Things start rolling when you press the power button on the computer (no! do tell!). Once the motherboard is powered up it initializes its own firmware – the chipset and other tidbits – and tries to get the CPU running. If things fail at this point (e.g., the CPU is busted or missing) then you will likely have a system that looks completely dead except for rotating fans. A few motherboards manage to emit beeps for an absent or faulty CPU, but the zombie-with-fans state is the most common scenario based on my experience. Sometimes USB or other devices can cause this to happen: unplugging all non-essential devices is a possible cure for a system that was working and suddenly appears dead like this. You can then single out the culprit device by elimination.
Newer systems boot up in a different ways, but this is an interesting read.
Biohacking — Two Blood Meters to Start Your Biohacking Adventure
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My, we have come a long way from toggling bootstraps in via the front panel or soldering together matrices of individual diodes to build a boot-ROM.
what i don’t really understand is why all computers (and phones now that i think about it) seem to take so long to boot up. obviosly OS are massive now, but my C64 is (near) instant on, whereas my netbook is the best part of a minute and my work pc takes ages! progress?!
Agreed! Do you remember when you could actually flick through TV channels? Even my CD player takes 20 seconds to boot up. Progress indeed.