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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Determine allowed interrupt routing for current timer and allocate an interrupt for it. Comparators are NOT required to specifictaion this mode; you must detect this capability when initializing a comparator. In standard mapping, each timer has its own interrupt routing control.

Enter search terms or a module, class or function name. This mapping mode will not be further discussed in this article.

specificatin When the interrupt is generated, however, the hardware will increase the value in comparator register by the last value written to it! Detailed explanation is provided further in the article. Compared to these older timer circuits, the HPET has higher frequency and wider bit counters although they can be driven in bit mode.

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Operators can indicate their desire to have HPET in the guest by specifying a placement trait trait: Namely, you probably want to use some of ISA interrupts – or, at very least, be able to use them at one point unambiguously. But we’ve also set bit 6. A few options to use Traits were considered as described in the next section, but we end up choosing the simpler approach due to the following reasons:. I believe that the wording could’ve been much better.


Writes of 0 have no effect. HPET is meant to supplement and replace the programmable interval timer and the Hoet periodic interrupt function. As an end user looking to migrate an existing appliance to run in a cloud environment I would like to be able to request a guest with HPET so that I can share common code between my virtualized and physical products.

High Precision Event Timer

This field is used to allow software to directly set periodic timer’s accumulator. Retrieved from ” https: Retrieved from ” https: 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. In one-shot mode the comparator fires an interrupt once when specificafion main counter reaches the value stored in the comparator’s register, while in the periodic mode the interrupts are generated at specified intervals.

This page is not meant as a full description of HPET, only as a lightweight introduction. This page has been accessed 35, times. Update User Documentation for image properties [2].

HPET – OSDev Wiki

Implementations have been observed in which the period register is off by parts per million or more. The following table and field descriptions can also be found in the specification. Keep in mind that allowed interrupt routing may be insane. HPET supports three interrupt mapping options: Reads will return current value of the main counter.

If 32 bit reads are performed on 64 bit counter, consult 2. This page was last edited on 24 Septemberat If another interrupt occurs before that bit is cleared, the interrupt will remain active. If the timer is set to 32 bit mode, it will also generate an interrupt when the counter wraps around. Bit 3 is also quite straightforward – 1 means periodic timer. While and RTC can be put into an HPET-like one-shot mode, the set-up process is so slow that their one-shot mode is not used in practice for tasks requiring precise scheduling.


Determine if timer N is periodic capable, save that information to avoid hpeet it every time. Periodic mode is more tricky than non-periodic mode. Webarchive template archiveis links Articles needing additional references from Specificatiom 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 Otherwise, this bit will be ignored and reading it will always return 0.

These comparators are or bit-wide.

The implicit transformation of a special extra spec into placement-isms is arcane. Once scheduled to a compute node, the virt driver looks for trait: Personal tools Log in. This article needs additional citations for verification. It can also be a nuisance that the ever-increasing processor speeds of newer processor designs make this usable time span shorter still.

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