POST(Power On Self Test)

The post refers to the Power On Self Test. This is the very first step that happens when you switch ON the Computer. As you can see from the figure below after we Power-On the Computer, the CPU first reads instructions from BIOS/ROM (which stores the information about the motherboard) and then, BIOS performs the POST(Power On Self Test), during which the following processes occur

• Counting and Testing of RAM are done,
• The keyboard is Tested.
• Hard Disk Drive is Detected and Initialized.

After POST, Boot up sequence occurs during which bootable files of the operating system (e.g windows) are loaded into RAM from a permanent storage device like a Hard disk.

After the successful boot-up sequence, the operating system of the system gets loaded and you reach the C:> prompt in DOS operating system and windows desktop in Windows Operating System. In the windows operating system, you can see the POST after pressing TAB on the keyboard.

POST is the key to the successful booting of the operating system. If the BIOS finds anything faulty on the computer, it will stop the booting and the message will be displayed on the screen. If there is a problem before the display is activated, POST will notify you about the problem by giving a beep or a combination of beeps. These audio beeps are BIOS-dependent. Every BIOS manufacturer has its own set of beeps to convey the problem. The most common of all BIOS standards is the IBM BIOS and you can know the cause of the problem from the Audio Beeps from your PC when it turns On and troubleshoots the computer accordingly.

Signal	Possible Cause
No Beep	Power supply bad or not plugged in or motherboard faulty
Continuous Beep	Keyboard Stuck
Repeating Short Beep	Power Supply Faulty
1 Long Beep followed by 1 short beep	Motherboard error
1 Long Beep, 2 short beeps	Display Card Faulty
1 short Beep, Blank screen	Check Display Cable or Display Card