What test does startup BIOS perform when you first turn on a computer to verify it can communicate with essential hardware devices?

The BiOS (Basic Input-Output System) loads a ROM (Read-Only Memory; a chip that stores permanent bootup instructions) that performs a POST (Power-On Self-Test) when the computer is first powered on, or when the CPU (Central Processing Unit) is soft reset via a triple fault or an untrapped reset command through the keyboard port (by sending the value 254 to the hardware port 100). The POST checks to make sure that there is system memory, and usually at least one drive device (disk or hard drive), and all the other various parts that make up the core functionality of the computer.