What happens when you click your PC's power button?
Tracing the hardware journey from closing an electrical circuit on your motherboard, through PSU voltage regulation, POST hardware checks, and UEFI bootloader handover to the OS kernel.
> *Published on Kernel Thoughts (Hashnode). Read original article at [kernel-thoughts.hashnode.dev](https://kernel-thoughts.hashnode.dev/what-happens-when-you-click-your-pc-s-power-button).*
Pressing the front-panel power button on your PC isn't a software action—it is a physical electrical event that initiates the entire system boot sequence.
The 4-Stage Power Boot Sequence
+------------------+ +------------------+ +------------------+
| 1. Short Circuit | ---> | 2. PSU Power-OK | ---> | 3. UEFI / POST |
| (Front Panel) | | (Voltage Stable) | | (Hardware Test) |
+------------------+ +------------------+ +------------------+
|
v
+------------------+
| 4. OS Bootloader |
| (Kernel Handover)|
+------------------+1. Closing the Front Panel Circuit
When you press the power button, you temporarily bridge two header pins on your motherboard (`PWR_BTN` and `GND`). This sends a momentary active-low signal to the Super I/O chip or Embedded Controller (EC).
2. PSU Activation & Power-OK Signal
The EC pulls the `PS_ON#` line down to ground, telling the Power Supply Unit (PSU) to turn on main rails (+12V, +5V, +3.3V). Once voltages stabilize, the PSU sends a `POWER_OK` (or `PWR_GOOD`) signal back to the motherboard.
3. CPU Reset & Firmware Execution (BIOS / UEFI)
Receiving `POWER_OK` releases the CPU reset line. The CPU initializes in real mode (or 64-bit flat mode on modern x86_64 chips) and executes its first instruction at the reset vector (`0xFFFFFFF0`), jumping to UEFI firmware stored in SPI flash memory.
1. **POST (Power-On Self-Test)**: Validates RAM integrity, GPU initialization, and peripheral buses. 2. **Boot Device Selection**: Reads the EFI System Partition (ESP) formatted as FAT32.
4. OS Kernel Handover
The UEFI firmware loads the bootloader executable (e.g. `systemd-boot`, `GRUB`, or Windows Boot Manager). The bootloader loads the kernel image (`vmlinuz`) and `initramfs` into RAM, invokes `start_kernel()`, and transfers total control to the operating system.
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*Note: Holding the power button for 4–5 seconds forces the EC to cut `PS_ON#` immediately, performing a hard shutdown that bypasses OS filesystem sync callbacks.*