bios vs cmos vs system rom

Discussion in 'A+' started by jmassey74, Jul 7, 2016.

  1. jmassey74

    jmassey74 New Member

    Hi all,I'm very new to the forums and also to studying for the A+ cert. I have a study guide but am having problems understanding the differences between bios, system rom, and cmos. If somebody could help clarify this I would be much appreciated.
    WIP: A+
  2. garycul

    garycul Nibble Poster

    The BIOS is a ROM chip on the motherboard which contains code that tells the CPU how to interact and control the other components in the computer.

    The CMOS is also a chip on the motherboard but is a RAM chip with volatile memory which stores information about the computer components and their settings.

    RAM chips lose the info stored in them when the computer is turned off so this is where the CMOS battery comes in to it to keep power to the CMOS chip so as to keep the settings.

    The BIOS code on the BIOS chip reads information from the CMOS chip when the computer starts during the boot up process.

    Hope this helps.
    Certifications: 70-697 Configuring Windows Devices, 70-680 Configuring Windows 7, 70-687 Configuring Windows 8.1, TCP/IP ON NT4, NT 4 Server, NT4 Workstation, Windows 95, Windows 3.1, ITIL SLM Practitioner, ITIL v2 and v3 Foundation
  3. dmarsh

    dmarsh Petabyte Poster

    BIOS = basic input/output system, Its boot code, low level IO routines etc. Most of the original IBM/PC BIOS is now legacy and redundant.
    The BIOS code must live somehere, normally on a ROM chip.

    ROM = Read Only Memory, a different type of memory chip thats non volatile, meaning it doesnt lose its content when power is lost.
    More common in the old days was a special type of erasable ROM called an EEPROM.

    CMOS = Complimentary Metal Oxide Semiconductor, a process technique for making a certain type of popular semiconductor.

    There are what are called 'CMOS Settings', this is config about how the computer should boot that are configured using a BIOS boot admin tool. Since these must persist but be changeable they were for a while stored in battery backed RAM with a watch battery supplying the charge.

    Nowadays we have flash ram, so there is no real need for CMOS NVRAM.

    The BIOS was already 80%+ legacy but now its been replaced by UEFI and OS vendors provide their own hardware routines.


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