Timer Interrupt Sources

X86 related clocks from Brendan post on http://forum.osdev.org/viewtopic.php?p=277285&sid=e5be2757dbd0ffdab02081df2bec8863#p277285

Overview

	Architecture	Discovery	Per CPU Core	Counter	Calibration Not Required	IRQ Frequency	Fixed Frequency IRQ	IRQ on terminal count	Access speed
RTC	x86 PC	{{No}	No	Yes	style="background: #ddffdd" | Yes	No	Slow ISA IO ports
PIT	x86 PC	6 PC		No	Yes	Local A	Yes	Slow ISA IO ports (or slow SMM emulation of ISA ports)
ACPI Power Management Timer	x86 PC	ACPI tables (No}}	}}	{	Yes	Yes	es}}		No	IO ports (may or may not be "faster, non-ISA IO ports")
HPET	x86 PC	ACPI tables (}}	Yes	Yes	Yes

= | style="background: #ddffdd" | Yes | style="background: #ddffdd" | Yes | 64-bit monotonically increasing at 10 MHz or better |y

Acce | style="background: #ddffdd" | Yes |Fast (typically memory mapped device) |- | TSC |x86 PC |ate/time)

Fixed frequency I| : Yes, | style="background: #ddffdd" | Yes | style="background: #ffdddd" | No | No IRQ avaialable | at fa | style="background: #ffdddd" | No |Very fast (built into CPU) |- |Local APIC timer (older) | x86 PC | to exist, but may be emulated by HPET)

Calibration: Not necessary

Access speed: Slow or ACPI tables ( slow SMM emul| ion of | style="background: #ffdddd" | No | style="background: #ffdddd" | No | Depends on CPU's bus / link speed, typically 100 Mhz or better (10 ns better) |nt: Yes | style="background: #ddffdd" | Yes |Very fast (built into CPU) |- |Local APIC timer (newer) |x86 PC |alibration: Not necessary

Access speed: IO ports (may or may not be "faster, non-ISA I or ACPI tables (r: Yes, 24-bit| r 32-bi | style="background: #ddffdd" | Yes | style="background: #ffdddd" | No | 64-bit monotonically increasing at CPU's clock speed (which can vary on old CPUs) | Timer] | style="background: #ddffdd" | Yes |Very fast (built into CPU) |- |Generic Timer |ARM | Always present |emory m | style="background: #ddffdd" | Yes | style="background: #ddffdd" | Yes |CNTP_CVAL and CNTV_CVAL system registers |Configurable | style="background: #ddffdd" | Yes |Very fast (built into CPU) |- | Local Timer | ARM | Always present |s, 100 | style="background: #ffdddd" | No | style="background: #ddffdd" | Yes | 38.4 MHz |]]

==T | style="background: #ddffdd" | Yes |Moderate (using MMIO) |- |System Timer |ARM/BCM | On BCM2835-2837 SoC (Raspberry Pi) |ncreas | style="background: #ddffdd" | Yes | style="background: #ddffdd" | Yes | Yes |hich ca | style="background: #ddffdd" | Yes |Moderate (using MMIO) |}

Precision

For counters; ignoring "emulated with something else", the options in order of best precision are: TSC, HPET, ACPI power management timer

For fixed frequency IRQ; ignoring "emulated with something else", the options in order of best precision are: local APIC timer, HPET, PIT, RTC

For IRQ on terminal count (e.g. needed for "tickless"); ignoring "emulated with something else", in order of best precision are: local APIC timer in TSC deadline mode, local APIC timer without TSC deadline mode, HPET, PIT

Emulation

Counters can be emulated in software using a fixed frequency IRQ (e.g. doing "tick++;" in the IRQ handler).

Fixed frequency IRQ can be emulated in software using IRQ on terminal count (e.g. just set the new count to the same value each time).

IRQ on terminal count can be emulated in software using a fixed frequency IRQ (e.g. doing "count--; if(count == 0)" in the IRQ handler).

Notes

A "counter" is something you poll when you need to know how much time has passed (e.g. for file system timestamps, measuring how much CPU time each thread consumed, etc).

For local APIC timer, there's one per CPU. This is important for scalability (rather than many CPUs fighting to access the same single timer).

A good OS would detect which timers exist and determine their capabilities; then use this information to select the best timers to use for each different purposes.