POST(Power On Self Test)

The post refers to 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 instruction from BIOS/ROM (which stores the information about motherboard) and then, BIOS performs the POST(Power On Self Test), during which 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 operating system (e.g windows) are loaded  into RAM from a permanent storage device like Hard disk.

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

POST is the key to 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 Audio Beeps from your PC when it turns On and troubleshoot 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