O que acontece entre o Power On e a inicialização da CPU?
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.
Fonte Iniciando um sistema de arquitetura Intel, Parte I: Inicialização antecipada
Qual é o sinal do relógio?
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.
Fonte Relógio