system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's actually happening within a standard computer as it's booting up?
gpt
### First Stage Boot Loader (Your Motherboard's Firmware)

1. Power comes on. Motherboard chip is physically programmed to go to a known location where it expects to find a boot loader.
2. The motherboard chip executes the boot loader, which tells it put some instructions into memory for the CPU. The instructions say look at the very beginning of Drive 1, Drive 2, DVD-drive, and USB port in that exact order.
2. Motherboard chip tells CPU to go execute those instructions that it just loaded into memory.
3. CPU reads instructions (which say go look at the beginning of Drive 1) and so it goes and looks at the beginning of Drive 1
4. If it finds a program at the beginning of drive 1 (ie. a Master Boot Record), it reads the MBR and does whatever the MBR says
5. If it doesn't find an MBR, it goes to the next instruction from step 2, which was look at the beginning of drive 2. This repeats until an MBR is found or there's no more instructions and the computer craps out
6. Once an MBR is found, it (the MBR) tells the CPU to load some more stuff into memory and then tells the CPU to execute that stuff. This stuff is the second-stage boot loader.

### Second-Stage Bootloader (GRUB, BOOTMGR, etc.)
1. The second-stage boot loader starts loading the kernel, where the device drivers live
2. It then figures out what operating system to load or may even hand off to another boot loader (like a dual-boot linux/windows machine)
3. It checks to see if the operating system told it to install some stuff before you shut it down last time, and if it did, will do those installs
4. It then loads the preliminary instructions for the operating system into memory.
5. It hands off to the OS by telling the CPU to go execute the operating system instructions it just loaded

### The Operating System (Ubuntu, Windows, OSX)

1. The operating system makes sure all the drivers loaded up
2. The OS checks itself to make sure everything is okay and that all the updates went through fine. It may end up needing to do more updates and will just restart your computer again, in which case start over from the top
3. Your OS loads things that it needs to work. This is usually the process manager, permissions stuff, watchdogs, and base services (like display service, time service, disk services, etc)
3. The OS starts to load services, that you, the user have set for your startup. This includes establishing your wireless connection, your anti virus, RealPlayer, chat service, Steam, etc.
4. The OS presents you with the loaded system (usually a login screen)