BIOS

The BIOS is what works when your computer is first turned on. It wakes up the rest of the system. The BIOS is programmed into a chip that usually is installed into a socket on the mainboard. In the past that was necessary so it could be easily replaced if it needed to be updated. On newer boards the BIOS can be updated by just using a special program and changing the code without removing the chip.

On modern mainboards the BIOS program is programmed into a special type of RAM chip called an EEPROM, or Electrically Eraseable Programmable Read Only Memory. The earlier BIOS chips were in PROMs, chips that could have a progam written into them one time only, so they had to be replaced rather than reprogrammed. There was yet another type of BIOS chip that you may still find on some older boards. It used a chip called an EPROM (an Eraseable PROM). This chip could be erased and reprogrammed but it took a strong UV light shining through a small glass window on the chip to do the erasing. There were special ovens designed to do the erasing - the chips had to be exposed for an hour, or more - and sometimes it wouldn't work, so you had to replace the chip anyway.

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