Time & Time Sync Technology FAQContents:Related links:Two time-of-day clocks reside in every IBM-compatible computer. These clocks go byseveral different names, but for simplicity, we'll call them the software and hardwareclocks. The software clock runs only when the computer is turned on. It stops when thecomputer is turned off. The hardware clock uses a battery and runs even while the computeris turned off.Every computer that runs DOS or Windows has a built-in software clock within theoperating system. On old PC or XT-compatibles, this clock was driven by the Intel 8253timer-counter chip or an equivalent device. AT-compatible computers use a deviceequivalent to an Intel 8254 timer-counter.
The timer-counter is programmed by the BIOS togenerate an interrupt every 54.936 milliseconds, or about 18.206 times per second. AnotherBIOS routine counts the interrupt requests and generates a time-of-day clock that can beread or set by other software programs. For example, Windows uses the information from thesoftware clock when it date and time stamps files.The software clock is useful, but it has several limitations. First, the software clockis a poor timekeeper. Its accuracy is limited by the stability of the interrupt requests.Any change in the interrupt request rate causes the clock to gain or lose time. If youleave your computer turned on for long periods, the software clock might be off by largeamounts, perhaps a minute or more for every day that the computer was left turned on.
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It'salso possible for an ill-behaved software program to use the timer-counter for anotherpurpose and change its interrupt rate. This could cause the clock to rapidly gain or losetime.Another problem with the software clock is that it cannot display all possibletime-of-day values. The resolution of the clock is limited to the interval betweeninterrupts, or about 55 milliseconds as stated earlier. Only times that are even multiplesof this interval can be displayed. For example, 00:00:01.00 could never be displayed bythe software clock.
The closest possible values it can display are 00:00:00.98 and00:00:01.04.The single biggest limitation of the software clock, however, is that when the computeris turned off the clock stops running. On the original IBM-PC, this meant that youmanually had to set the clock each time you turned the computer on. You could purchase anoptional battery-backed clock for the PC, but there were several different standards, andnot all of them worked with all software packages. This problem was addressed with theintroduction of the IBM-AT in 1984, which included a battery-backed hardware clock asstandard equipment.
An AT-compatible hardware clock is included with every 'WinTel'computer produced today.The hardware clock is based on the Motorola 146818 Real Time Clock Chip, or afunctionally equivalent device. The clock is supported by the BIOS, and BIOSservices are available that let software programs read and set the clock.The hardware clock is a CMOS device that consumes very little power. When the computeris turned off, it runs on batteries. When the computer is turned back on, the softwareclock starts running again and sets itself (within 1 second) to the hardware clock.Although the two clocks are synchronized at start-up, they may run at very different ratesand will probably gain or lose time relative to each other while the computer is running.The hardware clock is updated once per second and cannot display fractions of a second.For this reason, it cannot be read or set within better than a second. The accuracy of thehardware clock is determined by the quality of its timebase oscillator (typically a 32.768kHz crystal).
These crystals are economical, costing less than $1 in single quantities.However, they offer only marginal timekeeping performance. They are sensitive totemperature and other factors and are often not calibrated at the factory.
Even under thebest conditions, these oscillators are not likely to be stable to better than 1 part permillion (about 0.1 seconds per day). In actual operation, most hardware clocks seem togain or lose time at a rate of about 1 to 15 seconds per day, with 5 or 6 seconds per daybeing typical. Although the hardware clock usually outperforms the software clock by aconsiderable amount, its performance often pales in comparison to even a low-costwristwatch.As you can tell by now, neither the software nor hardware clock is suitable for accuratetimekeeping. Fortunately, however, there are several ways you can keep accurate time onyour PC if your application demands it.The following discusses howa PC keeps time and the ways you can improve on it.I.The Basic PC Clocking ModelTimekeeping is done at several pointswithin the modern PC. The motherboard contains an on-board clock maintained and accessedby the BIOS, the board level operating system. The BIOS clock keeps time and is readby the operating system at boot time. Application software that uses time and datesaccesses the operating system clock.
The advantages of this system are simplicity in itsstraightforward approach; the major disadvantages are the poor time keeping ability on theBIOS level and the vulnerability of the operating system clock to be thrown off bydemanding software or hardware. ClockWatchClockWatch coordinates time settings connecting with an external timestandard. It sets both the operating system and BIOS level clocks.Atomic ClockTheNational Institute of Standards and Technology maintains a precision time source based onradioactive isotopes.
It is accessible over the Internet or by direct connection viamodem.Periodicallychecking an internal timekeeper keeps the PC clocks at a more stable and predictabletime setting. ClockWatch provides a means to synchronize the operating system on aregular basis. ClockCardBeagle Software's ClockCard provides anadjunct, stable Real Time Clockbased on a crystal oscillator. Every time the system is booted or the operating systemmakes a BIOS time request, the ClockCard intercepts the request and returns the moreaccurate time.ClockWatchOn win 95/98 Systems, ClockWatch checks ClockCard periodically to update theoperating system clock - on Windows NT systems the checking is done by the operatingsystem every hour.III.The to precision time keepingCombining the Internal and Externalapproaches yields a powerful two-tiered approach to time synchronization. ClockCardprovides a stable clock to be used for frequent time checks. The external time sourceprovides a definitive time source and is used to set the entire system at standard time.This approach provides the most comprehensive approach to precision time keeping. Computerhardware (BIOS)Internal Time Synchronization(ClockCard)Computer operating system (Windows)Synchronization Software (ClockWatch)External Time Synchronization (Atomic Clock)We've all heard about the Year 2000problem and that it will cause problems with PCs.
Just how does it effect PCs, and morespecifically what can you do to prevent this from being a problem for your organization?Below is a discussion based on a White paper by Karl Feilder, of Greenwich MeanTime, an authority on Y2K problems.TheFive-Layered Approach to the Year 2000 PC IssueWe can model the interaction between the hardware and software of a PC as a five-layered'sandwich' - each layer representing a type of hardware or software activity. The mostimportant thing to understand about the Y2K problem is that it occurs across all fivelayers of the PC. 1.2.3.4.5.Each layer ispotentially affected by the Year 2000 PC problem, which increases in complexity as itprogresses from 1 to 5.
So the BIOS (hardware) layer is more quickly and easily resolvedthan the data sharing layer. Since each layer is built upon the other it is especiallyimportant that the lower layers (hardware and O/S) are correct.The ProblemThe BIOS (basic input/output system) initializes every PC on start up and passes date andtime information to the operating system and other software programs. The BIOS in each PCsets up the system date and time by readingand automatically correctingthetime in the battery-powered clock chip in the PC.
If the BIOS date and time are wrong (andnew BIOSs can be affected, even now), the data generated by accounting, spreadsheet,inventory, scheduling and payroll software programs cant be trusted.The Solution. Locate and audit PCs. Assess hardware status.
Fix BIOSs. Repair or replace BIOSs that can't be fixed. ClockWatch can:Audit the PC's it is running on -performing checks several times a day.
It will highlight issues that occur in the BIOS,fixing small errors and alerting you to larger errors.The ProblemThe operating system is sometimes responsible for feeding information to the other layers,so it is critical that it operates correctly. The standard installation of most commonoperating systems (including newer operating systems) is not optimized for 2000, whichputs successive layers at risk.The Solution. Discover what operating systems are in use and where.
Reconfigure to operate correctly. Upgrade/replace those operating systems that can't be corrected. ClockWatch can:Monitor for problems that may crop up inthe operating system, correcting the minor problems and alerting you to major problems.The ProblemEvery software program treats data a little differently. Independent research shows that64% of PC software programs exhibit potential Year 2000 problems in normal usage mode.This research has also identified no less than 73 shades of gray at thislayer. For instance, software programs may be forced to guess the century when the userenters only the last two digits of the yearand each software program may guess adifferent century.
Assumptions made by software programs are generally not obvious to theuser.The Solution. Identify what software programs are running within the organization and where theyreside. Find all custom software installed on the PC. Identify mission-critical software programs. Understand what risks they pose to your business if they can't operate correctlyin 2000. Repair, patch, replace or upgrade.The ProblemData is fed into a PC, where it is turned into information. If the data includes dates, aproblem arises because most PCs cannot allow dates to span a century; years are entered astwo digits (YY), yet most PC programs calculate in four-digit years (CCYY).
Most programsautomatically expand YY dates to CCYY dates, making a variety of century assumptions. Datais thus often turned into incorrect information, but this is not apparent to the user whodoesnt know what century the program is assuming. The key is to focus on mission-critical systems and data so the business can continue operating through 2000 and beyond.The Solution. Discover how many files exist in the organization. Pinpoint where they reside and how old they are. Establish how theyre affected by the Year 2000 problem. Define their role in the organization.
Prioritize what files need to be repaired first. Repair data so it can be used in 2000.The ProblemPC users constantly share data via disk, e-mail, the Internet, EDI, across networks, andeven when users 'cut and paste' or 'drag and drop.' When data isshared between one programor PCand another, it might be modified during thetransition. This is a bit like spreading a virus. In most cases it happens unknowingly andgoes unnoticed. Problems that occur at all of the other layers are compounded when data isexchanged.The Solution. Identify data sharing points.
Limit exposure to non-compliant data. Keep existing data clean.The system clock ona Windows 95-based computer may experience a cumulative loss of time.When you run the Date/Time tool in Control Panel, the computer's real-time clock (RTC)is updated using the MS-DOS system time.
Because the RTC is being updated by a device thatis less accurate, a cumulative loss of time can occur over a period of time.The MS-DOS system time is accurate to 1/18 of a second. Each time you use the Date/Timetool, an incorrect time is written to the computer's RTC. The amount of time lost dependson how often the Date/Time tool is opened and the amount of time since it was last opened.For standalone computers, this may not be much of a problem. However, with networkprograms that rely upon time and date stamps to ensure the proper file is accessed, thiscan cause problems over a period of time.Synopsis of Microsoft Knowledge Base Article ID: Q157619If the time is continuously read on an x86-based Windows NT 3.1 machine,the system appears to lose a second each hour.The Real Time Clock (RTC) in x86-based computers has a resolution of one second.Therefore, the Hardware Abstraction Layer (HAL) on x86 platforms uses an interval timer toincrease the resolution of the system clock into the millisecond range.
Windows NT'scalculation of real time based on this interval timer is inaccurate enough that it couldget out of sync by over a second every hour.Synopsis of Microsoft Knowledge Base Article ID: Q106434Every PC contains a Real Time Clock (RTC) implemented in the hardware.This clock runs continuously and provides time to the operating system clock when thecomputer is booted. The RTC runs off the same power source as the system BIOS.The System BIOS has information saved in a little piece (64 bytes) of CMOS RAM. TheCMOS power is supplied by a small battery, so its contents will not be lost after the PCis turned off. Therefore, there is a battery and a small RAM memory on board, which shouldnever lose its information. The memory was in earlier times a part of the clockchip; nowit's part of a highly integrated circuit.
CMOS technology needs little power so thecomputer's battery is not in use much. Actually, there is not a battery on new boards,but an accumulator (Ni-Cad in most cases).
It is recharged every time the computer isturned on. Some new motherboards have a technology called Dallas Nov-Ramwhich eliminateshaving an on-board battery as there is a 10 year lithium cell epoxyed directly in thechip.If your BIOS is powered by external batteries, be sure that they are ingood operating condition. Also, be sure that they do not leak - that may damage themotherboard and cause your BIOS to suddenly 'forget' its configuration (andits time) and you may be looking for a problem elsewhere.Some system BIOS adjust the computers built-in real-time clock tocompensate for daylight savings time automatically. Windows NT 3.5 can also be configuredto adjust for daylight savings time. If both the BIOS and Windows NT adjust the close fordaylight savings time, your clock will be one hour off.Windows NT updates its internal date and time every hour by reading the CMOS on themotherboard. In-between the one hour intervals, Windows NT increments its own clockapproximately every 10 milliseconds. Because Windows NT cannot detect when the systemBIOS/CMOS (hardware level) adjusts the clock for daylight savings time, it may change theclock by one hour, just as the BIOS does.
This causes your computer clock to be one hourahead in the Spring or one hour behind in the Fall after both your computer's BIOS andWindows NT adjust the clock for daylight savings time.If you boot your computer under both Windows 95 and Windows NT, both operating systemswill adjust for daylight savings time independently, also causing the clock to be off byan hour.Synopsis of Microsoft Knowledge Base Article ID: Q139452See also Knowledge Base article: Q129574, Time Stamp Changes with Daylight Savings.This information applies to Microsoft Windows 95/98 & MicrosoftWindows 98 Second Edition. Symptoms. When you first turn your computer on, the time as shown in the taskbarclock is correct. After you leave your computer on for an extended amount of time, the timemay lose from two minutes up to an hour per day.Cause. You configure your computer to use third-party anti-virus, system utility,and screen saver programs.ResolutionDisable the following types of third-party programs and utilities:. Anti-virus.
Screen savers. System utilitiesStatusMicrosoft has confirmed this to be a problem in the Microsoft products listedat the beginning of this article.
More informationThe CMOS does keep the correct time, and if you restart the computer theWindows clock is updated. Also, if you start your computer in Safe Mode, Windowsdoes not lose time.NOTE: This problem does not affect the clock in your computer's CMOS.Synopsis of Microsoft KnowledgeBase Article Q189706(Updated 11/16/99)SymptomsThe clock on the taskbar and in the Date/Time tool in Control Panel may experience aloss of time. CauseThis problem can occur for either of the following reasons:. You change the year in the Date/Timetool. When you click a different year in the Date/Time Properties dialog box, the clockstops.
When you click Apply or OK, the clock starts again, but it does not compensate forthe length of time it was stopped. You change the month or date in theDate/Time tool.
When you click a different month or date in the Date/Time Propertiesdialog box, the current time is decreased by 5 to 10 seconds. Over time, this can resultin a significant time loss.