Ask Anything: What Happens In Your Computer When You Turn It On?

No, the little mouse inside it does not start scrambling around.

Inner Workings

Studio shot of white mouse in exercise wheelJamie Grill/Getty Images

Before a computer can load its operating system and locate drivers for its components, it must recognize that it has been turned on at all. This task falls to a relatively simple device known as the power-on reset circuit. “In its very basic form, it’s simply a little capacitor and a resistor and a little inverter, something like that,” says David Blaauw, a professor in the Michigan Integrated Circuits Laboratory at the University of Michigan. The power-on reset circuit is hardwired to recognize a minimum voltage level. When voltage crosses that threshold, the circuit signals the processor to execute a set of commands that allow for the loading up of the system.

This “instruction zero” could be hardwired, says Blaauw, but it’s generally placed on a flash chip on the computer’s motherboard. That way, the details of what it says can be left to the people who put the computer together.

Power-on reset circuits tell a computer how to start but also how to stop. Left unsupervised, a loss of voltage can be very damaging. At zero voltage, a processor won’t do anything at all, but at very low voltages, its behavior turns erratic. In a worst-case scenario, it may start making unpredictable changes to memory locations. That’s where the power-on reset circuit comes in: “It will detect that you have lost power,” says Blaauw, “and then it will assert a reset signal that stops the processor from doing anything foolish.”

This article originally appeared in the March 2014 issue of Popular Science.