IA-PC HPET (High Precision Event Timer) is a specification which was jointly developed by Intel and Microsoft in the early part of this decade.. The latest version is dated October 2004. It’s stated purpose is to initially supplement and eventually replace the legacy 8254 Programmable Interval Timer and the Real Time Clock Periodic Interrupt generation functions that are currently used as the ‘de-facto’ timer hardware for IA-PCs.
The HPET architecture defines a set of timers that can be used by the operating system. A timer block is a combination of a single counter and up to 32 comparators and match registers. The comparator compares the contents of the match register against the value of a free running monotonic up-counter. When the output of the up-counter equals the value in the match register an interrupt is generated. Each of the comparators can output an interrupt. A maximum of 8 timer blocks are supported for a total of 256 timers. Each timer block can have different clocking attributes. Specific implementations may include only a subset of these timers. A minimum of three timers is required.
The specification contains the following block diagram of the HPET architecture.
Some of the timers may be enabled to generate a periodic interrupt. If a timer is set to be periodic, its period is added to the match register each time a match occurs, thus computing the next time for this timer to generate an interrupt.. An up-counter is usually 64 bits wide but 32-bit implementations are permitted by the specification and 64-bit up-counters can also be driven in 32-bit mode. Up-counters run at a minimum of 10 MHz. which is much faster than the older RTC (Real Time Clock) and can thus produce periodic interrupts at a much higher resolution. The registers associated with these timers are mapped to memory space.
The BIOS uses ACPI ( Advanced Configuration and Power Interface) functionality to inform the operating system of the location of the HPET memory-mapped register space. Here is an example of a disassembled ACPI HPET table from an Intel DX48BT2 (AKA BoneTrail) motherboard. $ cat /sys/firmware/acpi/tables/HPET > /var/tmp/hpet.out $ iasl -d /var/tmp/hpet.out $ cat /var/tmp/hpet.dsl /* * Intel ACPI Component Architecture * AML Disassembler version 20090123 * * Disassembly of /var/tmp/hpet.out, Sun Jul 5 19:34:47 2009 * * ACPI Data Table [HPET] * * Format: [HexOffset DecimalOffset ByteLength] FieldName : FieldValue */ [000h