Power On Self Test (POST) is the first executed command before booting the computer. After the personal computer is powered on, the processor checks the self test and all components connected to the system bus.
When a computer is turned on, the program counter present in the processor, which indicates the memory address of the instruction execution, has the Basic Input/Output System( BIOS) address. It checks the version of the BIOS, the system clock and the memories on the graphics cards. The subsequent test of the RAM is displayed on the screen. After checking other hardware components like the keyboard, the processor determines the connected drives.
If the POST test detects system errors, they are displayed on the monitor. If the POST test is not completed correctly, then the next system boot is from the BIOS with the default settings and a note about a setup check.