The POST begins when the computer boots up.
POST stands for power-on self-test. The POST diagnostic is a process that all computers complete during a boot sequence. Anytime a computer is booted or restarted a POST occurs. The POST is a very basic test of the basic input/output system (BIOS), the system that forms the foundation of the computer's operations. The POST begins immediately after the computer powers up. A POST can also be run from a command line. The POST normally occurs so quickly that the user does not notice it running. However, a computer must pass the POST each time it boots up.
Power
The BIOS verifies that the power supply is on and is sending a reset signal to the system.
CPU
The BIOS ensures that the processor [central processing unit (CPU)] has restarted and is prepared to receive instructions from the system.
BIOS Checksum
The BIOS analyzes the BIOS checksum, or hash sum. The checksum is an arbitrary block of data the system inspects in order to detect random errors.
CMOS Checksum
The BIOS also reviews the checksum of the complementary metal--oxide--semiconductor (CMOS). The CMOS is the integrated circuit that regulates the system clock. The checksum must be free of errors for the BIOS to proceed with the boot sequence.
Memory Connections
The BIOS scrutinizes the connections between the CPU and the memory and the memory modules' interconnections. The CPU must be able to detect the memory modules, the memory controller and the data bus.
Memory
The BIOS checks the first 64KB of system memory. This block of memory must be functioning and able to contain the POST code. The POST code will be written to this block of RAM.
I/O Bus
The input/output (I/O) bus or controller connects the CPU to all computer components, except memory. The BIOS confirms that the I/O bus is connected and functioning.
I/O Bus and Video
The BIOS verifies that the I/O bus can detect and interact with the graphics system and read any video memory.
Errors and Codes
If no errors are detected, the system will continue the boot sequence as normal. If an error is detected, the BIOS will attempt to continue the boot process. If the error is critical, the boot sequence will stop. The BIOS reports the POST outcomes with codes. Each manufacturer's POST codes are different. Computer users or technicians can refer to the BIOS Central: Dell BIOS POST Codes for codes specific to Dell computer models, or refer to the computer user's manual.
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