On an ISA compatible system, this clock is specified as part of the ISA standard. A control program can read or set this clock only to a whole second, but it can also detect the edges of the 1 second clock ticks, so the clock actually has virtually infinite precision.
-This clock is commonly called the hardware clock, the real time clock, the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its capitalized form, was coined for use by *hwclock*. The Linux kernel also refers to it as the persistent clock.
+This clock is commonly called the hardware clock, the real-time clock, the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its capitalized form, was coined for use by *hwclock*. The Linux kernel also refers to it as the persistent clock.
-Some non-ISA systems have a few real time clocks with only one of them having its own power domain. A very low power external I2C or SPI clock chip might be used with a backup battery as the hardware clock to initialize a more functional integrated real-time clock which is used for most other purposes.
+Some non-ISA systems have a few real-time clocks with only one of them having its own power domain. A very low power external I2C or SPI clock chip might be used with a backup battery as the hardware clock to initialize a more functional integrated real-time clock which is used for most other purposes.
*The System Clock:* This clock is part of the Linux kernel and is driven by a timer interrupt. (On an ISA machine, the timer interrupt is part of the ISA standard.) It has meaning only while Linux is running on the machine. The System Time is the number of seconds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number of seconds since 1969 UTC). The System Time is not an integer, though. It has virtually infinite precision.
-The System Time is the time that matters. The Hardware Clock's basic purpose is to keep time when Linux is not running so that the System Clock can be initialized from it at boot. Note that in DOS, for which ISA was designed, the Hardware Clock is the only real time clock.
+The System Time is the time that matters. The Hardware Clock's basic purpose is to keep time when Linux is not running so that the System Clock can be initialized from it at boot. Note that in DOS, for which ISA was designed, the Hardware Clock is the only real-time clock.
It is important that the System Time not have any discontinuities such as would happen if you used the *date*(1) program to set it while the system is running. You can, however, do whatever you want to the Hardware Clock while the system is running, and the next time Linux starts up, it will do so with the adjusted time from the Hardware Clock. Note: currently this is not possible on most systems because *hwclock --systohc* is called at shutdown.
The unit specifier may be longer: *+5min*, *+6hours*, *+2days*.
*-d*, *--device* _device_::
-Use the specified _device_ instead of *rtc0* as realtime clock. This option is only relevant if your system has more than one RTC. You may specify *rtc1*, *rtc2*, ... here.
+Use the specified _device_ instead of *rtc0* as real-time clock. This option is only relevant if your system has more than one RTC. You may specify *rtc1*, *rtc2*, ... here.
*-l*, *--local*::
Assume that the hardware clock is set to local time, regardless of the contents of the _adjtime_ file.
projects / thirdparty / util-linux.git / commitdiff
