POST or Power On Self Test

Power-on self-test (POST) refers to routines run immediately after power is applied, by PC. The routines are part of a device’s pre-boot sequence and may be stored in firmware or included as hardware. Once POST completes successfully, bootstrapping code or booting an operating system is invoked. POST protects the operating system from being interrupted by faulty hardware. Routines included during POST include routines to execute internal tests, as determined by the device maker and they test several components of PC like CPU, BIOS ROMs, DMA Controller or disk drive controllers, Interrupt Controller, System timing chip (it provides timing signals for the bus and CPU), Video Card, RAM, Keyboard, Floppy Drives and Other devices.-POST checks the and beeps, if problem is found.  POST beep codes varies for motherboard manufacturer.

POST also checks the Master Boot Record (MBR) from where the operating system starts loading.

Original IBM POST beep codes

BeepsMeaning
1 short beepNormal POST – system is OK
2 short beepsPOST error – error code shown on screen
No beepPower supply, system board problem, disconnected CPU or speaker
Continuous beepPower supply, system board, or keyboard problem
Repeating short beepsPower supply or system board problem or keyboard
1 long, 1 short beepSystem board problem
1 long, 2 short beepsDisplay adapter problem (MDA, CGA)

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