You press the power button. A light comes on. A fan spins. Somewhere inside the case, a cascade of events begins that will end with your operating system greeting you at a login prompt. But between the moment your finger touches that button and the moment the CPU executes its very first instruction, there is an entire world of electrical engineering that most people never think about.
This article covers the first few milliseconds of that journey. No software is running yet. No code has executed. This is pure physics: electrons moving through copper, capacitors charging, voltages stabilizing, and a clock beginning to tick.
The Power Button Is Not an On Switch
The power button on a modern computer does not directly connect the power supply to the motherboard. It is a momentary switch -- press it and release it. The button sends a short signal to the motherboard's power control circuit, which then tells the power supply to turn on.
Think of it like a doorbell. You press the button, but you are not opening the door. You are telling someone inside to open it for you.
On ATX-standard motherboards, the power button connects two pins on the front panel header. When those pins are briefly shorted together, the motherboard's standby power circuit sends a signal to the power supply unit (PSU) telling it to turn on. The PSU has been in standby mode this whole time -- a small 5-volt standby rail has been active since the power cable was plugged in. That standby power keeps the motherboard's power management logic alive, waiting for your button press.
The signal that flows from the motherboard to the PSU is called PS_ON# (the # means it is active-low -- the signal activates the PSU by being pulled to ground, not by going high). When the PSU receives this signal, it begins converting AC mains power into the DC voltages the computer needs.
The Voltage Rails
A computer does not run on a single voltage. Different components need different voltages, and the PSU provides them simultaneously on separate "rails." The three main rails on an ATX power supply are:
- +12V -- The heavy lifter. Powers the CPU (through a separate connector), graphics card, fans, and hard drives. Most of the power budget lives here.
- +5V -- Used by USB ports, some motherboard logic, and legacy components.
- +3.3V -- Powers RAM, some chipset logic, and various low-power ICs on the motherboard.
There is also -12V (rarely used on modern systems, historically for serial ports) and the +5V standby rail we already mentioned, which stays on whenever the PSU is plugged into a wall outlet.
When the PSU turns on, these rails do not instantly jump to their target voltages. Capacitors inside the PSU and on the motherboard need time to charge. The voltage on each rail climbs from zero toward its target over a period of milliseconds. This is called the ramp-up period.
Capacitors and the Ramp-Up
A capacitor is a component that stores electrical energy. Think of it as a small rechargeable battery that charges and discharges very quickly. Capacitors are everywhere in a computer -- on the motherboard, inside the PSU, on graphics cards, on RAM sticks.
During power-on, every capacitor in the system is discharged (assuming the machine has been off for a while). When the PSU begins outputting voltage, current flows into these capacitors, charging them. The voltage on each rail rises in a smooth curve, not a sudden jump.
This matters because digital logic circuits have minimum voltage requirements. A chip rated for 3.3V operation might malfunction if you try to run it at 2.1V. The system cannot begin operating until all voltages are stable and within specification.
The ramp-up is not instantaneous, and different rails stabilize at different rates. The ATX specification requires the PSU to bring all rails within tolerance before asserting a specific signal that tells the rest of the system: everything is ready.
The Power Good Signal
The PSU does not just dump voltage onto the motherboard and hope for the best. It monitors its own output. Once all voltage rails have stabilized within their specified tolerances (typically within 5% of the target voltage), the PSU asserts a signal called PWR_OK (also known as Power Good or PG).
The ATX specification says the PSU must wait between 100 and 500 milliseconds after turning on before asserting PWR_OK. This gives capacitors time to charge and voltages time to settle.
Until PWR_OK goes high, the motherboard holds the CPU in a reset state. The CPU literally cannot do anything. It sits there, held in reset by a signal from the motherboard's power management circuitry, waiting for permission to start.
This is a safety mechanism. If the CPU tried to execute instructions while voltages were still climbing, it would read garbage from memory, corrupt registers, and crash before it ever really started. The Power Good signal prevents that.
Clock Stabilization
Digital logic does not operate continuously like an analog circuit. It operates in steps, synchronized by a clock signal. Every operation the CPU performs -- reading a value, adding two numbers, writing a result -- happens on the tick of a clock.
The motherboard has a crystal oscillator that generates this clock signal. When power first arrives, the oscillator needs time to start vibrating at a stable frequency. The first few cycles are irregular -- the frequency wobbles, the amplitude is inconsistent. This is called the oscillator startup time, and it typically takes a few milliseconds.
The CPU's clock is derived from this base oscillator through a component called a Phase-Locked Loop (PLL). The PLL multiplies the base frequency up to the CPU's operating frequency. A base clock of 100 MHz might be multiplied by 35 to produce a 3.5 GHz CPU clock. The PLL also needs time to lock onto the correct frequency after power arrives.
No instruction can execute until the clock is stable. The CPU needs a predictable, regular tick to synchronize its internal operations. An unstable clock would cause timing violations -- signals arriving too early or too late -- which would produce unpredictable results.
The Sequence in Full
Let us put the whole sequence together. From the moment you press the power button to the moment the CPU is released from reset, here is what happens:
- You press the power button. The momentary switch shorts two pins on the front panel header.
- The motherboard pulls PS_ON# low. This tells the PSU to activate.
- The PSU begins converting AC to DC. Voltage rails start ramping up from zero.
- Capacitors charge. Throughout the PSU and motherboard, capacitors absorb current and smooth the rising voltages.
- Voltages stabilize. Each rail reaches its target and stays within tolerance.
- The crystal oscillator starts. It needs a few milliseconds to reach a stable frequency.
- The PLL locks. The CPU's clock multiplier circuit acquires lock on the base frequency.
- The PSU asserts PWR_OK. All rails are stable. The system has permission to start.
- The motherboard releases the CPU from reset. The reset line goes inactive.
- The CPU fetches its first instruction. The boot process has begun.
This entire sequence takes somewhere between 100 milliseconds and one second. In human terms, it is the time between pressing the button and hearing the first fan spin up. In computer terms, it is an eternity -- a modern CPU could execute billions of instructions in that time, if only it were allowed to start.
What Can Go Wrong
If the PSU cannot bring a voltage rail within tolerance, PWR_OK never asserts. The system stays in reset forever. From the outside, this looks like a dead computer -- you press the button, maybe a fan twitches, but nothing happens.
If PWR_OK is asserted too early (a defective or out-of-spec PSU), the CPU might start executing with unstable voltages. The result is unpredictable: it might boot and crash seconds later, it might corrupt data, or it might appear to work fine until the system is under heavy load and the marginal voltage sags below tolerance.
This is why a reliable power supply matters. The PSU is the foundation of every single thing the computer does. Every bit stored in RAM, every instruction the CPU executes, every packet sent over the network -- all of it depends on clean, stable voltage from the PSU.
What Comes Next
The CPU is out of reset. The clock is ticking. Voltages are stable. The processor is alive and ready to execute its first instruction.
But where does it find that first instruction? The CPU does not know what a hard drive is. It does not know about your operating system. It has no concept of files or programs. All it knows is one thing: a hardwired address in memory where it must look for its first instruction.
That address is called the reset vector, and it is the subject of the next article.
Next: The Reset Vector