Software Developers Hpet Spec 1 0a – Download as PDF File .pdf), Text File .txt ) or read online. Updated HPET web link, added WSPT and WDAT, updated WDRT description and web link. Clarified that the endian-ness of data value. High Precision Event Timer Driver for Linux The High Precision Event Timer ( HPET) hardware follows a specification by Intel and Microsoft, revision 1.

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Webarchive template archiveis links Articles needing additional references from February All articles needing additional references Articles to be expanded from February All articles to be expanded Articles using small message boxes All articles with unsourced statements Articles with unsourced statements from December I am enabling the timers only when I actually use them, so there’s no “real” initialization of comparators here.

This article needs additional citations for verification. I hope the above code is obvious.

The comparators can be put into one-shot mode or periodic mode, with at least one comparator supporting periodic mode and all of them supporting one-shot mode. The functionality is dependent of whether edge or level-triggered mode is used for timer n.

If she forgets hw: The following operating systems are known to be able to use HPET: But this device has no driver and is not used at all. This has the express advantage of being independent of the CPU frequency and specificxtion provides a very reasonable sub-microsecond resolution and accuracy.

For bit timer, if this field is set, the timer will be forced to work in bit mode.

The following is the procedure you need to perform to initialize main counter and comparators in order to receive interrupts. It consists of usually bit main counter which counts upas well as from 3 to 32 32 or 64 bit wide comparators.


This is a consequence of HPET’s main counter being up-counting. Compared to these older timer circuits, the HPET has specufication frequency and wider bit counters although they can be driven in bit mode. In one-shot mode the comparator fires an interrupt once when the main counter reaches the value stored in the comparator’s register, while in the periodic mode the interrupts are generated at specified intervals.

This field is used to allow software to directly set periodic timer’s accumulator.

High Precision Event Timer

Since the original specification for HPET in calls for a high resolution counter, which is then exposed by the QueryPerformanceFrequency and QueryPerformanceCounter API calls already available since Windowsit is the QueryPerformanceFrequency that can shed light on how this “high precision” counter is actually being provided. In non-periodic mode, the OS programs one of timer’s comparator registers specificatikn value of main counter that is to trigger an interrupt.

Formerly referred to by Intel as a Multimedia Timer[1] the term HPET was selected to avoid confusion with the multimedia timers as a software feature introduced in the MultiMedia Extensions to Windows 3. Some hardware has both. Be aware of this when choosing interrupt routing for timers. Support to query nova resources filter by changes-before.

INI spscification to enforce its use.

Comparators can be driven by the operating system, e. If we do get down to the virt driver and the trait is set, and the driver for whatever reason e. Setting this bit to 1 enables triggering of interrupts. Specificatio can help by adding to it. The difficulties are exacerbated if the comparator value specfication not synchronized with the timer immediately, but delayed by one or two ticks, as some chipsets do.

A popular secification is The operator has to remember to set both extra specs, which is kind of gross UX. In standard mapping, each timer has its own interrupt routing control. So the HPET is only there to satisfy the system’s high speed needs. The following table and field descriptions can also be found in the specification. If an illegal value is written, then value read back from this field will not match the written value.


The comparator register’s value is never written by the hardware, you are free to write to it and read from it at any time, therefore you can change at what value in the main counter the bpet will be generated.

Retrieved from ” https: Operators can indicate their desire to have HPET in the guest by specifying a placement trait trait: Views Read View source View history. Writes of 0 have no effect.

HPET – OSDev Wiki

Update User Documentation for image properties [2]. The following options to use Trait were considered, but ultimatedly we chose a simpler approach without using Trait. When a corresponding timer interrupt is active, this bit is set. A corresponding flavor extra spec will not be introduced since enabling HPET specificxtion really a per-image concern rather than a resource concern for capacity planning. Please help improve this article by adding citations to reliable sources.

Support High Precision Event Timer (HPET) on x86 guests — Nova Specs documentation

More information on this is provided further in the article. This section needs expansion with: Must not be zero, must be less or equal to 0x05F5E, or nanoseconds.

HPET supports three interrupt mapping options: Determine allowed interrupt routing for current timer and allocate an interrupt for it. This spceification mode will not be further discussed in this article.