Explain BIOS and the POST In Detail

BIOS and the POST :-

• When the computer first looks at the BIOS, it begins the power-on self-test (POST) sequence to make sure the components in the computer are present and functioning properly. 
• If the computer does not pass any of these tests, it will encounter an irregular POST. An irregular POST is a beep code that is different from the standard one or two beeps. 
• For example, an irregular POST could generate no beeps at all or a combination of different beeps to indicate the cause of the failure.
• If the computer passes the initial POST, it will next look at the first 64-bytes of memory located in the complementary metal oxide semiconductor (CMOS) chip, which is kept alive by the CMOS battery even when the computer is turned off. 
• This chip contains information such as the system time and date and information about all the hardware installed in your computer.
• After loading the CMOS information, the POST will begin inspecting and comparing the system settings with what is installed in the computer. 
• If no errors are found it will then load the basic device drivers and interrupt handlers for hardware such as the hard drive, keyboard, mouse, floppy drive. 
• These basic drivers allow the CPU to communicate with these hardware devices and allow the computer to continue its boot process.
• Next, the POST will check the real-time clock (RTC) or system timer and the computer system bus to make sure both of these are properly working on the computer. 
• Finally, you'll get a picture on your display after the POST has loaded the memory contained on the display adapter and has made it part of the overall system BIOS.
• Next, the BIOS will check to see if it's currently performing a cold boot or warm boot (reboot) by looking at the memory address 0000:0472, if it sees 1234h the BIOS knows that this is a reboot and will skip the remainder of the POST steps.
• If 1234h is not seen, the BIOS knows that this is a cold boot and will continue running additional POST steps. 
• Next, it tests the computer memory (RAM) installed in the computer by writing to each chip. 
• With many computers, you'll know it's performing this step if you see the computer counting the total installed memory as it's booting.
• Finally, the POST will send signals to the computer floppy, optical, and hard drive to test these drives. 
• If all drives pass the test, the POST is complete and instruct the computer to start the process of loading the operating system.