Where is the BIOS stored


From http://en.wikipedia.org/wiki/BIOS:

BIOS software is stored on a non-volatile ROM chip on the motherboard. … In modern computer systems, the BIOS contents are stored on a flash memory chip so that the contents can be rewritten without removing the chip from the motherboard. This allows BIOS software to be easily upgraded to add new features or fix bugs, but can make the computer vulnerable to BIOS rootkits.

Since ROM means Read-Only Memory, why can the BIOS contents be rewritten?

Does the "flash memory chip" mean the same as the "non-volatile ROM", both meaning where BIOS is stored?

Best Answer

To add to Varaquilex's answer, the BIOS software is stored in an Electrically Erasable and Programmable ROM (EEPROM) which enables firmware updates to be performed electronically. Very old BIOS chips were actually UV-EPROM chips which required erasing with UV light exposure before they could be reprogrammed.

Edit: As has been pointed out in the comments, even earlier than that single use Programmable ROM (PROM) chips were sometimes used which could not be reprogrammed at all once configured and required complete replacement to upgrade (although this was rarely required).