What happens after pressing the Power button in PCs?

Boot Sequences of the Operating System

1. When we press the power button, If there is proper voltage, a short is created and send a pulse command to PSU (Power supply unit) to give power to all the devices and to send reset signals to the processor(CPU).                                                                                                                                                                                          
2. CPU has the instruction pointer pointing to the reset vector within the system ROM and the CPU starts executing the small firmware or Jump instruction to load the BIOS.                                                                                   
3. BIOS  initialize the primary graphics through video BIOS code and also the other secondary devices. BIOS starts the POST (Power ON Self Test) process which will  run the basic test for the devices and sets the device to it's default values. If there is any error during POST, beep codes and light codes helps us to troubleshoot the error device.                                                                                                                                                    
4. According to the boot sequence setting in BIOS, it checks the first sector of the boot devices in sequence.                                                                                                                                    
5. In Hard disks, if BIOS finds the boot signature (2 bytes - 0x55 0xAA) in the first sector, MBR (Master Boot Record) will get  loaded.                                                                                                                                                                       
6. MBR checks for PBR(Partition Boot Record) and passes the control to PBR to decode the file system on the partition and locate the Bootmgr(Boot manager) into memory.                                                                                            
7. Bootmgr reads the Boot Configuration Data(BCD) and locates the file ntoskrnl.exe.                                                           
8. Windows kernel gains control and initializes the CPU into protected mode. Loads the device drivers and shows the boot logo.                                                                                                                                                                             
9. User space will get loaded and starts session using csrss.exe and wininit.exe. Runs the pending windows update processes.                                                                                                                                                                               
10. Finally, Login screen gets displayed.