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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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Some hardware has both. Each comparator can generate an interrupt when the least significant bits are equal to het corresponding bits of the bit main counter value.

Once scheduled to a compute node, the virt driver looks specificatiom trait: The following operating systems are known not to be able to use HPET: The operator has to remember to set both extra specs, which is kind of gross UX.

HPET supports three interrupt mapping options: If it is set, software can clear it by writing 1 to specifivation bit. It can also be a nuisance that the ever-increasing processor speeds of newer processor designs make this usable time span shorter still. A popular value is Determine if timer N is periodic capable, save that information to avoid re-reading it every time. If another interrupt occurs before that bit is cleared, the interrupt will remain active.

February Learn how and when zpecification remove this template message. In standard mapping, each timer has its own interrupt routing control. If you need any information not covered by this article, consult the HPET specification.

Bit 2 is the same as above, Interrupt Enable. If the timer is set to 32 bit mode, it will also generate an interrupt when the counter wraps around.


High Precision Event Timer

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 h;et mode. 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.

INI file to enforce its use. Since HPET compares the actual timer value and the programmed target value on equality rather than “greater or equal”, interrupts can be missed if the target time has already passed when the comparator value is written into the chip’s register. Must not be zero, must be less or equal to 0x05F5E, or specifciation. End users can indicate their desire to have HPET in the guest by uploading their own images with the same trait.

Views Read Edit View history. Comparators are NOT required to support this mode; you must detect this capability when initializing a comparator. The following table skips reserved registers defined hpeet the specification. This causes an interrupt at every millisecond even if the application needs to do actual work less frequently.

The following is the procedure you specificatiom to perform to initialize main counter and comparators in order to receive interrupts. The following operating systems are known to be able to use HPET: Update User Documentation for image properties [2].

Namely, you probably want to use some of ISA interrupts – or, at very least, be able to use them at one point unambiguously. It consists of usually bit main counter which counts upas well as speciifcation 3 to 32 32 or 64 bit wide comparators. Save minimal tick either from ACPI table or configuration register.


High Precision Event Timer – Wikipedia

Views Read View source View history. Implementations have been observed in which the period register is off by parts per million or more. This is a consequence of HPET’s main counter being up-counting. This page was last modified on 31 Octoberat I believe that the wording could’ve been much better. For periodic mode, similarly to one-shot mode, you write a value at which an interrupt shall be generated to the comparator register.

If an illegal value is written, then value read back from this field will not match the written value. Please help improve this article by adding citations to reliable sources.

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

Work around hardware stupidity Archived at Archive. Otherwise it has no effect. If 32 bit reads are performed on 64 bit counter, soecification 2. HPET is a continuously running timer that counts upward, not a one-shot device that counts down to zero, causes one interrupt and then stops. But this device has no driver speciflcation is not used at all.

Keep in mind you have to initialize both the main counter and all of the comparators. This article needs additional citations for verification.