CompTIA A+ Core
TechNote: BIOS, CMOS and POST
BIOS is short for Basic Input/Output System. This small program
is used to startup the computer and communicate with hardware
before an operating system is loaded.
BIOS is stored in ROM chips on the mainboard. Some chips contain
programs to support basic hardware such as parallel and serial
ports, keyboard and the speaker. Another ROM chip, called the
CMOS (complementary metal-oxide semiconductor) chip
stores information that is subject to change such as time/date,
power saving settings, and video adapter, hard drives and other
BIOS settings are applied at every startup. Modern PCs copy
the BIOS information to RAM for faster performance, this process
is called shadowing.
Here is a picture of a common BIOS CHIP:
CMOS settings can be changed in the CMOS Setup, which can mostly
be entered by pressing the DEL or F2 key during startup (depends
on manufacturer, other key or key combinations might apply).
Some of the most common CMOS settings are described later in
Most mainboards have a "CMOS restore to factory defaults"-
jumper which allows you to return to the default CMOS settings
configured by the manufacturer. This is useful when you cannot
access the CMOS Setup because of incorrect CMOS settings or
lost CMOS passwords. Make sure the power is completely off when
you shorten the jumper.
The information contained in the CMOS chip is maintained
by a battery. If the battery runs low, the CMOS content will be
lost and POST will display a "CMOS invalid" or "CMOS
checksum invalid" message. The first symptom for a battery
running low is time slowing down and eventually complete loss
of date and time. In some cases you might get a boot device
error because a boot device such as floppy disk or hard drive
can not be located. Almost always the CMOS battery can be replaced
Here is a picture of a common CMOS battery:
Most modern BIOSs are stored on flash memory, this
enables you to upgrade the BIOS software when needed, in some
cases this is necessary to support new hardware technologies/devices.
Upgrading the BIOS is called flashing, before you do
make sure you write down all settings and during the process
pray the power won't be interrupted. The BIOS version ID is
displayed during startup.
Some mainboards allow replacing the BIOS ROM, although this
is not done often.
During startup the BIOS also invokes The POST (Power
On Self Test), also stored in ROM, a program that runs multiple
self-diagnostic routines. When the computer does not pass the
POST it will display an error code or message, or generate a
beep code through the speaker when display is not functioning.
Failing the POST does not always mean the computer will not
be able to boot, required devices that need to pass the POST
are CPU, RAM, display adapter and boot device, but if the floppy
drive is missing the computer can still be able to continue
the boot process.
The meaning of the beep codes vary depending of the manufacturer
mostly they indicate memory, CPU, display and keyboard problems.
Most BIOSs beep once to check if the speaker is working or to
confirm that the post has passed successfully.
For the exact meaning of beep codes for AMI BIOSs click
here and for Phoenix and Award click
Although most BIOS manufacturers started using error text messages,
some use numeric POST error codes. Some of them are listed below:
|POST error code
||Mainboard related errors
||Memory related errors.
||Keyboard related errors
||Real-time clock failure
||BAD CMOS memory
||Floppy Disk related errors
||Hard drive controller
||Network Adapter related errors
here for a complete list of numeric error codes by IBM.
parallel port—Uni., bi-directional, disable/enable, ECP,
In the CMOS you are able to configure a parallel port to use
EPP or ECP. Enhanced Parallel Port and Extended Capabilities
Port are both bi-directional standards, operate in 8-bit, and
allow data transfer speed of approximately 2 MB/s. Some of the
main differences are that ECP supports Direct Memory Access
(DMA) and data compression, which enables higher transfer rates.
It is also possible to completely disable the parallel port
in the BIOS. Most BIOSs allows you to set the DMA channel, when
the port mode is set to ECP.
Most personal computers have 2 serial ports. In the BIOS you
can assign COM1/COM2/COM3/COM4 to serial port 1 or 2. Most BIOSs also allow you to set the I/O and IRQ but this is
mostly done automatically.
The floppy drive(s) can be enabled/disabled in the BIOS (e.g.
set to Not Installed). The BIOS also allows you to choose the
capacity of the media.
- 360 KB 5.25 inch
- 1.2 MB 5.25 inch
- 720 KB 3.5 inch
- 1.44 MB 3.5 inch
- 2.88 MB 3.5 inch
Some BIOSs also allows you to swap A: and B: and disable seeking
a floppy disk for a boot sector during startup.
Most modern BIOSs allow automatically detection of
disk parameters. These are some of the primary CMOS settings
that apply to hard drives (and CD/DVD-ROM drives, etc.), the
settings can be individually configured for the primary master
and slave device and the secondary master and slave device.
Common disk types are:
- User, User-defined CHS values
- Auto, automatically detects hard disks parameters at every
- 1-46, predefined combinations of CHS values.
- CDROM, used fot atapi CD-ROM drives
- ARMD, used fo atapi ZIP and LS 120 drives.
Determines the capacity of the drive.
- number of Cylinders
- number of Heads
- number of Sectors
LBA (Large Block
Addressing), technologie to overcome the 528 MB limit.
Another common CMOS setting related to hard drives is Boot Sector
Virus protection Enabled/Disabled, enabling this will make the
BIOS issue a warning message/beep if a write to the boot sector
or partition table of a hard disk is attempted.
Today's motherboards provide too many BIOS settings
regarding to memory to discuss here and most are beyond the
scope of the exam.
CompTIA does mention parity, non-parity in the exam objectives.
Parity adds an extra bit (odd or even) to the 8-bit data-string
to ensure data integrity in memory modules. Its successor, ECC,
provides even better ways to ensure the data integrity by adding
information about individual bits.
This setting is used to determine in which order devices
(e.g. CD, floppy or hard disks) the computer should look for
a boot sector.
The Date and Time is set in the BIOS, stored in CMOS,
maintained by CMOS battery.
In most cases a user (startup) password and a supervisor
(setup) password can be set in the CMOS. When a Setup password
is required the computer will prompt for it when you try to
access the BIOS setup. When a Startup password is configured
the computer will prompt for it at every startup.
The CMOS password can be reset by shortening the "CMOS
restore to factory defaults jumper" or by temporarily removing
the CMOS battery.
Plug & Play BIOS
Today's BIOSs are PnP-aware. This means they are able
to automatically assign resources such as IRQ and DMA to Plug
and Play devices. Information about these PnP devices is stored in a separate
area of non-volatile CMOS memory, called the ESCD (Extended
System Configuration Database). The PnP BIOS and the operating
system can both access this area so they can communicate with
each other about resource settings assigned to PnP devices and
also to non-plug and play devices. For example, when a fixed
IRQ is manually assigned to a particular device using Device
Manager, Windows will write this information to the ESCD on
shutdown preventing the BIOS from assigning the same IRQ to
a PnP device at startup. You can also reserve IRQ's for non-plug and play devices in
the CMOS setup, this will prevent the BIOS from assigning these
reserved resources to PnP devices, a common example is a legacy
sound card that needs IRQ 5.
Modern mainboards provide ACPI (Advanced Configuration
and Power Management Interface) settings such as wake-up, power
button function and standby/suspend timers, these are also configured
in the CMOS Setup.
related exam objectives for the 2002 A+ Core exam.
| 1.1 Identify
basic terms, concepts, and functions of system modules, including
how each module should work during normal operation and during
the boot process.
Examples of concepts and modules are:
1.8 Identify hardware methods of upgrading system performance,
procedures for replacing basic subsystem components, unique
components and when to use them.
Content may include the following:
- Upgrading BIOS
- When to upgrade BIOS
2.1 Identify common symptoms and problems associated with each
module and how to troubleshoot and isolate the problems.
Content may include the following:
4.4 Identify the purpose of CMOS (Complementary Metal-Oxide
Semiconductor), what it contains and how to change its basic
Example Basic CMOS Settings:
- Printer parallel port—Uni., bi-directional, disable/enable,
- COM/serial port—memory address, interrupt request, disable
- Floppy drive—enable/disable drive or boot, speed, density
- Hard drive—size and drive type
- Memory—parity, non-parity
- Boot sequence
- Plug & Play BIOS
here for the complete list of exam objectives.
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