POST process (Power-On Self Test)

Short for power-on self-test, the POST is a test the computer must complete to verify all hardware is working properly before starting the remainder of the boot process. The POST process checks computer hardware, like RAM (random-access memory), hard drive, CD-ROM (compact disc read-only memory) drive, keyboard, etc., to ensure everything works correctly.

If all hardware passes the POST, the computer continues the boot up process and may generate a single beep sound. If POST is unsuccessful, it generates a beep code to indicate the error encountered, and the computer will not boot up. All POST errors relate to hardware issues with one of the components in the computer.

A power-on self-test (POST) is a set of routines performed by firmware or software immediately after a computer is powered on, to determine if the hardware is working as expected. The process would proceed further only if the required hardware is working correctly, else the BIOS(Basic Input Output Software) would issue an error message. POST sequence is executed irrespective of the Operating System and is handled by the system BIOS. Once the tests are passed the POST would generally notify the OS with beeps while the number of beeps can vary from system to system. When POST is successfully finalized, bootstrapping is enabled. Bootstrapping starts the initialization of the OS.

The Role of POST in the Boot Sequence

The boot sequence is the process of starting a computer/system. The boot process is initiated when the power button is pressed, it sends power to the boot-loader in the cache memory. The Boot loader performs POST as a preboot sequence and if everything is working well without any errors the BIOS(Basic Input Output System) is activated which finds and loads the operating system.

Finally the software has to interact with the hardware units to complete the process. To avoid any hardware errors while executing a software program, the pre-boot sequence would test the hardware and initiate the OS if and only if the basic hardware units are functioning as expected. he principal duties of the main BIOS during POST are as follows:

1. Find, size, and verify the system main memory.
2. Initialize BIOS.
3. Identify, organize, and select which devices are available for booting.
4. Verify CPU registers.
5. Verify the integrity of the BIOS code itself.
6. Verify some basic components like DMA, timer, interrupt controller.
7. Pass control to other specialized extensions BIOS (if installed).

Progress and error reporting[edit]

BIOS POST card for ISA bus

BIOS POST card for PCI bus.

Professional BIOS POST card for PCI bus.

Two POST seven-segment displays integrated on a computer motherboard

The original IBM BIOS made POST diagnostic information available by outputting a number to I/O port 0x80 (a screen display was not possible with some failure modes). Both progress indication and error codes were generated; in the case of a failure which did not generate a code, the code of the last successful operation was available to aid in diagnosing the problem. Using a logic analyzer or a dedicated POST card‍—‌an interface card that shows port 0x80 output on a small display‍—‌a technician could determine the origin of the problem. Once an operating system is running on the computer the code displayed by such a board may become meaningless, since some OSes, e.g. Linux, use port 0x80 for I/O timing operations. The actual numeric codes for the possible stages and error conditions differ from one BIOS supplier to another. Codes for different BIOS versions from a single supplier may also vary, although many codes remain unchanged in different versions.

Later BIOSes used a sequence of beeps from the motherboard-attached PC speaker (if present and working) to signal error codes. Some vendors developed proprietary variants or enhancements, such as MSI's D-Bracket. POST beep codes vary from manufacturer to manufacturer.

What are the POST code indicators and their meaning?

1. Short Beep
Normal post: the system is OK.
2. Sort Beeps
Post errors error code is shown on the screen.
3. No BEEP
Power supply, system problem, disconnected CPU, or disconnected speaker.
4. Continuous beep
Ram or keyboard problem.
5. Repetitive short beeps
Power supply, system board, or keyboard problem.
6. 1 long and 1 short beep
System board problem.
7. 1 long and 2 short beeps
Display adapter problem
8. 1 long and 3 short beeps
enhanced the graphic editor problem.
9. 3 long beeps

keyboard card problem