What happens between Power On and the CPU initializing?
Hardware Power Sequences: The Pre-Pre-Boot
When someone pushes the power button, the CPU can't simply jump up and
start fetching code from flash memory. When external power is first
applied, the hardware platform must carry out a number of tasks before
the processor can be brought out of its reset state.
The first task is for the power supply to be allowed to settle down to
its nominal state. Once the primary power supply settles, there are
usually a number of derived voltage levels needed on the platform. For
example, on the Intel Architecture reference platform the main input
supply is a 12-volt source, but the platform and processor require
voltage rails of 1.5, 3.3, 5, and 12 volts. Voltages must be provided
in a particular order, a process known as power sequencing. The power
is sequenced by controlling analog switches, typically field-effect
transistors. The sequence is often driven by a Complex Program Logic
Device (CPLD).
Platform clocks are derived from a small number of input clock and
oscillator sources. The devices use phase-locked loop circuitry to
generate the derived clocks used for the platform. These clocks take
time to converge.
It is only after all these steps have occurred that the
power-sequencing CPLD can de-assert the reset line to the processor,
as illustrated in Figure 1. Depending on integration of silicon
features, some of this logic may be on chip and controlled by
microcontroller firmware that starts prior to the main processor.
Once the processor reset line has been de-asserted, the processor
begins fetching instructions.
Source Booting an Intel Architecture System, Part I: Early Initialization
What is the clock signal?
So, what is clock anyway? Clock is a signal used to sync things inside
the computer. Take a look at Figure 2, where we show a typical clock
signal: it is a square wave changing from “0” to “1” at a fixed rate.
On this figure you can see three full clock cycles (“ticks”). The
beginning of each cycle is when the clock signal goes from “0” to “1”;
we marked this with an arrow. The clock signal is measured in a unit
called Hertz (Hz), which is the number of clock cycles per second. A
clock of 100 MHz means that in one second there is 100 million clock
cycles.
In the computer, all timings are measured in terms of clock cycles.
Source Clock
Further reading