Grou Power ON self-test and basic-

input/output-system (BIOS)
configuration
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When power is turned on, POST (Power-On Self-Test) is the diagnostic testing
sequence that a computer's basic input/output system (or "starting program") runs to
determine if the computer keyboard, random access memory, disk drives, and other
hardware are working correctly.

What is It

Power-on self-test

A power-on self-test (POST) is a process performed by firmware or software

routines immediately after a computer or other digital electronic device is powered on.
This article mainly deals with POSTs on personal computers, but many other
embedded systems such as those in major appliances, avionics, communications, or
medical equipment also have self-test routines which are automatically invoked at
power-on.
The results of the POST may be displayed on a panel that is part of the device,
output to an external device, or stored for future retrieval by a diagnostic tool. Since a
self-test might detect that the system's usual human-readable display is nonfunctional,
an indicator lamp or a speaker may be provided to show error codes as a sequence of
flashes or beeps. In addition to running tests, the POST process may also set the initial
state of the device from firmware.
 In the case of a computer, the POST routines are part of a device's pre-boot
sequence; if they complete successfully, the bootstrap loader code is invoked to load
an operating system.

IBM-compatible PC POST
In IBM PC compatible computers, the main duties of POST are handled by the
BIOS, which may hand some of these duties to other programs designed to initialize
very specific peripheral devices, notably for video and SCSI initialization. These other
duty-specific programs are generally known collectively as option ROMs or individually
as the video BIOS, SCSI BIOS, etc.
The principal duties of the main BIOS during POST are as follows:

• verify CPU registers

• verify the integrity of the BIOS code itself
• verify some basic components like DMA, timer, interrupt controller
• find, size, and verify system main memory
• initialize BIOS
• pass control to other specialized extension BIOSes (if installed)
• identify, organize, and select which devices are available for booting The
functions above are served by the POST in all BIOS versions back to the very
first. In later BIOS versions, POST will also:
• discover, initialize, and catalog all system buses and devices
• provide a user interface for system's configuration
• construct whatever system environment is required by the target operating
system
Original IBM POST beep codes
Beeps Meaning

1 short beep Normal POST – system is OK

2 short beeps POST error – error code shown on screen

Power supply, system board problem, disconnected
No beep
CPU, or disconnected speaker
Continuous beep Power supply, system board, RAM or keyboard problem

Repeating short Power supply, system board or keyboard problem

beeps
1 long, 1 short beep System board problem

1 long, 2 short beeps Display adapter problem (MDA, CGA)

1 long, 3 short beeps Enhanced Graphics Adapter problem (EGA)
3 long beeps 3270 keyboard card error
POST AMI BIOS beep codes
Beeps Meaning

1 Memory refresh timer error

2 Parity error in base memory (first 64 KiB block)

3 Base memory read/write test error

Motherboard timer not operational (check all PSU to MB

4
connectors seated)

5 Processor failure

6 8042 Gate A20 test error (cannot switch to protected mode)

7 General exception error (processor exception interrupt error)

8 Display memory error (system video adapter)

9 AMI BIOS ROM checksum fix

10 CMOS shutdown register read/write fix

11 Cache memory test failed

continuous Motherboard does not detect a RAM module (continuous

beeping beeping)

How BIOS Works

The basic input-output system BIOS is the first thing you see when you turn on
your computer.
One of the most common uses of Flash memory is for the basic input/output
system of your computer, commonly known as the BIOS (pronounced "bye-ose"). On
virtually every computer available, the BIOS makes sure all the other chips, hard
drives, ports and CPU function together.
Every desktop and laptop computer in common use today contains a
microprocessor as its central processing unit. The microprocessor is the hardware
component. To get its work done, the microprocessor executes a set of instructions
known as software (see How Microprocessors Work for details). You are probably very
familiar with two different types of software:
• The operating system - The operating system provides a set of services for the
applications running on your computer, and it also provides the fundamental
user interface for your computer. Windows 98 and Linux are examples of
operating systems. (See How Operating Systems Work for lots of details.)
• The applications - Applications are pieces of software that are programmed to
perform specific tasks. On your computer right now you probably have a browser
application, a word processing application, an e-mail application and so on. You
can also buy new applications and install them.
It turns out that the BIOS is the third type of software your computer needs to
operate successfully. In this article, you'll learn all about BIOS -- what it does, how to
configure it and what to do if your BIOS needs updating.

BIOS uses Flash memory, a type of ROM.

The BIOS software has a number of different
roles, but its most important role is to load the
operating system. When you turn on your computer and
the microprocessor tries to execute its first
instruction, it has to get that instruction from
somewhere. It cannot get it from the operating
system because the operating system is located on a
hard disk, and the microprocessor cannot get to it
without some instructions that tell it how. The BIOS
provides those instructions. Some of the other
common tasks that the BIOS performs include:
• A power-on self-test (POST) for all of the different hardware components in the
system to make sure everything is working properly
• Activating other BIOS chips on different cards installed in the computer - For
example, SCSI and graphics cards often have their own BIOS chips.
• Providing a set of low-level routines that the operating system uses to interface
to different hardware devices - It is these routines that give the BIOS its name.
They manage things like the keyboard, the screen, and the serial and parallel
ports, especially when the computer is booting.
• Managing a collection of settings for the hard disks, clock, etc.
• The BIOS is special software that interfaces the major hardware components of
your computer with the operating system. It is usually stored on a Flash memory
chip on the motherboard, but sometimes the chip is another type of ROM.
When you turn on your computer, the BIOS does several things. This is its usual
sequence:

1. Check the CMOS Setup for custom settings

2. Load the interrupt handlers and device drivers
3. Initialize registers and power management
4. Perform the power-on self-test (POST)
5. Display system settings
6. Determine which devices are bootable
7. Initiate the bootstrap sequence
 The first thing the BIOS does is check the information stored in a tiny (64 bytes)
amount of RAM located on a complementary metal oxide semiconductor (CMOS) chip.
The CMOS Setup provides detailed information particular to your system and can be
altered as your system changes. The BIOS uses this information to modify or
supplement its default programming as needed. We will talk more about these settings
later.
Interrupt handlers are small pieces of software that act as translators between the
hardware components and the operating system. For example, when you press a key
on your keyboard, the signal is sent to the keyboard interrupt handler, which tells the
CPU what it is and passes it on to the operating system. The device drivers are other
pieces of software that identify the base hardware components such as keyboard,
mouse, hard drive and floppy drive. Since the BIOS is constantly intercepting signals to
and from the hardware, it is usually copied, or shadowed, into RAM to run faster.

This is the message you receive if a disk is in the drive when you restart your
computer.
Whenever you turn on your computer, the first thing you see is the BIOS
software doing its thing. On many machines, the BIOS displays text describing things
like the amount of memory installed in your computer, the type of hard disk and so on.
It turns out that, during this boot sequence, the BIOS is doing a remarkable amount of
work to get your computer ready to run. This section briefly describes some of those
activities for a typical PC.
After checking the CMOS Setup and loading the interrupt handlers, the BIOS
determines whether the video card is operational. Most video cards have a miniature
BIOS of their own that initializes the memory and graphics processor on the card. If
they do not, there is usually video driver information on another ROM on the
motherboard that the BIOS can load.
Next, the BIOS checks to see if this is a cold boot or a reboot. It does this by
checking the value at memory address 0000:0472. A value of 1234h indicates a
reboot, and the BIOS skips the rest of POST. Anything else is considered a cold boot.
If it is a cold boot, the BIOS verifies RAM by performing a read/write test of each
memory address. It checks the PS/2 ports or USB ports for a keyboard and a mouse. It
looks for a peripheral component interconnect (PCI) bus and, if it finds one, checks all
the PCI cards. If the BIOS finds any errors during the POST, it will notify you by a series
of beeps or a text message displayed on the screen. An error at this point is almost
always a hardware problem.
The BIOS then displays some details about your system. This typically includes
information about:

• The processor
• The floppy drive and hard drive
• Memory
 • BIOS revision and date
• Display
Any special drivers, such as the ones for small computer system interface (SCSI)
adapters, are loaded from the adapter, and the BIOS displays the information. The
BIOS then looks at the sequence of storage devices identified as boot devices in the
CMOS Setup. "Boot" is short for "bootstrap," as in the old phrase, "Lift yourself up by
your bootstraps." Boot refers to the process of launching the operating system. The
BIOS will try to initiate the boot sequence from the first device. If the BIOS does not
find a device, it will try the next device in the list. If it does not find the proper files on
a device, the startup process will halt. If you have ever left a disk when you restarted
your computer, you have probably seen this message.
The BIOS has tried to boot the computer off of the disk left in the drive. Since it did
not find the correct system files, it could not continue. Of course, this is an easy fix.
Simply pop out the disk and press a key to continue.