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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Created using Sphinx 1. This section needs expansion with: Unsourced material may be challenged and removed. Keep in mind you have to initialize both the main counter and all of the comparators.

If another interrupt occurs before that bit is cleared, the interrupt will remain active. In standard mapping, each timer has its own interrupt routing control. The following operating systems are known to be able to use HPET: Periodic mode is more tricky than non-periodic mode. It can also be a nuisance that the ever-increasing processor speeds of newer processor designs make this usable time span shorter still.

So the HPET is only there to satisfy the system’s high speed needs. I hope the above code is obvious.

High Precision Event Timer

Writes of 0 have no effect. In non-periodic mode, speciffication OS programs one of timer’s comparator registers with value of main counter that is to trigger an interrupt.

This page has been accessed 35, times. Some hardware has both. Besides mentioning the race condition discussed above, a VMware document also lists some other drawbacks: 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 interrupt will be generated.


By using this site, you agree to the Terms of Use and Privacy Policy. Be aware of this when choosing interrupt routing for timers.

Detailed explanation is provided further in the article. If the timer is set to 32 bit mode, it will also generate an interrupt when the counter wraps around. It was developed jointly by Intel and Microsoft and has been incorporated in PC chipsets since circa Keep in mind that allowed interrupt routing may be insane. The implicit transformation of a special extra spec into placement-isms is arcane.

Bit 2 is the same as above, Interrupt Enable. Namely, you probably want to use some upet ISA interrupts – or, at very least, be able to use them at one point unambiguously. If 32 bit reads are performed on 64 bit counter, consult 2. Comparators can be driven by the operating system, e.

High Precision Event Timer – Wikipedia

The following table skips reserved registers defined in the spevification. The driver uses the hw: If it’s not, please analyze the meaning of specific fields specfiication registers used above. Once scheduled to a compute node, the virt driver looks for trait: This has the express advantage of being independent of the CPU frequency and still provides a very reasonable sub-microsecond resolution and accuracy.

The following is the procedure you need to perform to initialize main counter and comparators in order to receive interrupts.


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Comparators are NOT required to support this mode; you must detect this capability when initializing a comparator.

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. The following table and field descriptions can also be found specificatio the specification.

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

Views Read Edit View history. Since the original specification for HPET in calls for a specitication resolution het, 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.

But we’ve also set bit 6. Last updated on Fri Dec 21 Compared to these older timer circuits, the HPET has higher frequency and wider bit counters although they can be driven in bit mode.

More information on this procedure is provided further in the text. A popular value is