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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While and RTC can be put into an HPET-like one-shot mode, the set-up process is so specificcation that their one-shot mode is not used in practice for tasks requiring precise scheduling.

HPET is meant to supplement and replace the programmable interval timer and the RTC’s periodic interrupt function. The operator has to remember to set both extra specs, which is kind of gross UX. This specificatin needs additional citations for verification. Reads will return current value of the main counter.

HPET – OSDev Wiki

The functionality is dependent of whether edge or level-triggered mode is used for timer n. Compared to these older timer circuits, the HPET has higher frequency and wider bit counters although they can be driven in bit mode. If 32 bit reads are performed on 64 bit counter, consult 2.

Comparators can be driven by the operating system, e. When a corresponding timer interrupt is active, this bit is set.

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

Each comparator can generate an interrupt when the least significant bits are equal to the corresponding bits of the bit main counter value. Personal tools Log in.

The driver uses the hw: If we do get down to the virt driver and the trait is set, and the driver for whatever reason e. A benchmark in that environment for 10 million event counts found that TSC took about 0. If the timer is set to 32 bit mode, it will also generate an interrupt when the counter wraps around. This section needs expansion with: Detailed explanation is provided further in the article.


Note A corresponding flavor extra spec will not be introduced since enabling HPET is really a per-image concern rather than a resource concern for capacity planning.

This means that next write to timer N comparator register will have the usual meaning, while second next write will write directly to the accumulator. But we’ve specificatiln set bit 6.

Since Secification 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.

I am enabling the timers only when I actually use them, so there’s no “real” initialization of comparators here.

High Precision Event Timer

Bit 2 is the same as above, Interrupt Enable. From Wikipedia, the free encyclopedia.

The following operating systems are known to be able to use HPET: If it’s not, please analyze the meaning of specific fields in registers used above. Once scheduled to a compute node, the virt driver looks for trait: It h;et recommended to use 32 bit counter when on bit only software.

Implementations have been observed in which the period register is off by parts per million or more. It is always set to 0. These comparators are or bit-wide. Save minimal tick either from ACPI table or configuration register.

INI file to enforce its use. Unsourced material may be challenged and removed. It was developed jointly by Intel and Microsoft and has been incorporated in PC chipsets since circa Update User Documentation for image properties [2]. HPET supports three interrupt mapping options: End users can indicate their desire to have HPET in the guest by uploading their own images with the same trait.


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

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 specificatiom. This has the express advantage of being independent of the CPU frequency and still provides a very reasonable sub-microsecond resolution and accuracy.

So the HPET is only there to satisfy the system’s high speed needs. Also, the routing as well as allowed routing of comparator interrupts is independent, so you have to detect and set it up for each of them individually.

It can also be a nuisance that the ever-increasing processor speeds of newer processor designs make this usable time span shorter still.

More information on this is specirication further in the article. Work around hardware stupidity Archived at Archive. A popular value is Operators can indicate their desire to have HPET in the guest by specifying a placement trait trait: Writes of 0 have no effect.

The following operating systems are known not to be able to use HPET: I hope the above code is obvious. February Learn how and when to remove this template message. If she forgets hw: Views Read View source View history.