How a Computer Restarts

Have you ever wondered how a computer restarts? When your computer is seemingly powered down for a second or two, how does it roar back to life? Better yet, how does the hardware actually restart? Do you just cut the power to various processors, memory chips, and controllers, or is it far more complicated

This fundamental question, which most computer users have probably wondered about but never bothered to research, was recently asked over on Superuser, a Stack Overflow site.The answers, as you would expect, are comprehensive — and if you’re into computers, they’re really damn interesting, too.

The basic gist is this: we started off with AT power, which is “hard power”: when you flip the switch, power is instantly cut to the motherboard. Then, with ATX, “soft power” — software-controlled power — made it possible for the operating system to power off or reboot the system. ATX brought a ton of power management changes, but fundamentally it introduced the concept of standby power, where a 5-volt line to the motherboard is always switched on, even when you’re powered down. This 5V current keeps your BIOS, network adaptors, and other basic components ticking over — which is how you turn a computer on with magic packets. In other words, while a computer is plugged in at the wall, it is never truly off — it’s just resting.

But how does a modern, ATX-powered computer actually reboot? Because the 5V current is always there, the motherboard doesn’t physically turn off and on — rather, a reset command is sent to the computer’s power control system: the Advanced Configuration and Power Interface (ACPI). When this command is received, the ACPI is then tasked with sending reset commands to all logic chips, including the CPU and memory controller. For the most part, this just involves firing a signal down each chip’s reset (RST) pin. Once the computer’s logic has been reset, ACPI proceeds with the standard boot process, as if the power button had just been pressed. Voila, a rebooted computer!

As a corollary, do laptops, tablets, and smartphones reboot in the same way? Do laptops, even in their powered-off state, have a 5V standby line? We can only hope that someone chimes in over at Superuser — or perhaps a knowledgeable reader knows?

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