The moment you push the power button to open your computer, the following steps occur. Hence, when the power button is pressed, a signal is sent to computer power supply, which performs the conversion of alternating current into direct current, providing the necessary electricity and voltage to the computer. After that, a signal is sent to the computer processor (CPU), which cleans the memory registers and it is ready to receive and process the instructions contained by BIOS.
Then the computer runs it's self test to check if all it;s components function properly. If the computer passes the beep code POST, then it loads CMOS data, which are compared to system settings installed on the computer.
Then, POST verifies the system timer, after that BIOS checks if it sees 1234h, to know if it i a reboot. If BIOS does not see 1234h, then it will run next POST steps.
After hard drive, computer floppy and optical are tested by POST, by means of signals, the computer may start loading operating system.
The communication between hardware and CPU is enabled after the operating system is loaded.