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Power-On Self-Test (POST) refers to routines which run immediately after many digital electronic devices are powered on. Perhaps the most widely-known usage pertains to computing devices (personal computers, PDAs, networking devices such as routers, switches, intrusion detection systems and other monitoring devices). Other devices include kitchen appliances, avionics, medical equipment, laboratory test equipment—all embedded devices. The routines are part of a device's pre-boot sequence. Once POST completes successfully, bootstrap loader code is invoked.
POST includes routines to set an initial value for internal and output signals and to execute internal tests, as determined by the device manufacturer. These initial conditions are also referred to as the device's state. They may be stored in firmware or included as hardware
Test results may either be displayed on a panel that is part of the device, or output via bus to an external device. They may also be stored internally, or may exist only until the next power-down. In some cases, such as in aircraft and automobiles, only the fact that a failure occurred may be displayed (either visibly or to an on-board computer) but may also upload detail about the failure(s) when a diagnostic tool is connected.
POST protects the bootstrapped code from being interrupted by faulty hardware. Diagnostic information provided by a device, for example when connected to an engine analyzer, depends on the proper function of the device's internal components. In these cases, if the device is not capable of providing accurate information—which ensures that the device is safe to run—subsequent code (such as bootstrapping code) may not be permitted to run.
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