Boot & BIOS glossary

POST stands for Power-On Self-Test. Each time you switch on the PC, the motherboard checks that the processor, memory and graphics work before it loads Windows. If a check fails, the PC stops there, and you usually see no picture.

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The BIOS (on modern PCs officially called UEFI) is the small program built into the motherboard. It starts the hardware before Windows loads and holds settings like boot order and memory speed. You usually enter it by pressing Delete or F2 right after switching on.

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Motherboard makers release new BIOS versions to fix bugs and add support for newer processors. A board made before your processor came out may need an update before it can run it. Updates come from the motherboard maker's website.

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Some motherboards have a special button on the back panel (called BIOS Flashback, Q-Flash Plus or Flash BIOS, depending on the brand). It updates the BIOS from a USB stick using only the power supply, so it works even if the processor isn't supported yet. Follow the maker's instructions exactly.

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Clearing the CMOS resets all BIOS settings to factory defaults, undoing any setting that stops the PC from starting. You do it with a button on the back panel, two pins on the board, or by removing the round coin battery for a few minutes. Your files and Windows are not affected.

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Most modern motherboards have four small lights labeled CPU, DRAM, VGA and BOOT. They light up one after another as the PC starts. If one stays on, startup got stuck at that step, which points you at the part to check.

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Some higher-end motherboards have a tiny two-digit display that shows a code during startup. If the PC gets stuck, the code tells you where. Look it up in your motherboard manual, because codes differ between brands.

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The boot order is a setting in the BIOS that tells the PC which drive to load the operating system from. If it points to the wrong drive, or to an empty one, Windows won't start even though the hardware works.

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If a small speaker is connected to the motherboard, it beeps in a pattern when startup fails (for example one long and two short beeps). Each pattern means a different problem, listed in your motherboard manual.

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Standoffs are small brass or metal spacers screwed into the case. The motherboard sits on them so it never touches the case directly. A missing standoff, or an extra one where the board has no hole, can cause a short circuit.

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A small block of pins, usually at the bottom-right of the motherboard, where tiny cables from the case connect: the power button (POWER SW), the reset button and the case lights. If the power button cable is on the wrong pins, the PC won't turn on.

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A short circuit happens when electricity flows somewhere it shouldn't, for example from the motherboard into the metal case through a loose screw. The power supply detects it and shuts off, so the PC turns off immediately or won't start.

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The I/O shield is the thin metal plate that surrounds the ports on the back of the PC. Its small springy tabs can sometimes bend into a port and cause a short.

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The socket is the square holder on the motherboard where the processor sits. On Intel boards the tiny pins are in the socket; on older AMD processors they're on the chip itself. A bent pin can stop the PC starting or make memory go missing.

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A minimal boot test means disconnecting everything except the motherboard, processor, cooler, one stick of RAM and the power supply. If the PC starts like that, you add parts back one at a time until it fails.

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