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http://muhdzamri.blogspot.com/ Clocks in a Linux System There are two main clocks in a Linux system: The Hardware Clock: This is a clock that runs independently of any control program running in the CPU and even when the machine is powered off. On an ISA system, this clock is specified as part of the ISA standard. The control program can read or set this clock to a whole second, but the control program 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 because all of the other names are inappropriate to the point of being misleading. So for example, 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 Time: This is the time kept by a clock inside the Linux kernel and 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). 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 in a Linux system is to keep time when Linux is not running. You initialize the System Time to the time from the Hardware Clock when Linux starts up, and then never use the Hardware Clock again. 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(1L) program to set it while the system is run- ning. 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. You can also use the program adjtimex(8) to smoothly adjust the System Time while the system runs. A Linux kernel maintains a concept of a local timezone for the system. But don’t be misled -- almost nobody cares what timezone the kernel thinks it is in. Instead, programs that care about the timezone (perhaps because they want to display a local time for you) almost always use a more traditional method of determining the timezone: They use the TZ environment variable and/or the /usr/share/zoneinfo directory, as explained in the man page for tzset(3). However, some programs and fringe parts of the Linux kernel such as filesystems use the kernel timezone value. An example is the vfat filesystem. If the kernel timezone value is wrong, the vfat filesystem will report and set the wrong timestamps on files. hwclock sets the kernel timezone to the value indicated by TZ and/or /usr/share/zoneinfo when you set the System Time using the --hctosys option. The timezone value actually consists of two parts: 1) a field tz_minuteswest indicating how many minutes local time (not adjusted for DST) lags behind UTC, and 2) a field tz_dsttime indicating the type of Daylight Savings Time (DST) convention that is in effect in the locality at the present time. This second field is not used under Linux and is always zero. (See also settimeofday(2).) How hwclock Accesses the Hardware Clock hwclock Uses many different ways to get and set Hardware Clock values. The most normal way is to do I/O to the device special file /dev/rtc, which is pre- sumed to be driven by the rtc device driver. However, this method is not always available. For one thing, the rtc driver is a relatively recent addition to Linux. Older systems don’t have it. Also, though there are versions of the rtc driver that work on DEC Alphas, there appear to be plenty of Alphas on which the rtc driver does not work (a common symptom is hwclock hanging). Moreover, recent Linux systems have more generic support for RTCs, even systems that have more than one, so you might need to override the default by specifying /dev/rtc0 or /dev/rtc1 instead. On older systems, the method of accessing the Hardware Clock depends on the system hardware. On an ISA system, hwclock can directly access the "CMOS memory" registers that constitute the clock, by doing I/O to Ports 0x70 and 0x71. It does this with actual I/O instructions and consequently can only do it if running with superuser effective userid. (In the case of a Jensen Alpha, there is no way for hwclock to execute those I/O instructions, and so it uses instead the /dev/port device special file, which provides almost as low-level an interface to the I/O subsystem). This is a really poor method of accessing the clock, for all the reasons that user space programs are generally not supposed to do direct I/O and disable interrupts. Hwclock provides it because it is the only method available on ISA and Alpha systems which don’t have working rtc device drivers available. On an m68k system, hwclock can access the clock via the console driver, via the device special file /dev/tty1. hwclock tries to use /dev/rtc. If it is compiled for a kernel that doesn’t have that function or it is unable to open /dev/rtc (or the alternative special file you’ve defined on the command line) hwclock will fall back to another method, if available. On an ISA or Alpha machine, you can force hwclock to use the direct manipulation of the CMOS registers without even trying /dev/rtc by specifying the --directisa option. The Adjust Function The Hardware Clock is usually not very accurate. However, much of its inaccuracy is completely predictable - it gains or loses the same amount of time every day. This is called systematic drift. hwclock’s "adjust" function lets you make systematic corrections to correct the systematic drift. It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical information. This is called the adjtime file. Suppose you start with no adjtime file. You issue a hwclock --set command to set the Hardware Clock to the true current time. Hwclock creates the adjtime file and records in it the current time as the last time the clock was calibrated. 5 days later, the clock has gained 10 seconds, so you issue another hwclock --set command to set it back 10 seconds. Hwclock updates the adjtime file to show the current time as the last time the clock was calibrated, and records 2 seconds per day as the systematic drift rate. 24 hours go by, and then you issue a hwclock --adjust command. Hwclock consults the adjtime file and sees that the clock gains 2 seconds per day when left alone and that it has been left alone for exactly one day. So it subtracts 2 seconds from the Hardware Clock. It then records the current time as the last time the clock was adjusted. Another 24 hours goes by and you issue another hwclock --adjust. Hwclock does the same thing: subtracts 2 seconds and updates the adjtime file with the current time as the last time the clock was adjusted. Every time you calibrate (set) the clock (using --set or --systohc ), hwclock recalculates the systematic drift rate based on how long it has been since the last calibration, how long it has been since the last adjustment, what drift rate was assumed in any intervening adjustments, and the amount by which the clock is presently off. A small amount of error creeps in any time hwclock sets the clock, so it refrains from making an adjustment that would be less than 1 second. Later on, when you request an adjustment again, the accumulated drift will be more than a second and hwclock will do the adjustment then. It is good to do a hwclock --adjust just before the hwclock --hctosys at system startup time, and maybe periodically while the system is running via cron. The adjtime file, while named for its historical purpose of controlling adjustments only, actually contains other information for use by hwclock in remem- bering information from one invocation to the next. The format of the adjtime file is, in ASCII: Line 1: 3 numbers, separated by blanks: 1) systematic drift rate in seconds per day, floating point decimal; 2) Resulting number of seconds since 1969 UTC of most recent adjustment or calibration, decimal integer; 3) zero (for compatibility with clock(8)) as a decimal integer. Line 2: 1 number: Resulting number of seconds since 1969 UTC of most recent calibration. Zero if there has been no calibration yet or it is known that any previous calibration is moot (for example, because the Hardware Clock has been found, since that calibration, not to contain a valid time). This is a dec- imal integer. Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set to Coordinated Universal Time or local time. You can always override this value with options on the hwclock command line. You can use an adjtime file that was previously used with the clock(8) program with hwclock. Automatic Hardware Clock Synchronization By the Kernel You should be aware of another way that the Hardware Clock is kept synchronized in some systems. The Linux kernel has a mode wherein it copies the System Time to the Hardware Clock every 11 minutes. This is a good mode to use when you are using something sophisticated like ntp to keep your System Time syn- chronized. (ntp is a way to keep your System Time synchronized either to a time server somewhere on the network or to a radio clock hooked up to your sys- tem. See RFC 1305). This mode (we’ll call it "11 minute mode") is off until something turns it on. The ntp daemon xntpd is one thing that turns it on. You can turn it off by running anything, including hwclock --hctosys, that sets the System Time the old fashioned way. To see if it is on or off, use the command adjtimex --print and look at the value of "status". If the "64" bit of this number (expressed in binary) equal to 0, 11 minute mode is on. Otherwise, it is off. If your system runs with 11 minute mode on, don’t use hwclock --adjust or hwclock --hctosys. You’ll just make a mess. It is acceptable to use a hwclock --hctosys at startup time to get a reasonable System Time until your system is able to set the System Time from the external source and start 11 minute mode. ISA Hardware Clock Century value There is some sort of standard that defines CMOS memory Byte 50 on an ISA machine as an indicator of what century it is. hwclock does not use or set that byte because there are some machines that don’t define the byte that way, and it really isn’t necessary anyway, since the year-of-century does a good job of implying which century it is. If you have a bona fide use for a CMOS century byte, contact the hwclock maintainer; an option may be appropriate. Note that this section is only relevant when you are using the "direct ISA" method of accessing the Hardware Clock. ACPI provides a standard way to access century values, when they are supported by the hardware. |
