When I hit DEL during start up, am I entering CMOS or BIOS?

Question

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?

Now which piece of software is actually already running and reads the keyboard and decides to enter CMOS setup? :) — Hagen von Eitzen, May 05 '15 at 18:32
Technically nobody uses BIOS anymore, it has been superseded by [tag:UEFI]. — , May 06 '15 at 01:34
like: when I start my pc, am I entering to windows(or any other os) or hard disk? — RogUE, May 06 '15 at 09:30
@RogUE, you are entering a firmware program (BIOS or UEFI or Whatever) that initiates boot on a specific harddisk by locating and the boot sector (MBR or whatever), which, when executed, boots the OS. — Frank Thomas, May 06 '15 at 11:31

score 60 · Accepted Answer · edited May 05 '15 at 04:41

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.

If it can be "flashed" it's an EEPROM. – Andrew May 05 '15 at 06:37 — Andrew, May 05 '15 at 06:37

score 3 · Answer 2 · edited May 05 '15 at 10:49

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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.

edited May 05 '15 at 10:49

Richie Frame

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answered May 05 '15 at 06:26

William Russell

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'Complementary' ;) – Space Ghost May 05 '15 at 06:28
Aww, that's right! Thanks for catching me on that. – William Russell May 05 '15 at 06:31
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Actually BIOS is written in *ROM, not in CMOS. BIOS write in CMOS to store some parameters. Sorry, you loose my upvote :( – Romeo Ninov May 05 '15 at 06:34
@RomeoNinov CMOS is the semiconductor technology used to build EEPROM chips – Richie Frame May 05 '15 at 10:22
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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 – Romeo Ninov May 05 '15 at 10:23
@RomeoNinov CMOS technology is used in the RTC, the BIOS memory chip, and even the CPU and RAM. Technically the transistors floating gate holds the BIOS program data, but it is still CMOS based technology – Richie Frame May 05 '15 at 10:25
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@RichieFrame: Technically you are right but many years ago the [CMOS RAM backed up by a battery](https://en.wikipedia.org/wiki/Nonvolatile_BIOS_memory) 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. – pabouk - Ukraine stay strong May 05 '15 at 11:38

When I hit DEL during start up, am I entering CMOS or BIOS?

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