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Wikipedia says

Originally, BIOS firmware was stored in a ROM chip on the PC motherboard. In modern computer systems, the BIOS contents are stored on flash memory so it can be rewritten without removing the chip from the motherboard.

When BIOS is stored in either a ROM chip or a flash memory, does BIOS need to be loaded into main memory to be executed by CPU? Or can CPU execute BIOS directly in ROM or flash memory?

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  • BIOS flash is a type called "NOR" (as opposed to NAND) that supports byte-level updating and "execute in place." en.wikipedia.org/wiki/Flash_memory
    – LawrenceC
    Commented Feb 20, 2019 at 0:46
  • flash is ROM. <--. You can say lash is EEPROM, but EEPROM is ROM, by definition
    – barlop
    Commented Jun 30, 2019 at 0:32

1 Answer 1

Reset to default
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Early BIOS executes directly from the flash chip. The address of the flash chip is mapped to overlap with the Reset Vector. Later, for second and third stage bootloaders, BIOS will copy itself into processor cache and finally RAM.

A very detailed guide of modern processor booting is available here. This is to some extent reflected in the Intel Software Development Manual and the Intel BIOS Writer's Guide. AMD, ARM, VIA, etc. produce equivalent guides for their processors.

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