Before issuing the shutdown command, the BIOS sets a value into the shutdown byte in the CMOS, which is checked after a reset, so the BIOS can branch to the relevant code and continue where it left off. The Shutdown command, on the other hand, just forces the CPU to leave protected mode for real mode, so the system behaves differently after each one. The Reset command stops the current operation and begins fetching instructions from the BIOS, as if the power has just been switched on. Consistent failures point indicate a bad battery backup system.
System Initialisation involves loading configuration from the CMOS, and failures will generate a text message. Early POST failures are generally fatal and will produce a beep code, because the video will not be active in fact, the last diagnostic during Early POST is usually on the video, so that Late failures can actually be seen. The POST checks at three levels, Early, Late and System Initialisation. Having obtained a POST code, identify the manufacturer of the chipset on the motherboard, then refer to the Chipsets section to find the chip(s) that control whatever's not working. In this chapter, some general procedures are described that may help if you have no POST card. POST Diagnostic cards can read what's sent to whichever port and display the codes on a pair of numerical displays, so you can check the progress of the POST and hopefully diagnose errors when it stops, though a failure at any given location does not necessarily mean that part has the problem treat it as a guidepost for further troubleshooting (a good card is the POSTmortemâ„¢ contact the author for details, or check Useful Numbers. Those at 50h are chipset or custom platform specific. IBM PS/2s use 90, whilst some EISA machines send them to 300H as well. Some computers may use a different port, such as 84 for the Compaq, or 378 (LPT1) for Olivettis. During the POST on AT-compatibles and above, special signals are output to I/O port 80H at the beginning of each test (genuine PCs and XTs don't issue POST codes, although some machines with compatible BIOSes do).