From Button Press to Desktop
This system has an invisible motherboard. If it were visible, the components would not be clearly seen, so it remains invisible.
0) While “off” (standby)
The PSU’s 5VSB rail is still alive. Tiny chips (RTC clock, USB charge, wake features) are idling.
1) You press the power button
- The button is just a tiny switch. It signals the motherboard’s Super I/O/EC: “turn on!”
- Motherboard tells the PSU:
PS_ON# → ON.
- PSU ramps up 12V, 5V, 3.3V rails, checks itself, then raises
PWR_OK = “voltage stable.”
2) Power flows to the board
- VRMs on the motherboard turn those rails into clean CPU/GPU/RAM voltages.
- Fans spin, LEDs blink. The CPU is released from reset.
3) First instructions (UEFI/BIOS)
- CPU jumps to a fixed address (the reset vector) and starts running code from the SPI flash (your UEFI/BIOS firmware).
- Firmware loads CPU microcode, sets up the basics.
4) Memory comes alive (the big POST step)
- Firmware reads each RAM stick’s SPD chip (what speed/timings it supports).
- Does memory training (tiny calibrations so the CPU’s memory controller and RAM talk reliably).
- If RAM fails here, you’ll get beep/LED error codes and no display.
5) Core POST checks (health & hardware)
- Checks CPU, RAM, chipset temps/fans/sensors.
- Initializes graphics (iGPU or discrete GPU) → you see the logo.
- Enumerates PCIe devices (NVMe, GPU, Wi-Fi), USB, SATA drives, etc.
- If anything critical is missing/faulty, you get a halt + code (varies by board).
6) Pick something to boot
- UEFI boot order is consulted (NVMe → SATA → USB → Network…).
- Finds an EFI System Partition, loads a boot manager (Windows Boot Manager / GRUB).
7) Hand off to the OS
- Bootloader loads the kernel (and initrd), passes control.
- The OS initializes drivers, mounts the root disk, starts services, and shows login/lock screen.
8) Desktop/Start screen
Your auto-start apps load. You’re ready.
🔎 What POST actually “tests”
- Power rails & VRMs (are voltages OK?)
- CPU present & microcode loaded
- RAM presence + training + quick integrity
- Graphics init (so you can see errors)
- Storage & PCIe/USB enumeration
- Fans/temps sanity
- Failures: boards show beeps, Q-LEDs, or hex codes. The pattern meanings are in the motherboard manual.
⏱️ Quick Timing Math (typical cold boot)
- PSU on + rails stable: 0.2–0.8 s
- Early firmware + CPU init: 0.2–0.5 s
- RAM training (DDR4): 0.8–2 s | (DDR5): 1.5–6 s
- Devices/PCIe/USB scan: 1–3 s
- Bootloader found: 0.3–1 s
- OS to login (NVMe): 2–8 s | (HDD): 8–25 s
- Total cold boot: ~5–20 s (fast rigs) up to 30+ s (slow RAM training/HDD).
🧰 Minimal parts needed to POST (show a logo)
- PSU + motherboard + CPU + at least one RAM stick
- Graphics (iGPU or a discrete GPU)
- Optional: a tiny speaker for beeps (or rely on Q-LEDs)
- Keyboard, mouse, and storage aren’t required just to POST.
🛠️ If it won’t boot—human checklist
- Nothing powers? Check PSU switch, wall, 24-pin + CPU 8-pin cables, front-panel header.
- Fans spin, no display? Try 1 RAM stick only, correct slot; reseat GPU; clear CMOS.
- Beep/LED codes? Look them up in the board manual (RAM vs GPU vs CPU).
- Loops/restarts on DDR5? Let RAM training run 2–3 cycles; load XMP/EXPO conservatively or update BIOS.
🧠 Tiny mnemonic: P-V-F-M-D-B-O
Power button → Voltages (PSU/VRM) → Firmware (UEFI) → Memory (train & test) → Devices (PCIe/USB/Storage) → Boot target (EFI/Bootloader) → OS (kernel → drivers → desktop)