eli5: How does “restart” work? when a device, e.g. windows PC is turned off, how does in know to turn itself back on?

Question

Edit: Typo in title🤦‍♀️

How does it* know to turn itself back on?

In: 114

Answer 1

In older PCs and similar devices, there was a physical power switch. You’d flip it or press it and it would break the physical connection between the wall current and the power supply for the device.

In more modern systems there are circuits that wait for a small signal, like the voltage generated by a button being pressed. When they detect it, they’ll send another signal into the control circuitry of the device to either flip a physical relay (you’ll hear a “click” sound in these ones) or to cause other solid-state circuitry that carries much higher currents to be enabled and send power to the main device.

When a computer is first powered on its memory, registers (tiny bits of memory used to work on something directly) and other devices are usually in a random state, so in order to clear that up, initialize the system and put all the pieces in place- set the stage, essentially- the system must “bootstrap” itself, meaning it has to lift itself by its own boot straps. That’s since become shortened to “boot”.

When a system boots up, there will be some hard-coded instructions somewhere in the form of read-only-memory, that can’t be altered. The CPU is physically wired to start at a specific, hard-wired set of instructions. Those instructions are extremely basic, and do very little except tell the computer how to start, and usually to load the “Basic Input/Output System” (aka BIOS), which is also stored in non-volatile, but modifiable if you have the right tools, memory.

The BIOS then takes over and starts the process of initializing devices that the CPU hasn’t yet (keyboard, mouse, hard drive interfaces, USB bus, etc) and then starts the process of loading the actual operating system- Windows, MacOS, Linux, whatever.

Other devices that rely on microprocessors will all work in a similar way, and while they may use slightly different processes or call things by different names, do them in different order, it’s all variations on a theme.