Whenever a computer starts up, a series of tests are automatically performed to check the primary components in the system, such as the CPU, ROM, memory, and motherboard support circuitry. The routine that carries out this function is POST. POST, as mentioned in Chapter 2 "How Computers Work", is a hardware diagnostics routine that is built into the system BIOS. The basic function of the POST routine is to make sure that all the hardware the system needs for startup is there and that everything is functioning properly before the boot process begins. The POST routine, therefore, ensures that the computer is ready to begin the boot sequence. POST also provides some basic troubleshooting to determine what devices have failed or have problems initializing during this pre-startup hardware check.

Post Errors, Troubleshooting
The POST routine provides error or warning messages whenever it encounters a faulty component. Post error codes take the form of a series of beeps that identify a faulty hardware component. If everything has been installed correctly during the assembly process and the new system is functioning normally, one short beep will usually be heard at the completion of POST. If a problem is detected, a different number of beeps will be heard, sometimes in a combination of short and long tones. These are mainly BIOS-dependent codes. They vary according to BIOS manufacturer and even between different versions of BIOS.

The beep codes can be used to troubleshoot hardware failures occurring during the POST routine. Although the POST routine is not very thorough compared to existing disk-based diagnostics, it is a first line of defense, especially in detecting severe motherboard problems. POST typically provides three types of output messages. These include audio codes (beeps), onscreen text messages, and hexadecimal numeric codes that are sent to an I/O port address. POST generally continues past non-fatal problems, but fatal problems cause POST to halt the boot process. If problems occur early, before any drivers are loaded for the monitor, for example, then POST can only signal that a problem exists using beeps. Beeps are issued through the computer system speaker. Conversely, if the POST and the boot sequence can advance up to a point where the system can use the system video to display messages, then a message can be displayed on the screen. The message indicates what problems occurred and the probable cause. These are referred to as visual error codes. These error messages are usually in the form of a numeric code, for example, 1790-Disk 0 Error.

In many instances, the BIOS manual or the manufacturer's website will need to be consulted for charts to help decode some of the more detailed error codes. Table gives a summary of the major groups of visual error codes frequently encountered. They make up some of the major groups of POST hardware diagnostics messages commonly used on PC systems. Although most of the major BIOS manufacturers use many of these codes, not one uses all of these codes. Consult the manual for a specific system BIOS.

Problems that occur during the POST are usually caused by incorrect hardware configuration or installation. Actual hardware failure is rare. A POST error may indicate that power has to be turned off from the system. Unplug the system from the wall, and carefully double-check the assembled computer to make sure that one or all of the steps in Figure were properly carried out.