X86 timing

• PIT -- Programmable Interval Timer (eg Intel 8253)
• ACPI counter
• LAPIC counter
• HPET
• TSC since Pentium
• RTC (Linux docs)
  • ACPI standardized the cmos-rtc type, extending the Motorola MC146818 (CONFIG_RTC_DRV_CMOS at the LKDD)

[recombinator](0) $ cat /proc/driver/rtc 
rtc_time	: 07:43:30
rtc_date	: 2009-06-27
alrm_time	: **:00:00
alrm_date	: ****-**-**
alarm_IRQ	: no
alrm_pending	: no
24hr		: yes
periodic_IRQ	: no
update_IRQ	: no
HPET_emulated	: yes
DST_enable	: no
periodic_freq	: 1024
batt_status	: okay
[recombinator](0) $ 

• Linux clocksources vs clockevents