For the longest time I assumed I was entering the BIOS setup and CMOS is a chip that holds the settings I set in BIOS in its memory.

I recently read somewhere that when I am configuring the boot order and etc, that that's actually CMOS setup.

I'm a little confused now, could someone explain this to me?

  • You were right. May 4, 2015 at 22:49
  • 5
    CMOS is hardware, BIOS is code
    – Ron
    May 5, 2015 at 5:46
  • Now which piece of software is actually already running and reads the keyboard and decides to enter CMOS setup? :) May 5, 2015 at 18:32
  • Technically nobody uses BIOS anymore, it has been superseded by uefi.
    – user76225
    May 6, 2015 at 1:34
  • like: when I start my pc, am I entering to windows(or any other os) or hard disk?
    – RogUE
    May 6, 2015 at 9:30

2 Answers 2


Both. CMOS stores BIOS configuration information. When you "enter setup", you are running the BIOS's configuration program, which loads the settings defined in CMOS. You are "setting up" the CMOS, by providing configuration information the BIOS will use as it runs.

The BIOS is a program written as Firmware onto a ROM, so it cannot be written to (except by an all-or-nothing flash operation, which is dangerous, so not an everyday operation). The BIOS ROM stores its configuration info on to the CMOS when you hit F10. That's why clearing the CMOS restores your BIOS settings to default, and is why it doesn't delete the BIOS iteslf, leaving you with an expensive paperweight.

In example, the BIOS has a subroutine that will load the OS per the boot order. the boot order information however, (eg use first CD-ROM as first device) is stored in the CMOS. If you clear the CMOS, the BIOS will use a default, usually the first disk on the first disk controller that is populated.

  • 6
    If it can be "flashed" it's an EEPROM.
    – Andrew
    May 5, 2015 at 6:37

BIOS stands for Basic Input Output System and CMOS stands for Complementary Metal Oxide Semiconductor. The BIOS system is written onto the CMOS microchip. You can't have a BIOS without a CMOS to hold it, so it's not uncommon for the two terms to be used interchangeably.

  • 1
    'Complementary' ;) May 5, 2015 at 6:28
  • Aww, that's right! Thanks for catching me on that. May 5, 2015 at 6:31
  • 3
    Actually BIOS is written in *ROM, not in CMOS. BIOS write in CMOS to store some parameters. Sorry, you loose my upvote :( May 5, 2015 at 6:34
  • 1
    In this case by CMOS is used in sense of realtime clock where are stored BIOS settings. ANd it is not the same place as where BIOS code reside May 5, 2015 at 10:23
  • 2
    @RichieFrame: Technically you are right but many years ago the CMOS RAM backed up by a battery began to be called simply "CMOS". So nowadays, stupidly, the additional meaning of the term CMOS is the integrated circuit containing the non-volatile RAM with BIOS settings (and the RTC circuits). Certainly a better term is NVRAM (non-volatile RAM). --- Anyway, as Romeo Ninov wrote, the BIOS is not stored in this CMOS. Only the settings of BIOS are there. May 5, 2015 at 11:38

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