How Your Computer Displays Correct Time When Turned On After Years?

Short Bytes: Your computer can remember the date and time for years. It does so by using a small chip called Real-time Clock or CMOS chip which is powered by a battery. For Macs, the same task is done using a battery-powered chip called PRAM.

It was once when I was in 7th grade. I turned on my desktop computer after almost a year. I glanced at the screen a few moments after the PC booted and I was surprised to know it displayed correct date and time after a year of no operation. Also, the computer wasn’t connected to the internet as I forgot to turn on the router so that it could sync the time.

To satisfy my curiosity, I researched on the web and came to know that there is a semiconductor chip present on the motherboard which is assigned the job of keeping a note of the time when the computer disconnects from the power source.

The RTC chip remembers the date and time.

This timekeeper chip is called the Real-time Clock (RTC) or the CMOS ( Complementary Metal-oxide Semiconductor) chip. The CMOS chip (known as CMOS RAM) is a memory which also stores the BIOS information of the computer. And it is a volatile memory, that’s why It is driven by a low voltage battery having a life of around five to ten years. On desktop computers, the battery looks like a silver colored coin.

real-time-clock-computer — Image: Wikipedia Commons

The RTC chips come in a compact form, often not user-replaceable, in newer computers including laptops. In fact, the need for such chips in machines is declining as high-speed connectivity is being adopted at a tremendous rate. The computers can easily sync time using the internet. Single board computers like the Raspberry Pi don’t feature a real-time clock chip.

Single board computers like the Raspberry Pi don’t feature a real-time clock chip. Other compact devices mobile phones have the real-time clock built on the chip itself but maybe without a battery in order to save space. The RTC is then powered by the mobile phone’s main battery. That’s the reason the time on your mobile resets when you remove the battery.

What about the Macintosh?

You know, Apple has its own breed of computers. The CMOS chip is mostly found on the computers running non-Apple operating systems like Windows. For Macintosh machines, the chip is PRAM or Parameter RAM.

PRAM is also a battery-powered chip using keep date and time information on the Macintosh machines. On newer Macs, the place of PRAM is taken by NVRAM (Non-Volatile RAM) which doesn’t require any battery to keep the data alive.

The NVRAM can also store information, quickly like the screen resolution, speaker volume, startup disk selection, etc.

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