201, #5 Exhibit 301, #6 Exhibit 302, #7 Exhibit 304, #8 Exhibit 305, #9 Exhibit 501, #10 Exhibit 502, #11 Exhibit 504, #12 Exhibit 601, #13 Exhibit 801 (Part 1), #14 Exhibit 801 (Part 2), #15 Exhibit 901, #16 Exhibit 902, #17 Exhibit 903, #18 Exhibit 904, #19 Exhibit 905, #20 Exhibit 1101, #21 Exhibit 1102, #22 Exhibit 1105, #23 Exhibit 1110, #24 Exhibit 1114, #25 Exhibit 1115, #26 Exhibit 1116, #27 Exhibit 1201, #28 Exhibit 1301, #29 Exhibit 1302, #30 Exhibit 1303, #31 Exhibit 1304, #32 Exhibit 1305, #33 Exhibit 1401, #34 Exhibit 1402, #35 Exhibit 1403, #36 Exhibit 1404, #37 Exhibit 1405, #38 Exhibit 1406, #39 Exhibit 1407, #40 Exhibit 1408, #41 Exhibit 1409)(Miner, Curtis)

Exhibit 503

IA-PC HPET (High Precision Event Timers)
Specification
Revision:
Date:

1.0a
October 2004

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
1

LEGAL DISCLAIMER
THIS SPECIFICATION IS PROVIDED "AS IS" WITH NO WARRANTIES
WHATSOEVER, INCLUDING ANY WARRANTY OF MERCHANTABILITY, NONINFRINGEMENT, FITNESS FOR ANY PARTICULAR PURPOSE, OR ANY WARRANTY
OTHERWISE ARISING OUT OF ANY PROPOSAL, SPECIFICATION OR SAMPLE.
Intel disclaims all liability, including liability for infringement of any proprietary rights,
relating to use of information in this specification. No license, express or implied, by
estoppels or otherwise, to any intellectual property rights is granted herein.
This specification is a preliminary draft provided for comment and informational purposes
only, and is subject to change without any notice, obligation or liability. Readers should
not rely on this specification in any way for product design purposes.
It is Intel's intent to provide a Version 1.0 of this specification, that will be made available
subject to an appropriate license agreement. However, Intel is under no obligation or
liability to do so.

The contents of the areas marked as "reserved" in this specification will not be
licensed under the Intel license for this specification.

IA-PC HPET Specification
Copyright © 1999-2004 Intel Corporation
All rights reserved.

*THIRD-PARTY BRANDS AND NAMES MAY BE CLAIMED AS THE PROPERTY OF OTHERS.

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
2

Table of Contents
1.

IA-PC HPET........................................................................................................................................... 4
1.1
Revision History:............................................................................................................................ 4
1.2
Scope .............................................................................................................................................. 5
1.3
Terminology ................................................................................................................................... 6
2. Hardware Overview................................................................................................................................ 7
2.1
Register Model Overview............................................................................................................... 8
2.1.1
Memory Map .......................................................................................................................... 8
2.2
Minimum Recommended Hardware Implementation .................................................................... 9
2.3
Register Definitions...................................................................................................................... 10
2.3.1
Register Overview ................................................................................................................ 10
2.3.2
Programming Requirements ................................................................................................. 10
2.3.3
Power Management Considerations ..................................................................................... 10
2.3.4
General Capabilities and ID Register ................................................................................... 11
General Capability and ID Register Addressing................................................................................... 11
2.3.5
General Configuration Register ............................................................................................ 12
2.3.6
General Interrupt Status Register.......................................................................................... 14
2.3.7
Main Counter Register.......................................................................................................... 15
2.3.8
Timer N Configuration and Capabilities Register ................................................................ 16
2.3.9
Timer N Comparator Register .............................................................................................. 19
2.3.9.1 Register Definition and Usage Model .............................................................................. 20
2.3.9.2 Periodic vs. Non-Periodic Modes ..................................................................................... 21
2.3.9.2.1 Non-Periodic Mode.................................................................................................... 21
2.3.9.2.2 Periodic Mode............................................................................................................ 21
2.3.9.2.3 Read/Write Paths for Periodic Mode Vs One-Shot Mode ......................................... 22
2.3.10 Timer N FSB Interrupt Route Register................................................................................. 23
2.4
Theory Of Operation .................................................................................................................... 23
2.4.1
Timer Accuracy Rules .......................................................................................................... 23
2.4.2
Interrupt Mapping................................................................................................................. 24
2.4.2.1 Mapping Option #1: LegacyReplacement Option ........................................................... 24
2.4.2.2 Mapping Option #2: Standard Option ............................................................................. 24
2.4.2.3 Mapping Option #3: FSB Option..................................................................................... 24
2.4.3
Periodic vs. Non-Periodic Modes ......................................................................................... 24
2.4.3.1 Non-Periodic Mode .......................................................................................................... 24
2.4.3.2 Periodic Mode................................................................................................................... 25
2.4.4
Enabling the Timers.............................................................................................................. 25
2.4.5
Interrupt Levels..................................................................................................................... 25
2.4.6
Handling Interrupts............................................................................................................... 26
2.4.7
Issues related to 64-bit Timers with 32-bit CPUs ................................................................. 26
3. Enumeration & Configuration of HPET............................................................................................... 27
3.1
Initial State of Event Timer Hardware.......................................................................................... 27
3.2
BIOS Initialization........................................................................................................................ 27
3.2.1
Assign memory to Timer Block(s) ....................................................................................... 27
3.2.2
HPET Block Interrupt Routing ............................................................................................. 27
3.2.2.1 Routing Interrupts for HPET Blocks that do not support 8254/RTC IRQ Routing .......... 27
3.2.2.2 Routing Interrupts for HPET Blocks that support 8254/RTC IRQs ................................. 28
3.2.3
Considerations for Platforms without Legacy Timers .......................................................... 29
3.2.4
Create ACPI 2.0 HPET Description Table (HPET).............................................................. 30
3.2.5
Describe Event Timer(s) in ACPI Name space .................................................................... 32
3.2.5.1 ACPI Name Space Example ............................................................................................. 32
3.2.6
Recommendations for OS Initialization code ....................................................................... 32

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IA-PC HPET Specification
Rev 1.0a
3

1. IA-PC HPET
1.1 Revision History:
Version

Comments

0.97

Last Updated: 03/07/2000

0.97a

Last Updated: 05/18/2000

0.98

01/20/2002

•
•

Incorporated technical editing changes, released for external feedback.
Incorporated various non-technical and legal feedbacks.

•

Technologic term changed: from Legacy Mode to LegacyReplacement Mode for clarity purpose

•

ETDT ACPI table changed: ETDT (Event Timer Descriptor Table) is changed to HPET table and its content of the table
has been updated.

•
0.98a

Product name changed: from Multimedia Timer to HPET (High Precision Event Timer)

•

IA64 platform support: Use GAS(Generic Address Structure) format in HPET table and up to 64KB timer block.

08/31/2001
•

1.0

Modified the accuracy of clock frequency drift to 0.05%

•

Add “write lock” note to the programming requirement

6/8/2004
•

Removed color-code for read-only fields

•
1.0a

Added programming notes for 64-bit register access in a 64-bit platform

•

Explicitly mark “Reserved” in reserved fields of FSB Registers

6/8/2004
•

Clarifications to 64 bit accesses.

•

General cleanup

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
4

1.2 Scope
This specification provides register model and programming interface definitions for new event timer
hardware for use on Intel Architecture-based Personal Computers. In this specification, the terms ‘IA-PC
HPET and ‘Event Timers’ refer to the same timer hardware.
The IA-PC HPET Specification defines timer hardware that is intended to initially supplement and
eventually replace the legacy 8254 Programmable Interval Timer and the Real Time Clock Periodic
Interrupt generation functions that are currently used as the ‘de-facto’ timer hardware for IA-PCs.
This new timer hardware can be used by system software for:
•

Synchronizing
o Real-Time Digital Audio & Video Streams
64-bit free running up-counter

•

Scheduling
o Threads, Tasks, Processes, etc.
Fixed Rate (Periodic) Interrupt Generation
• System Heart Beat
• Non-Real Time Thread Scheduler
Variable Rate (One-Shot) Interrupt Generation
• Scheduling real time tasks associated with host-based signal processing
applications
Time Stamping
o On Multiprocessor platforms
64-Bit free running up-counter can be utilized as DIG64 “platform timer” for
Time Stamping Applications. This provides a time-base that is insensitive to
clock frequency drifts on individual CPU’s on a N-Way MP systems.

•

Note:
The name of the timer block has been changed from Multimedia Timer to HPET (High Precision
Event Timer). However, before the new name was adopted, many related documents continue to use
or reference the term of “Multimedia Timer”. Therefore, for the purposes of designing products to
this specification, the terms HPET, Multimedia Timer, MMT and MM Timer should be treated as
the same timer hardware.

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
5

1.3 Terminology
Term
IA
PC
IA-PC
PIT
RTC
SCI
FSB
MM
•
Timer
•
Event Timer
•
HPET
•
MM Timer
•
MMT
Timer Block
32-Bit Timer
64-Bit Timer

Definition for this document
Intel Architecture
Personal Computer
Intel Architecture-based PC
8254 Programmable Interval Timer
Real Time Clock
System Configuration Interrupt
Front Side Bus
Multimedia
The terms Timer, Event Timer, HPET, MMT and MM Timer refer to the combination of
a Counter, Comparator, and Match Register. The Comparator compares the contents of
the Match Register against the value of a free running up-counter. When the output of the
up-counter equals the value in the match register an interrupt is generated. The IA-PC
HPET Architecture allows up to 32 compare/match registers per counter. Each of the 32
comparators can output an interrupt.
Each Timer Block consists of a single counter that feeds up to 32 comparators. Each
Timer Block in the system can have different clocking attributes.
Comparator Register is 32 bits wide. Main Counter can be 32 or 64 bits wide for a ’32 bit
Timer’.
Comparator Register is 64 bits wide. Main Counter must be 64 bits wide for a ’64 bit
Timer’.

Note: For better legibility, the fields of HPET internal registers are color-coded as following:
R

Purple background indicates reserved fields

<b1:b2>: Represents bit filed from bit b1 to bit b2
<b1>: Represents a single bit field

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
6

2. Hardware Overview
The IA-PC HPET Architecture defines a set of timers that can be used by the operating system. The timers
are defined such that in the future, the OS may be able to assign specific timers to be used directly by
specific applications. Each timer can be configured to generate a separate interrupt. This specification
allows for a block of 32 timers, with support for up to 8 blocks, for a total of 256 timers. However, specific
implementations can include only a subset of these timers.
The timers are implemented as a single up-counter with a set of comparators. The counter increases
monotonically. When software does two consecutive reads of the counter, the second read will never return
a value that is less than the first read unless the counter has actually rolled over. Each timer includes a
match register and a comparator. Each individual timer can generate an interrupt when the value in its
match register equals the value of the free-running counter. Some of the timers can be enabled to generate
a periodic interrupt.
The registers associated with these timers are mapped to memory space (much like the I/O APIC).
However, it is not implemented as a standard PCI function. The BIOS reports to the operating system the
location of the memory-mapped register space consumed by the timers. The hardware can support relocatable address decode space, however the BIOS will set this space prior to handing it over to the OS. It
is not expected that the OS will move the location of these timers once it is set by the BIOS.

Clock
Gen

64/32 Bit
Up Counter

Current_Count
64/32 Bit
Compare
C0

32 Bit
Compare
=

C1

32 Bit
Compare
=

C2

=

Data Bus
32 Bit
Adder
IRQ En &
Status Reg
8254

RTC

FSB
INTI[X:Y]

Interrupt Routing Logic
INTI_8
INTI_2

Sticky Bits

I
O
A
P
I
C

IOAPIC

C
P
U

Enable Bits
IRQ8

Config/Enable/Status Regs
Counter Register

> All registers are memory-mapped
> Multimedia Timer Subsystem is exposed to OS
using ACPI Tables

IRQ0

8
2
5
9

Legacy

Compare Registers
Period Register

Figure 1 Hardware Block Diagram

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
7

2.1 Register Model Overview
2.1.1 Memory Map
The Event Timer registers are memory mapped in a non-indexed scheme. This allows the CPU to directly
access each register without having to use an index register. The timer register space is 1024 bytes. The
registers are generally aligned on 64-bit boundaries to simplify implementation with IA64 processors. For
IA64 platform, the timer register space can be up to 64K bytes with page protection capability. The register
model allows each timer block to contain up to 32 timers, where each ‘timer’ consists of a comparator plus
a match register.

Register Model
& Programming Interface

63

0
General Capabilities ID
General Configuration
General IRQ Status

Memory
Mapped
IO
Interface

‘Main’ Counter ( 32 or 64bit)

T0_Config_&_ Caps

> BIOS selects 1 of N
location in system memory

BIOS BAR
Timer
Registers
(1K)

T0_Comparator
T0_FSB_IRQ_Routing

> BIOS reports Base Address
of Timer Reg Block via
ACPI Tables and ASL Code
> Base Address can be
4K aligned

Memory-Mapped
64-bit Aligned
Extensible
Allows up to 32 Timers to be supported

Capabilities Registers are Read Only
Configuration Registers are R/W

T31_Config_&_ Caps
T31_Comparator
T31_FSB_IRQ_Routing

Figure 2 Register Model Overview

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
8

2.2 Minimum Recommended Hardware Implementation
Item

Recommendation

Main Counter

Required to be an upcounter

Main Counter Width

64-bits

Clock Frequency

Fmin = 10 MHz

Clock Frequency Drift

+- .05 % (500 ppm )
+- .2 % (2000 ppm)

Number of Comparators

3

Width of Comparators

32 bits (Minimum)

Number of Periodic
Capable Timers

1 of 3

Width of Adder on
Periodic Capable
Timers

32 bits (Minimum)

Number of One-shot
Capable Timers

All 3

Interrupt Delivery via
8259

Optional

Interrupt Delivery via
IOxAPIC

Required

Interrupt Delivery via
CPU FSB

Comments

Optional

Over any interval >= 1 Millisecond
Over any interval <= 100 Microseconds

If 64 Bits, must have 32-bit mode for IA32
platforms

LegacyReplacement IRQ Routing required
for systems that intend to replace/supplement
8254/RTC legacy timers with this new timer
architecture.
LegacyReplacement IRQ Routing required
for systems that intend to replace/supplement
8254/RTC legacy timers with this new timer
architecture.

Table 1 Minimum Recommended Hardware Implementation

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
9

2.3 Register Definitions
2.3.1 Register Overview
Offset
000-007h
008-00Fh
010-017h
018-01Fh
020-027h
028-0EFh
0F0-0F7h
0F8-0FFh
100-107h
108-10Fh
110-117h
118-11Fh
120-127h
128-12Fh
130-137h
138-13Fh
140-147h
148-14Fh
150-157h
158-15Fh
160-3FFh

Register
General Capabilities and ID Register
Reserved
General Configuration Register
Reserved
General Interrupt Status Register
Reserved
Main Counter Value Register
Reserved
Timer 0 Configuration and Capability Register
Timer 0 Comparator Value Register
Timer 0 FSB Interrupt Route Register
Reserved
Timer 1 Configuration and Capability Register
Timer 1 Comparator Value Register
Timer 1 FSB Interrupt Route Register
Reserved
Timer 2 Configuration and Capability Register
Timer 2 Comparator Value Register
Timer 2 FSB Interrupt Route Register
Reserved
Reserved for Timers 3-31

Type
Read Only
Read-Write
Read/Write Clear
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write

Table 2 Memory-Mapped Registers
2.3.2 Programming Requirements
1.

2.
3.
4.

Software must not attempt to read or write across register boundaries. For example, a 32-bit access
must be to offset 00h, 04h, 08h, or 0Ch. 32-bit accesses must not be to 01h, 02h, 03h, 05h, 06h, 07h,
09h, 0Ah, 0Bh, 0Dh, 0Eh, or 0Fh. 64-bit accesses can only be to 00h or 08h and must not cross 64-bit
boundaries.
Software should not write to read-only registers.
Software should not expect any particular or consistent value when reading reserved registers or bits.
Software should perform read-modify-write operations on reserved bits.

Note:
Host controllers are not required to support exclusive-access mechanisms (such as PCI
LOCK) for accesses to the memory-mapped register space. Therefore, if software attempts
exclusive-access mechanisms to the host controller memory-mapped register space, the
results are undefined.
2.3.3 Power Management Considerations
It is the Operating System’s responsibility to save and restore Event Timer hardware context if this needs to
be preserved through ACPI System Sleep State transitions.
General behavioral rules for Event Timer hardware regarding sleep state transitions:
1.

The Event Timer registers (including the main counter) are not expected to be preserved through an
S3, S4, or S5 state.

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
10

2.
3.

The features and functions associated with these registers are not expected to be used in an S1 state.
Prior to going to an S1 state, all interrupts associated with this function should be disabled.
The main counter is permitted, but not required, to run during S1 or S2 states. This allows mobile
systems to stop clock generators feeding the main counter during S1 or S2 states.

2.3.4 General Capabilities and ID Register

63:32

31:16

15 14 13

12:8

7:0

R
REV_ID
NUM_TIM_CAP
COUNT_SIZE_CAP
Reserved
LEG_ROUTE_CAP
VENDOR_ID
COUNTER_CLK_PERIOD
Offset: 00h

Reserved

R

Figure 3 General Capability and ID Register
General Capability and ID Register Addressing
Offset
Attribute
Size
General Behavioral
Rules

0x00
Read-Only
64-bits
1.

Writes to this register should not be attempted by software.

2.

Software can read the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 00h or 04h, but not to offsets 01h, 02h, 03h, 05h, 06h, or 07h. 64bit accesses can only be done to 00h.

General Capabilities and ID Register Bit Definitions
Bit
63:32

COUNTER_CLK_PERIOD

31:16

VENDOR_ID

15

LEG_RT_CAP

14

Reserved:

13

COUNT_SIZE_CAP

Description
Main Counter Tick Period: This read-only field indicates the period at which
the counter increments in femptoseconds (10^-15 seconds). A value of 0 in this
field is not permitted. The value in this field must be less than or equal to
05F5E100h (10^8 femptoseconds = 100 nanoseconds). The resolution must be in
femptoseconds (rather than picoseconds) in order to achieve a resolution of 50
ppm.
This read-only field will be the same as what would be assigned if this logic was a
PCI function.
LegacyReplacement Route Capable: If this bit is a 1, it indicates that the
hardware supports the LegacyReplacement Interrupt Route option.
In order to preserve usage of these bits in the future, software should always write
a 0 to these bits until they are defined.
Counter Size:
•
This bit is a 0 to indicate that the main counter is 32 bits wide (and
cannot operate in 64-bit mode).
•
This bit is a 1 to indicate that the main counter is 64 bits wide (although

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
11

12:8

7:0

this does not preclude it from being operated in a 32-bit mode).
Number of Timers: This indicates the number of timers in this block. The
number in this field indicates the last timer (i.e. if there are three timers, the value
will be 02h, four timers will be 03h, five timers will be 04h, etc.).
This indicates which revision of the function is implemented. The value must
NOT be 00h.

NUM_TIM_CAP

REV_ID

2.3.5 General Configuration Register

63:16

15:8

R

R

7:2

1

0

R

ENABLE_CNF
LEG_RT_CNF
Reserved
Reserved for Manufacturer
Reserved

Offset: 00h

R

Reserved

Figure 4 General Configuration Register
General Configuration Register Addressing
Offset
Attribute
Size
General Behavioral
Rules

0x10
Read-Write
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 010h or 014h, but not to offsets 011h, 012h, 013h, 015h, 016h, or
017h. 64-bit accesses can only be done to 010h.

General Configuration Register Bit Definitions
Bit
63:16

Reserved

15:8

Reserved for NonOS

7:2

Reserved:

1

LEG_RT_CNF

Description
In order to preserve usage of these bits in the future, software should not modify the value in
these bits until they are defined. This is done by doing a “read-modify-write” to this
register.
These bits are reserved for the manufacturer. Future revisions of this spec will not use these
bits. OS-based drivers must not modify the value in these bits. This is done by doing a
“read-modify-write” to this register.
In order to preserve usage of these bits in the future, software should not modify the value in
these bits until they are defined. This is done by doing a “read-modify-write” to this
register.
LegacyReplacement Route:
•
0 – Doesn’t support LegacyReplacement Route
•
1 – Supports LegacyReplacement Route
If the ENABLE_CNF bit and the LEG_RT_CNF bit are both set, then the interrupts will be
routed as follows:

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
12

Timer 0 will be routed to IRQ0 in Non-APIC or IRQ2 in the I/O APIC
Timer 1 will be routed to IRQ8 in Non-APIC or IRQ8 in the I/O APIC
Timer 2-n will be routed as per the routing in the timer n config registers.
If the LegacyReplacement Route bit is set, the individual routing bits for timers 0 and 1
(APIC or FSB) will have no impact.

0

ENABLE_CNF

If the LegacyReplacement Route bit is not set, the individual routing bits for each of the
timers are used.
Overall Enable: This bit must be set to enable any of the timers to generate interrupts. If
this bit is 0, then the main counter will halt (will not increment) and no interrupts will be
caused by any of these timers.
•
0 – Halt main count and disable all timer interrupts
•
1 – allow main counter to run, and allow timer interrupts if enabled

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
13

2.3.6 General Interrupt Status Register

63:32

31:3

R

2

1

0

R

T0_INT_STS
T1_INT_STS
T2_INT_STS
Reserved
Reserved

Offset: 20h

R

Reserved

Figure 5 General Interrupt Status Register
General Interrupt Status Register Addressing
Offset
Attribute
Size
General Behavioral
Rules

020h
Read-Write Clear
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 20h or 24h, but not to offsets 21h, 22h, 23h, 25h, 26h, or 27h. 64bit accesses can only be done to offset 20h.

General Interrupt Status Register Field Definitions
Bit
63:32

Field Name
Reserved

31:3
2
1
0

Tn_INT_STS
T2_INT_STS
T1_INT_STS
T0_INT_STS

Description
In order to preserve usage of these bits in the future, software should always write a 0 to
these bits until they are defined.
Timer n Interrupt Active (where xx is 31:3): Same functionality as for Timer 0
Timer 2 Interrupt Active: Same functionality as Timer 0.
Timer 1 Interrupt Active: Same functionality as Timer 0.
Timer 0 Interrupt Active: The functionality of this bit depends on whether the edge or
level-triggered mode is used for this timer:
If set to level-triggered mode:
This bit defaults to 0. This bit will be set by hardware if the corresponding timer
interrupt is active. Once the bit is set, it can be cleared by software writing a 1
to the same bit position. Writes of 0 to this bit will have no effect. For example,
if the bit is already set a write of 0 will not clear the bit.
If set to edge-triggered mode:
This bit should be ignored by software. Software should always write 0 to this bit.

Note: Software uses Tn_INT_TYPE_CNF bit (bit <1> of Timer N Configuration and Capability Register)
to select Level vs Edge operation.

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
14

2.3.7 Main Counter Register

63

0

Counter value

Offset: 0F0h

Figure 6 Main Counter Register
Main Counter Register Addressing
Offset
Attribute
Size
General Behavioral
Rules

0F0h
Read/Write
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 0F0h or 0F4h. 64-bit accesses can be done to 0F0h. 32-bit
accesses must not be done starting at: 0F1h, 0F2h, 0F3h, 0F5h, 0F6h, or 0F7h.

2.

Writes to this register should only be done while the counter is halted.

3.

Reads to this register return the current value of the main counter.

4.

32-bit counters will always return 0 for the upper 32-bits of this register.

5.

If 32-bit software attempts to read a 64-bit counter, it must be aware of timer rollover. See
section 2.4.7 for details on safely accessing 64-bit counters using software or hardware which
only supports 32-bit accesses. It is strongly recommended that 32-bit software only operate the
timer in 32-bit mode.

6.

Reads to this register are monotonic. No two consecutive reads will return the same value,
except when the reads happen at less than the resolution of the counter or if the counter has
rolled over. (See section 2.4.1)

Main Counter Register Field Definitions
Bit
63:0

Field Name
MAIN_COUNTER_VAL

Description
Bits 63:0 of the counter.

______________________________________________________________________________________
IA-PC HPET Specification
Rev 1.0a
15

2.3.8 Timer N Configuration and Capabilities Register
63:32

31:16

15 14

13:9

R

8

7

6

5

4

3

2

R

1

0
R

Reserved
TN_INT_TYPE_CNF
TN_INT_ENB_CNF
TN_TYPE_CNF
TN_PER_INT_CAP
TN_SIZE_CAP
TN_VAL_SET_CNF
Reserved
TN_32MODE_CNF
TN_INT_ROUTE_CNF
TN_FSB_EN_CNF
TN_FSB_INT_DEL_CAP
Reserved
TN_INT_ROUTE_CAP

Offset: 0100h
(20h*N) + 100h

R

Reserved

Figure 7 Timer N Configuration and Capability Register

T0_FSB_EN
LEG_RT_EN

Main
Counter

FSB
IRQ
Router

FSB

APIC
IRQ
Router

Compare

Overall Enable
ENABLE_CNF

IOAPIC

=
Timer 0
Comparator
Register
T0_INT_ENB

M
U
X

C
P
U

INTI#2
IOAPIC

8254
PIC
IRQ0

Example of Timer 0 Configuration and Capacities Diagram

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Timer N Configuration and Capability Register Addressing
Offset
Attribute
Size
General Behavioral
Rules

Timer 0: 100h – 107h, Timer 1: 120h – 127h, Timer 2: 140h – 147h
Timer n: (20h * n) + 100h through (20h * n) + 107h
Read-Write
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 1x0h or 1x4h. 64-bit accesses can be done to offset 1x0h. 32-bit
accesses must not be done to offsets 1x1h, 1x2h, 1x3h, 1x5h, 1x6h, or 1x7h.

2.

Reads or writes to unimplemented timers should not be attempted.

Timer N Configuration and Capability Register Field Definitions
Bit
64:32

31:16

Field Name
Tn_INT_ROUTE_CAP

Reserved

15

Tn_FSB_INT_
DEL_CAP

14

Tn_FSB_EN_CNF

13:9

Tn_INT_ROUTE_CNF

Description
Timer n Interrupt Routing Capability: (where n is the timer number: 00 to
31) This 32-bit read-only field indicates to which interrupts in the I/O (x) APIC
this timer’s interrupt can be routed. This is used in conjunction with the
Tn_INT_ROUTE_CNF field.
Each bit in this field corresponds to a particular interrupt. For example, if this
timer’s interrupt can be mapped to interrupts 16, 18, 20, 22, or 24, then bits 16,
18, 20, 22, and 24 in this field will be set to 1. All other bits will be 0.
In order to preserve usage of these bits in the future, software should always
write a 0 to these bits until they are defined.
Timer n FSB Interrupt Delivery: (where n is the timer number: 00 to 31). If
this read-only bit is 1, then the hardware supports a direct front-side bus delivery
of this timer’s interrupt.
Timer n FSB Interrupt Enable: (where n is the timer number: 00 to 31). If the
Tn_FSB_INT_DEL_CAP bit is set for this timer, then the software can set the
Tn_FSB_EN_CNF bit to force the interrupts to be delivered directly as FSB
messages, rather than using the I/O (x) APIC. In this case, the
Tn_INT_ROUTE_CNF field in this register will be ignored. The
Tn_FSB_ROUTE register will be used instead.
Timer n Interrupt Route: (where n is the timer number: 00 to 31). This 5-bit
read/write field indicates the routing for the interrupt to the I/O APIC. A
maximum value of 32 interrupts are supported. Default is 00h Software writes
to this field to select which interrupt in the I/O (x) will be used for this timer’s
interrupt. If the value is not supported by this prarticular timer, then the value
read back will not match what is written. The software must only write valid
values.
Note: If the LegacyReplacement Route bit is set, then Timers 0 and 1 will have
a different routing, and this bit field has no effect for those two timers.

8

Tn_32MODE_CNF

7

Reserved

6

Tn_VAL_SET_CNF

Note: If the Tn_FSB_INT_DEL_CNF bit is set, then the interrupt will be
delivered directly to the FSB, and this bit field has no effect.
Timer n 32-bit Mode: (where n is the timer number: 00 to 31). Software can
set this read/write bit to force a 64-bit timer to behave as a 32-bit timer. This is
typically needed if the software is not willing to halt the main counter to read or
write a particular timer, and the software is not capable of doing an atomic 64bit read to the timer. If the timer is not 64 bits wide, then this bit will always be
read as 0 and writes will have no effect.
In order to preserve usage of these bits in the future, software should always
write a 0 to these bits until they are defined.
Timer n Value Set: (where n is the timer number: 00 to 31). Software uses this
read/write bit only for timers that have been set to periodic mode. By writing
this bit to a 1, the software is then allowed to directly set a periodic timer’s
accumulator.
Software does NOT have to write this bit back to 0 (it automatically clears).

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5

Tn_SIZE_CAP

4

Tn_PER_INT_CAP

3

Tn_TYPE_CNF

Software should not write a 1 to this bit position if the timer is set to nonperiodic mode.
Timer n Size: (where n is the timer number: 00 to 31). This read-only field
indicates the size of the timer. 1 = 64-bits, 0 = 32-bits.
Timer n Periodic Interrupt Capable: (where n is the timer number: 00 to
31). If this read-only bit is 1, then the hardware supports a periodic mode for
this timer’s interrupt.
Timer n Type: (where n is the timer number: 00 to 31).
If the corresponding Tn_PER_INT_CAP bit is 0, then this bit will always
return 0 when read and writes will have no impact.

2

Tn_INT_ENB_CNF

1

Tn_INT_TYPE_CNF

0

Reserved

If the corresponding Tn_PER_INT_CAP bit is 1, then this bit is read/write, and
can be used to enable the timer to generate a periodic interrupt.
•
Writing a 1 to this bit enables the timer to generate a periodic
interrupt.
•
Writing a 0 to this bit enables the timer to generate a non-periodic
interrupt.
Timer n Interrupt Enable: (where n is the timer number: 00 to 31). This
read/write bit must be set to enable timer n to cause an interrupt when the timer
event fires.
Note: If this bit is 0, the timer will still operate and generate appropriate status
bits, but will not cause an interrupt.
Timer n Interrupt Type: (where n is the timer number: 00 to 31)
•
0 = The timer interrupt is edge triggered. This means that an edgetype interrupt is generated. If another interrupt occurs, another edge
will be generated.
•
1 = The timer interrupt is level triggered. This means that a leveltriggered interrupt is generated. The interrupt will be held active until
it is cleared by writing to the bit in the General Interrupt Status
Register. If another interrupt occurs before the interrupt is cleared, the
interrupt will remain active.
In order to preserve usage of these bits in the future, software should always
write a 0 to these bits until they are defined.

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2.3.9 Timer N Comparator Register

63:32

31:0

TN Comparator Value
Offset: (20h * N) + 108h

Figure 8 Timer N Comparator Register
Non-Periodic Mode Operation:

TN_Comparator Register Value

Counter
Compare
=
IRQ

Compare
Register

Periodic Mode Operation:
Main
Counter

TN_Comparator Register Value

Compare
=
+

IRQ

Adder
Comparator
Register

Update retgister
at match

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2.3.9.1 Register Definition and Usage Model
Timer N Comparator Register Addressing
Offset

Attribute
Size
General Behavioral
Rules

Timer 0: 108h – 10Fh
Timer 1: 128h – 12Fh
Timer 2: 148h – 14Fh
Timer n: (20h * n) + 108h - (20h * n) + 10Fh
Read-Write
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 1x8h or 1xCh. 64-bit accesses can be done to 1x8h. 32-bit
accesses must not be done to offsets 1x9h, 1xAh, 1xBh, 1xDh, 1xEh, or 1xFh.

2.

Reads to this register return the current value of the comparator.

3.

If the timer is configured to non-periodic mode:
•
•

When the main counter equals the value last written to this register, the corresponding
interrupt can be generated (if so enabled).

•
4.

Writes to this register load the value against which the main counter should be compared
for this timer.

The value in this register does not change based on the interrupt being generated.

If the timer is configured to periodic mode:
•

When the main counter equals the value last written to this register, the corresponding
interrupt can be generated (if so enabled).

•

After the main counter equals the value in this register, the value in this register is increased
by the value last written to the register.
For example, if the value written to the register is 00000123h, then
1.
2.
3.
4.

•

5.

An interrupt will be generated when the main counter reaches 00000123h.
The value in this register will then be adjusted by the hardware to 00000246h.
Another interrupt will be generated when the main counter reaches 00000246h
The value in this register will then be adjusted by the hardware to 00000369h

As each periodic interrupt occurs, the value in this register will increment. When the
incremented value is greater than the maximum value possible for this register
(FFFFFFFFh for a 32-bit timer or FFFFFFFFFFFFFFFFh for a 64-bit timer), the value will
wrap around through 0. For example, if the current value in a 32-bit timer is FFFF0000h
and the last value written to this register is 20000, then after the next interrupt the value
will change to 00010000h

Default value for each timer is all 1’s for the bits that are implemented. For example, a 32-bit
timer will have a default value of 00000000FFFFFFFFh. A 64-bit timer will have a default
value of FFFFFFFFFFFFFFFFh.

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2.3.9.2 Periodic vs. Non-Periodic Modes

2.3.9.2.1 Non-Periodic Mode
This mode change can be thought of as creating a one-shot timer.
When a timer is set up for non-periodic mode, it will generate a value in the main counter that matches the
value in the timer’s comparator register. If the timer is set up for 32-bit mode, then it will generate another
interrupt when the main counter wraps around.
During run-time, the value in the timer’s comparator value register will not be changed by the hardware.
Software can of course change the value.
WARNING: Software developers must be careful when programming the comparator registers. If the
value written to the register is not sufficiently set far enough ahead of the current register value, then the
counter may pass the value before it reaches the register and the interrupt will be missed.
Every timer is required to support the non-periodic mode of operation.

2.3.9.2.2 Periodic Mode
When a timer is set up for periodic mode, the software writes a value in the timer’s comparator value
register. When the main counter value matches the value in the timer’s comparator value register, an
interrupt can be generated. The hardware will then automatically increase the value in the comparator
value register by the last value written to that register.
To make the periodic mode work properly, the main counter is typically written with a value of 0 so that
the first interrupt occurs at the right point for the comparator. If the main counter is not set to 0, interrupts
may not occur as expected.
During run-time, the value in the timer’s comparator value register can be read by software to find out
when the next periodic interrupt will be generated (not the rate at which it generates interrupts). Software
is expected to retain the last value written to the comparator’s value register (the rate at which interrupts are
generated).
If software wants to change the periodic rate, it should write a new value to the comparator value register.
At the point when the timer’s comparator indicates a match, this new value will be added to derive the next
matching point. So as to avoid race conditions where the new value is written just as a match occurs,
either the main counter should be halted or the comparator disabled when the new periodic rate is written.
If the software resets the main counter, the value in the comparator’s value register needs to reset as well.
This can be done by setting the Tn_VAL_SET_CNF bit. Again, to avoid race conditions, this should be
done with the main counter halted. The following usage model is expected:
1) Software clears the GLOBAL_ENABLE_CNF bit to prevent any interrupts
2) Software Clears the main counter by writing a value of 00000000h to it.
3) Software sets the TIMER0_VAL_SET_CNF bit.
4) Software writes the new value in the TIMER0_COMPARATOR_VAL register
5) Software sets the GLOBAL_ENABLE_CNF bit to enable interrupts.

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2.3.9.2.3 Read/Write Paths for Periodic Mode Vs One-Shot Mode
One-Shot Mode of Operation

Clock
Gen

N-bit Bit
Up Counter
=

N- Bit
Compare
Read/
Write

=

IRQ

T0_C

N- Bit
Compare

Read/
Write

Data Bus

T1_C

IRQ

Read/
Write

Periodic Mode of Operation
Clock
Gen

Data Bus

N-bit Bit
Up Counter
N Bit
Adder

Read/
Write

Writes

T0_C

=

IRQ

Reads

Figure 9 Read/Write paths for One-shot Vs Periodic modes of operation
For Event Timers that provide hardware support (i.e. the adder logic) for periodic mode of operation, the
Timer N Comparator Register is overloaded as shown in Figure 9. In the periodic mode of operation, writes
to this register will program the periodic interval value to be added to the contents of the match register at
the next interrupt. Reads from this register will return the current contents of the match register at which the
next interrupt will occur.
If it is necessary to save/restore the context of a periodic timer through ACPI Sleep state transitions, system
software is expected to ‘remember’ the last value written to this register.

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2.3.10 Timer N FSB Interrupt Route Register

63:32

31:0

TN_FSB_INT_VAL
TN_FSB_INT_ADDR

Offset: (20h * N) + 110h

Figure 10 Timer N FSB Interrupt Route Register

Offset

Attribute
Size
General Behavioral
Rules

Timer 0: 110h – 117h,
Timer 1: 130h – 137h,
Timer 2: 150h – 157h
Timer n: (20h * n) + 110h - (20h * n) + 117h
Read-Write
64-bits
1.

Software can access the various bytes in this register using 32-bit or 64-bit accesses. 32-bit
accesses can be done to offset 1x0h or 1x4h. 64-bit accesses can be done to 1x0h. 32-bit
accesses must not be done to offsets 1x1h, 1x2h, 1x3h, 1x5h, 1x6h, or 1x7h.

2.

Reads or writes to unimplemented timers should not be attempted.

Bit
64:32

Field Name
Tn_FSB_INT_ADDR

31:0

Tn_FSB_INT_VAL

Description
Software sets this 32-bit field to indicate the location that the FSB interrupt
message should be written to.
Software sets this 32-bit field to indicate that value that is written during the FSB
interrupt message

2.4 Theory Of Operation
2.4.1 Timer Accuracy Rules
1.

The timers are expected to be accurate over any 1 ms period to within 0.05% (500 ppm)of the time
specified in the timer resolution fields.

2.

Within any 100-microsecond period, the timer is permitted to report a time that is up to 2 ticks too
early or too late. Each tick must be less than or equal to 100 ns, so this represents an error of less than
0.2% (2000 ppm).

3.

The main counter must be an up-counter. Two consecutive reads to the main counter may return the
same value if the access latency to the timer is less than the clock period that feeds it. For back-back
reads, the 2nd read must never return a value that is less than the 1st read, unless the counter has rolled
over and actually reached the same value.

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2.4.2 Interrupt Mapping
The interrupts associated with the various timers have several interrupt mapping options.

2.4.2.1 Mapping Option #1: LegacyReplacement Option
If the Legacy Replacement Route bit (LEG_RT_CNF) is set (‘1’), the following mapping is forced:

Timer

8259 Mapping

APIC Mapping

Comment

0

IRQ0

IRQ2

In this case, the 8254 timer will not cause any interrupts

1

IRQ8

IRQ8

In this case, the RTC will not cause any interrupts.

2

As per IRQ
Routing Field

As per IRQ Routing
Field

2.4.2.2 Mapping Option #2: Standard Option
If the LegacyReplacement Route bit (LEG_RT_CNF) is clear (‘0’), each timer has its own routing control.
The interrupts can be routed to various interrupts in the I/O APIC. A capability field indicates which
interrupts are valid options for the routing.
If a timer is set for edge-triggered mode, the timers should not be shared with any PCI interrupts.
2.4.2.3 Mapping Option #3: FSB Option
In this case, the interrupts are mapped directly to the FSB interrupts without going to the 8259 or I/O (x)
APIC. To use this mode, the interrupt must be configured to edge-triggered mode. A separate configure
bit must be set to enable this mode.
When the interrupt is delivered to the FSB, the message is delivered to the address indicated in the
Tn_FSB_INT_ADDR field. The data value for the write cycle is specified in the Tn_FSB_INT_VAL
field.
Notes:
•

The FSB interrupt deliver option has HIGHER priority and is mutually exclusive to the standard
interrupt delivery option. Thus, if the Tn_FSB_EN_CNF bit is set, the interrupts will be delivered via
the FSB, rather than via the APIC or 8259.

•

The FSB interrupt delivery can be used even when the LegacyReplacement mapping is used.

2.4.3 Periodic vs. Non-Periodic Modes
2.4.3.1 Non-Periodic Mode
This mode of operation provides a one-shot timer.
When a timer is set for non-periodic mode, it will generate an interrupt when the value in the main counter
matches the value in the timer’s comparator register. If the timer is set up for 32-bit mode, then it will
generate another interrupt when the main counter wraps around.
During run-time, the value in the timer’s comparator value register will not be changed by the hardware.
Software can of course change the value.
WARNING:
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Software developers must be careful when programming the comparator registers. If the value written to
the register is not sufficiently set far enough ahead of the current register value, then the counter may pass
the value before it reaches the register and the interrupt will be missed.
Every timer is required to support the non-periodic mode of operation.
2.4.3.2 Periodic Mode
When a timer is set for periodic mode, the software writes a value in the timer’s comparator register. When
the main counter value matches the value in the timer’s comparator register, an interrupt can be generated.
The hardware will then automatically increase the value in the compare register by the last value written to
that register.
To make the periodic mode work properly, the main counter is typically written with a value of 0 so that
the first interrupt occurs at the right point for the comparator. If the main counter is not set to 0, interrupts
may not occur as expected.
During run-time, the value in the timer’s comparator value register can be read by software to find out
when the next periodic interrupt will be generated.
Software can also write to the comparator’s match register to select a different period. A write to the
register will not immediately be loaded into the comparator. It will only be added at the time the
comparator triggers.
There is no mechanism to immediately change the periodic rate.
Each timer is NOT required to support this mode of operation. A capabilities bit indicates if the particular
timer supports periodic mode. The reason for this is that supporting the periodic mode adds a significant
amount of gates.

2.4.4 Enabling the Timers
The BIOS or OS PnP code should:
1.

Assign Base Address to each Timer Block in the System.

2.

Route the interrupts. This includes the LegacyReplacement Route bit, Interrupt Route bit (for each
timer), interrupt type (to select the edge or level type for each timer).

3.

If system BIOS is used to enable the timers, then report memory and interrupt resources consumed by
each Timer Block to OS Configuration Manager using HPET.

The Device Driver code should do the following for an available timer:
1.

Set the timer type field (selects one-shot or periodic).

2.

Set the interrupt enable

3.

Set the comparator match

4.

Set the Overall Enable bit (Offset 04h, bit 0). This starts the main counter and enables comparators to
deliver interrupts.

2.4.5 Interrupt Levels
The interrupts are all active high.
If the interrupts are mapped to the I/O APIC and set for level-triggered mode, they can be shared with PCI
interrupts.

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2.4.6 Handling Interrupts
If each timer has a unique interrupt and the timer has been configured for edge-triggered mode, then there
are no specific steps required. No read is required to process the interrupt.
If a timer has been configured to level-triggered mode, then its interrupt must be cleared by the software.
This is done by reading the interrupt status register and writing a 1 back to the bit position for the interrupt
to be cleared.
Independent of the mode, software can read the value in the main counter to see how much time has passed
from when the interrupt was generated and when it was first serviced.
If a timer is set up to generate a periodic interrupt, the software can check to see how much time remains
until the next interrupt by reading the main counter.
2.4.7 Issues related to 64-bit Timers with 32-bit CPUs
A 32-bit timer can be read directly using processors that are capable of 32-bit or 64-bit instructions.
However, a 32-bit processor may not be able to directly read a 64-bit timer. A race condition comes up if a
32-bit CPU reads the 64-bit register using two separate 32-bit reads. An accuracy problem may be arise if
just after reading one half, the other half rolls over and changes the first half.
Software has several ways of reading the 64 bit counter using 32 bit reads. The first is that it may halt the
counter, read the high and low 32 bits, and then restart the counter. This has the obvious drawback of
shifting the counter timebase.
If software does not want to halt the timer, it can use the 64-bit timer as a 32-bit timer by setting the
TIMERn_32MODE_CNF bit. This will cause the timer to behave as a 32-bit timer. The upper 32-bits will
always be 0.
Alternatively, software may do a multiple read of the counter while it is running. Software can read the
high 32 bits, then the low 32 bits, then the high 32 bits again. If the high 32 bits have not changed between
the two reads, then a rollover has not happened and the low 32 bits are valid. If the high 32 bits have
changed between reads, then the multiple reads are repeated until a valid read is performed.
Note: On a 64-bit platform, if software attempts a 64 bit read of the 64-bit counter, software must
be aware that some platforms may break the 64 bit read into two 32 bit reads, and therefore the
read may be inaccurate if the low 32 bits roll over between the high and low part reads.

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3. Enumeration & Configuration of HPET
Operating System software must discover/configure platform timers early in the OS boot process.
Typically, the OS has to establish basic timer services before it can begin loading drivers. This requires
enumeration of timer hardware to be handled by system BIOS tables (ie ACPI Tables) versus ACPI name
space. System Resources consumed by the Event Timer hardware (memory and interrupts) should be
reported by the System BIOS using ACPI Name space.

3.1 Initial State of Event Timer Hardware
•
•
•

Main Counter is Halted and Zeroed
Comparator Match Registers reset to all 1’s.
All interrupts are disabled
• General Configuration and Capability Register [Offset 0x010]<1:0> = 00
• Global IRQ Enable bit comes up disabled…no comparators can deliver interrupts
• LegacyReplacement IRQ Routing Enable bit comes up disabled…8254 is on IRQ0, RTC is
on IRQ8

3.2 BIOS Initialization
3.2.1 Assign memory to Timer Block(s)
Map each HPET (MMT) block to CPU memory space using implementation specific Base Address
Registers. Each HPET block in the system will consume 1K of system memory.
3.2.2 HPET Block Interrupt Routing
Except for the case where HPET are being used to replace 8254/RTC functionality, all HPET interrupts are
disabled. The OS is responsible for establishing interrupt routing/delivery metrics prior to utilizing any
given comparator within a Timer Block.
3.2.2.1 Routing Interrupts for HPET Blocks that do not support 8254/RTC IRQ Routing

In general, system BIOS is not required to assign or report HPET interrupts in system name space. The
Power-On-Default state of all compare interrupts should be disabled as shown in the following table.
Device
8254

Interrupt Routing
IRQ0, INTI2

RTC

IRQ8, INTI8

Compare 0

INTI [xx]

Compare 1

INTI [xx]

Compare 2

INTI [xx]

…
Compare 31

…
INTI [xx]

Comments
8254 signals via PIC/APIC using IRQ0/INTI2
LegacyReplacement IRQ Routing Disabled for Comparator_0
RTC signals via PIC/APIC using IRQ8/INTI8
LegacyReplacement IRQ Routing Disabled for Comparator_1
BIOS leaves Disabled
OS responsible for establishing irq routing prior to using this comparator
BIOS leaves Disabled
OS responsible for establishing irq metrics prior to using this comparator
BIOS leaves Disabled
OS responsible for establishing irq metrics prior to using this comparator
BIOS leaves Disabled
OS responsible for establishing irq metrics prior to using this comparator

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3.2.2.2 Routing Interrupts for HPET Blocks that support 8254/RTC IRQs
The exception to this rule is for the single HPET block in the system that may optionally support legacy
8254 & LegacyReplacement RTC irq routings for the compare interrupts. Assuming platform does not
have 8254/RTC hardware or does not want to support this legacy timer hardware, for this case, System
BIOS should set the LegacyReplacement Route bit and report IRQ0 & IRQ8 as being consumed by the
HPET block in system name space:
Device
8254

Interrupt Routing
Not connected

RTC

Not connected

Compare 0

IRQ0, INTI2

Compare 1

IRQ8, INTI8

Compare 2
…
Compare 31

Comments
BIOS sets LegacyReplacement Route bit (LEG_RT_CNF)
> LegacyReplacement IRQ Routing Enabled for Comparator_0
•
If present, 8254 will not cause any interrupts
•
If present, 8254 will still consume legacy i/o range
BIOS sets LegacyReplacement Route bit (LEG_RT_CNF)
> LegacyReplacement IRQ Routing Enabled for Comparator_1
•
If present, RTC Periodic Interrupt Function will not cause any interrupts.
•
RTC Alarm function (still required) will signal interrupts via SCI
•
RTC CMOS function (still required) will consume i/o range
BIOS sets LegacyReplacement Route bit (LEG_RT_CNF)
•
Comparator_0 replaces 8254 PIT Function
BIOS sets LegacyReplacement Route bit (LEG_RT_CNF)
•
Comparator_0 replaces 8254 PIT Function

INTI [xx]
INTI [xx]

Note 1: The use of LegacyReplacement IRQ Routing for C0 & C1 does not preclude delivery of
IRQ0/IRQ8 via FSB.

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3.2.3 Considerations for Platforms without Legacy Timers
If it is necessary to maintain DOS compatibility at INT 19 (or if necessary by OS to have periodic timer
ticks running when hand-over occurs from BIOS to OS for IPL), BIOS can use Timer_0 in periodic mode
(Vs 8254).
IA-PC HPET do not replace RTC Time of Day, RTC Alarm, and RTC CMOS functionality. IA-PC
Multimedia Event Timer architecture supplements/replaces only the RTC Periodic Interrupt function.
When the event timer is using IRQ8, RTC Alarm function will signal interrupts using SCI.
I
O
A
P
I
C

INTI_8

Legacy IRQ
Route Enable
Bit for Compare_X

SCI

Counter
CX_IRQ
Enable Bit
IRQ

2:1 Mux
8
2
5
9
PIC

CX_IRQ
Status Bit

IRQ8
SCI

PM1_EN
Reg

=

Value to
Compare

RTC Periodic

IRQ8#

SCI
Router

N Bit
Compare

PM1_STS
Reg

RTC Alarm
Sleep/
Wake
SM

RTC_EN

RTC_STS

I/O Based
Index/Data
Interface

RTC TOD

CMOS NVS

Figure 11 RTC Functionality replaced by IAPC HPET
The IA-PC HPET Architecture switches on or off both the 8254 and the RTC Periodic Interrupt Functions
together.
For the platforms without Legacy Timers, System BIOS needs to mark the 8254 and RTC Periodic
functions as ‘missing’ using the ACPI 2.0 proposed ‘legacy free’ flags.

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3.2.4 The ACPI 2.0 HPET Description Table (HPET)
The HPET Description Table is a means to report the Base Addresses of each Event Timer Block early in
the OS boot process. The table is needed to allow Operating Systems to discover event timers and establish
basic timer services for driver load.
Table 3 HPET Description Table
Field

Byte
Length
decimal

Byte Offset
decimal

Description

Header
Signature
Length

4
4

0
4

Revision

1

8

Checksum
OEMID
OEM Table ID

1
6
8

9
10
16

OEM Revision

4

24

Creator ID

4

28

Creator Revision

4

32

Event Timer Block ID

4

36

12

40

‘HPET’. Signature for the HPET Description Table.
Length, in bytes, of the entire Event Timer Description
Table.
The revision of the HPET Description Table definition;
currently 01h.
Entire table must sum to zero.
OEM ID.
For the Event Timer Description Table, the table ID is the
manufacturer model ID.
OEM revision of Event Timer Description Table for
supplied OEM Table ID.
Vendor ID of utility that created the table. For the DSDT,
RSDT, SSDT, and PSDT tables, this is the ID for the
ASL Compiler.
Revision of utility that created the table. For the DSDT,
RSDT, SSDT, and PSDT tables, this is the revision for
the ASL Compiler.
Hardware ID of Event Timer Block:
Contents of General_Cap&ID Reg of Timer Block
[31:16] = PCI Vendor ID of 1st Timer Block
[15] = LegacyReplacement IRQ Routing Capable
[14] = Reserved
[13] = COUNT_SIZE_CAP counter size
[12:8] = Number of Comparators in 1st Timer Block
[7:0] = Hardware Rev ID
The lower 32-bit base address of Event Timer Block.
Each Event Timer Block consumes 1K of system
memory, regardless of how many comparators are
actually implemented by the hardware.

1

52

2

53

*Note #1

BASE_ADDRESS
Lower 32-bit

*Note #2
HPET Number

Main Counter Minimum
Clock_tick in Periodic
Mode
*Note#3

This one byte field indicates the HPET sequence number.
0 = 1st table, 1 = 2nd table and so forth. This field is
written by BIOS at boot time and should not be altered by
any other software.
Unit: Clock tick
The minimum clock ticks can be set without lost
interrupts while the counter is programmed to operate in
periodic mode

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Field

Page Protection And OEM
Attribute

Byte
Length
decimal
1

Byte Offset
decimal

Description

55

The lower 4-bit ( bit <0..3> ) of this field describes the
timer hardware capability to guarantee the page
protection. This information is required for the OSes that
want to expose this timer to the user space:
0 = no guarantee for page protection.
1 = 4KB page protected, access to the adjacent 3KB
space will not generate machine check or
compromise the system security.
2 = 64KB page protected, access to the adjacent
63KB space will not generate machine check or
compromise the system security.
3~ 15 = Reserved for future use.
The upper 4-bit (bits <4..7> of this field is reserved for
OEM attributes:
OEM can use this field for its implementation specific
attributes.

*Note #1: This field provides a quick access for software which needs to know the HPET implementation.
*Note #2: This is a 12-byte ACPI address format:
GAS (Generic Address Structure) -- ACPI Address Format:
Byte #1 – Address_Space_ID : 0 – System Memory
1 – System I/O
Byte #2 – Register_Bit_Width
Byte #3 – Register_Bit_Offset
Byte #4 – Reserved
Byte #5 to 12 – 64-bit address
* Note #3: This field is written by BIOS and may be chipset and/or platform dependent. This indicates the
minimum value that must be used for any counter programmed in periodic mode to avoid lost interrupts. For any
counter x that has been configured for periodic mode, the number can be programmed in any Tx_Compare
Register must be greater than P, where
P = (Minimum Period) / (Main counter period) in order to avoid lost interrupts.
* Note 4: The HPET Number field must match the number in the _UID field in the ACPI Namespace table
(section 3.2.5.1) for the specific counter being described.

For the case where there may be additional Event Timer Blocks implemented in the system, their base
addresses would be described in ACPI Name space.
Only one Event Timer Block needs to be described in the HPET in order to boot strap the OS.
For “legacy free” platforms that do not implement the 8254 Timer/RTC Periodic Interrupt logic, the Event
Timer Block described in the HPET would be the one that provides functionality to replace the 8254/RTC
Periodic Interrupt Logic.

Object Description

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Named object that provides the interface’s Plug and Play identifier. This value
is set to PNP0103. _HID is a standard device configuration control method
defined in ACPI 2.0 Spec section 6.1.4, "_HID (Hardware ID)."
Named object (optional) that evaluates to a Unicode string that may be used by
an OS to provide information to an end user describing the device. __STR is a
standard device configuration control method defined in ACPI 2.0 Spec section
6.1.5, "_STR (String)."
Named object that returns the Event Timer’s current resource settings. Event
Timer is considered static resources; hence only return its defined resources.
_CRS is a standard device configuration control method defined in ACPI 2.0 Spec section 6.2.1,
"_CRS (Current Resource Settings)."
Object that specifies the page protection capability, as defined in the HPET
Description Table.
Object that specifies the timer attributes, as defined in the HPET
Description Table

_HID

_STR

_CRS

PAGE
ATTR

3.2.5 Describing Event Timer(s) in ACPI Name space
System BIOS must report memory and interrupt resources consumed by each Event Timer block in ACPI
Name space.
Event Timer(s) memory assignments are established by the system BIOS on per Timer Block basis. Event
Timer(s) memory assignments are reported to the OS by the BIOS using HPET Table and in ACPI Name
space.
Event Timer IRQ assignments need only be established for the single Timer Block that may optionally
provide hardware support to supplement/replace legacy 8254 and legacy RTC hardware with these new
HPET. For this case, system bios reports interrupt resources consumed by the Timer Block in ACPI Name
space. For the case where the Timer Block does not provide legacy 8254/legacy RTC hardware
replacement, system bios is not required to establish or report Event Timer Interrupt assignments. The OS
is expected to assign event timer interrupts prior to utilizing any given comparator in the Timer Block.
PNP0103 is Microsoft assigned generic PNPID for IA-PC HPET blocks.

3.2.5.1 ACPI Name Space Example
Device (HPET)
_HID

PNP0103

// newly assigned PNPID for IAPC HPET

_UID

0

// Optional : used if there are more than 1 timer blocks

memory range consumed
IRQs consumed
)

// Report 1K of memory consumed by this Timer Block
// Optional : only used if BIOS allocates Interrupts1

_CRS

(

Notes:
1.
For case where Timer Block is configured to consume IRQ0/IRQ8 AND Legacy 8254/Legacy RTC hardware still
exists, the device objects associated with 8254 & RTC devices should not report IRQ0/IRQ8 as “consumed
resources.”
2. If one or more HPET Description tables exist (see section 3.2.4), the _UID in the ACPI name space description for
the counter must correspond to the “HPET Number” field for the counter described in the in the HPET Description
Table(s).

3.2.6 Recommendations for OS Initialization code
From ACPI HPET table, the OS initialization code extracts the base address assigned by the BIOS for each
timer block implemented in the system. The OS initialization code can then directly query the timer
hardware to discover detailed attributes and configurations supported by the timer hardware
implementation.

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IA-PC HPET architecture has configuration and capability reporting mechanisms designed in at the register
level. OS initialization code can query these registers to determine, establish, or override (for example:
default IRQ assignments established by the BIOS) Event Timer hardware configurations.
The following registers provide enumeration info to system software:
General Capabilities & ID Register
Offset 00h
• Clock Frequency
• Width of Main Counter
• Vendor ID/Hardware ID
• LegacyReplacement Timer IRQ Routing Capable (or not)
• Number of Comparator’s Implemented
To determine / override the default IRQ assignments established by the BIOS:
General Configuration Register
Offset 04h
• Global Interrupt Enable Bit
• LegacyReplacement Timer IRQ Routing Enable Bit
Timer N Configuration & Capabilities Register
1 for each Comparator implemented:
Offsets:

•
•

•

108h – 10Fh
128h – 12Fh
148h – 14Fh
(20h * n) + 108h - (20h * n) + 10Fh

Width of Comparator
• Comparator Configuration bit which allows 64bit comparator to behave like a 32bit
comparator.
Per Comparator Interrupt Enable
• Edge Vs Level selection
• IOAPIC IRQ Routing capabilities reporting
• IO APIC IRQ Router settings
• FSB IRQ Delivery Capable (or not)
• FSB IRQ Delivery Enable bit
Operating Modes : Periodic Vs One-Shot

Timer_N_FSB_Route Register
•
•

Timer 0:
Timer 1:
Timer 2:
Timer n:

Offset: 1 for each Comparator Implemented

Address to which the FSB interrupt message should be written.
Data Value/Vector to be used with FSB interrupt message.

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