The boot sequence today is far more complex than it was even a decade ago. Here's a detailed, low-level, step-by-step walkthrough of the boot up.
Taking a lot of little steps along a path is a good analogy for understanding boot flow in Intel Architecture-based systems. The minimum firmware requirements for making a system operational and for booting an operating system are presented in this article. The vast majority of systems perform these steps in this article to do a full or cold boot. Depending on the architecture of the BIOS, there may be multiple software phases to jump through with different sets of rules, but the sequence for waking up the hardware is, at least in the early phases, very much the same.
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.
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