Timer Interrupt Sources: Difference between revisions
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From Brendan post on http://forum.osdev.org/viewtopic.php?p=277285&sid=e5be2757dbd0ffdab02081df2bec8863#p277285 |
From Brendan post on http://forum.osdev.org/viewtopic.php?p=277285&sid=e5be2757dbd0ffdab02081df2bec8863#p277285 |
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'''RTC''' |
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== RTC == |
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Discovery: Standard (assumed to exist) |
Discovery: Standard (assumed to exist) |
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IRQ on terminal count: No |
IRQ on terminal count: No |
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[[RTC]] |
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== PIT == |
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Discovery: Standard (assumed to exist, but may be emulated by HPET) |
Discovery: Standard (assumed to exist, but may be emulated by HPET) |
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==ACPI Power Management Timer== |
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Discovery: ACPI tables (FADT) |
Discovery: ACPI tables (FADT) |
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==HPET== |
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Discovery: ACPI tables (HPET) |
Discovery: ACPI tables (HPET) |
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==TSC== |
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Discovery: CPUID |
Discovery: CPUID |
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==Local APIC timer (older)== |
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Discovery: MP specification table, ACPI tables (APIC/MADT) |
Discovery: MP specification table, ACPI tables (APIC/MADT) |
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==Local APIC timer (newer, with TSC deadline mode)== |
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Discovery: MP specification table, ACPI tables (APIC/MADT) |
Discovery: MP specification table, ACPI tables (APIC/MADT) |
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==Precision== |
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For counters; ignoring "emulated with something else", the options in order of best precision are: TSC, HPET, ACPI power management timer |
For counters; ignoring "emulated with something else", the options in order of best precision are: TSC, HPET, ACPI power management timer |
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==Emulation== |
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Counters can be emulated in software using a fixed frequency IRQ (e.g. doing "tick++;" in the IRQ handler). |
Counters can be emulated in software using a fixed frequency IRQ (e.g. doing "tick++;" in the IRQ handler). |
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==Notes== |
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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). |
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). |
Revision as of 01:53, 4 July 2017
From Brendan post on http://forum.osdev.org/viewtopic.php?p=277285&sid=e5be2757dbd0ffdab02081df2bec8863#p277285
RTC
Discovery: Standard (assumed to exist)
Calibration: Not necessary
Access speed: Slow ISA IO ports
Counter: Not really (1 Hz if you use RTC's date/time)
Fixed frequency IRQ: Yes, up to 8 KHz (125 us) becoming unreliable at faster frequencies
IRQ on terminal count: No
PIT
Discovery: Standard (assumed to exist, but may be emulated by HPET)
Calibration: Not necessary
Access speed: Slow ISA IO ports (or slow SMM emulation of ISA ports)
Counter: No
Fixed frequency IRQ: Yes, up to 596.591 KHz (1.676 us)
IRQ on terminal count: Yes, 828 ns precision
ACPI Power Management Timer
Discovery: ACPI tables (FADT)
Calibration: Not necessary
Access speed: IO ports (may or may not be "faster, non-ISA IO ports")
Counter: Yes, 24-bit or 32-bit, monotonically increasing at 3.579545 MHz
Fixed frequency IRQ: No
IRQ on terminal count: No
HPET
Discovery: ACPI tables (HPET)
Calibration: Not necessary
Access speed: Fast (typically memory mapped device)
Counter: Yes, 64-bit monotonically increasing at 10 MHz or better
Fixed frequency IRQ: Yes, up to 10 Mhz or better (100 ns or better)
IRQ on terminal count: Yes, 100 ns or better precision
TSC
Discovery: CPUID
Calibration: Yes
Access speed: Very fast (built into CPU)
Counter: Yes, 64-bit monotonically increasing at CPU's clock speed (which can vary on old CPUs)
Fixed frequency IRQ: No
IRQ on terminal count: No
Local APIC timer (older)
Discovery: MP specification table, ACPI tables (APIC/MADT)
Calibration: Yes
Access speed: Very fast (built into CPU)
Counter: No
Fixed frequency IRQ: Yes, depends on CPU's bus/link speed, typically 100 Mhz or better (10 ns or better)
IRQ on terminal count: Yes, depends on CPU's bus/link speed, typically 10 ns or better
Local APIC timer (newer, with TSC deadline mode)
Discovery: MP specification table, ACPI tables (APIC/MADT)
Calibration: Yes
Access speed: Very fast (built into CPU)
Counter: Yes, 64-bit monotonically increasing at CPU's clock speed (which can vary on old CPUs)
Fixed frequency IRQ: No
IRQ on terminal count: Yes, depends on CPU's clock speed, typically 1 ns or better precision
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.