HPET
- This page is not meant as a full description of HPET, only as a lightweight introduction. If you need any information not covered by this article, consult the HPET specification.
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
The 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 and you should fall back to PIT.
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));
HPET - timer vs comparators
There is just one up counting main counter in the timer, but interrupt generation is handled by *comparators*. There's from 3 to 256 comparators, and the exact amount is indicated by comparator_count field in the above hpet structure. Keep in mind you have to initialize both the main counter and all of the comparators. 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. More information on this procedure is provided further in the text.
HPET operating modes
HPET offers three operating modes; as of now, this article describes two of them: one-shot (also called "non-periodic" by the specification) and periodic mode.
One-shot mode
In non-periodic mode, the OS programs one of timer's comparator registers with value of main counter that is to trigger an interrupt. If the timer is set to 32 bit mode, it will also generate an interrupt when the counter wraps around. 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.
Every comparator in HPET **must support** non-periodic mode.
Periodic mode
Periodic mode is more tricky than non-periodic mode. For periodic mode, similarly to one-shot mode, you write a value at which an interrupt shall be generated to the comparator's match register. When the interrupt is generated, however, the hardware will **increase the value of comparator's register by the last value written to it**! This is a consequence of HPET's main counter being up-counting.
So, if the main counter's value is 12345 when we set the timer up, and we write 12456 to comparator (i.e. the interrupt should trigger 111 time units from now), when the interrupt triggers, 12456 will be *added* to the comparator register, so it will become 24912, which is 12456 time units from the first interrupt. There are two techniques to deal with this problem; they will both be described in later part of the article.
Comparators are **NOT required** to support this mode; you must detect this capability when initializing a comparator. More information on this is provided further in the article.