HPET

This page is a work in progress.

This page may thus be incomplete. Its content may be changed in the near future.

This page, right now, is going to be my diary of my adventures with HPET, but somewhere in the future, when support for HPET is complete in my kernel, I'll turn it into regular wiki page.

Preface

HPET, or High Precision Event Timer, is a piece of hardware designed by Intel and Microsoft to replace older PIT and RTC. It consists of (usually 64-bit) main counter (which counts up), as well as from 3 to 256 32 or 64 bit wide comparators. HPET is programmed using memory mapped IO, and the base address of HPET can be found using ACPI.

Detecting HPET using ACPI

HPET specification defines an ACPI 2.0 table that is to be used to detect presence, address and capabilities of HPET present in the system. If this table doesn't exist, you should assume there is no HPET.

    struct address_structure
    {
        uint8_t address_space_id;    // 0 - system memory, 1 - system I/O
        uint8_t register_bit_width;
        uint8_t register_bit_offset;
        uint8_t reserved;
        uint64_t address;
    } __attribute__((packed));

    struct description_table_header
    {
        char signature[4];    // 'HPET' in case of HPET table
        uint32_t length;
        uint8_t revision;
        uint8_t checksum;
        char oemid[6];
        uint64_t oem_tableid;
        uint32_t oem_revision;
        uint32_t creator_id;
        uint32_t creator_revision;
    } __attribute__((packed));

    struct hpet : public description_table_header
    {
        uint8_t hardware_rev_id;
        uint8_t comparator_count:5;
        uint8_t counter_size:1;
        uint8_t reserved:1;
        uint8_t legacy_replacement:1;
        pci_vendor_t pci_vendor_id;
        address_structure address;
        uint8_t hpet_number;
        uint16_t minimum_tick;
        uint8_t page_protection;
    } __attribute__((packed));

See also

• HPET article on Wikipedia
• Intel's HPET Specification v1.0a