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
| Beeps | Meaning |
| 1 short beep | Normal POST – system is OK |
| 2 short beeps | POST error – error code shown on screen |
| No beep | Power supply, system board problem, disconnected CPU or speaker |
| Continuous beep | Power supply, system board, or keyboard problem |
| Repeating short beeps | Power supply or system board problem or keyboard |
| 1 long, 1 short beep | System board problem |
| 1 long, 2 short beeps | Display adapter problem (MDA, CGA) |
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