eli5: How does a phone/computer process the power button being pressed if it’s off?

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I tried google and all I got was “pressing the power button to turn it off won’t damage it anymore.”

In: Technology

A modern phone or computer is never off. It’s just in a deep sleep, waiting for your demands. That’s also why the clock can keep up when it’s ‘off’.

There’s a little bit of standby power in every device, as long as it’s plugged in.

Pressing the power button completes a circuit; that circuit uses the standby power to energize other circuits, which energizes still *other* circuits until the device is operating at full capacity again.

There are two basic solutions, but in both the button is really just a signal, it’s not the actual source of power to the entire device.

In the first, the device is not 100% off, but rather there is a tiny circuit still awake drawing a **very** small amount of power, waiting for the button to be pressed. When it sees the button pressed, it will activate the rest of the machine into waking.

The other solution has the power button run from the power source to a small circuit that is off until briefly powered up when the button is pressed. When it wakes, it will wake the rest of the circuitry and the device will power up.

It’s kind of like someone listening for a bell to ring in a shutdown factory, when they hear it, they will start powering up everything in the building (lights, machinery, etc) and getting it all going. That little bell didn’t ’t do anything, but it woke up the one guy who could turn it all on.

For computers:

When you power the computer, there’s a single hardwired instruction to the CPU that is executed (a JUMP). It tells the CPU to load the first instruction of the BIOS. The BIOS is a chip with ROM memory that has the set of instructions, it first test if the devices are ok, and if everything is ok, it will load the first instructions stored on where it is booting (mainly the HD, but the user can also boot from another device, like a flash drive, or a DVD), those first instructions basically tells the the CPU where everything is stored, and then loads a program called bootloader. The bootloader will then load the kernel, and from there, the kernel will take control of the computer.