Datasheet

32
Chapter 1
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Personal Computer System Components
The POST process can end with a beep code or displayed code that indicates the issue dis-
covered. Each BIOS publisher has its own series of codes that can be generated. Figure 1.25
shows a simplified POST display during the initial boot sequence of a computer.
FIGURE 1.25 An example of a BIOS boot screen
CMOS and CMOS Battery
Your PC has to keep certain settings when it’s turned off and its power cord is unplugged.
These settings include the following:
Date
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Time
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Hard drive configuration
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Memory
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Integrated ports
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Boot sequence
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Power management
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Your PC keeps these settings in a special memory chip called the complementary metal
oxide semiconductor (CMOS) memory chip. Actually, CMOS (usually pronounced see-moss)
is a manufacturing technology for integrated circuits. The first commonly used chip made
from CMOS technology was a type of memory chip, the memory for the BIOS. As a result,
the term CMOS is the accepted name for this memory chip.
The BIOS starts with its own default information and then reads information from the
CMOS, such as which hard drive types are configured for this computer to use, which
drive(s) it should search for boot sectors, and so on. Any overlapping information read
from the CMOS overrides the default information from the BIOS. A lack of corresponding
information in the CMOS does not delete information that the BIOS knows natively. This
process is a merge, not a write-over. CMOS memory is usually not upgradable in terms of
its capacity and might be integrated into the BIOS chip or some other chip.
To keep its settings, integrated circuit-based memory must have power constantly. When
you shut off a computer, anything that is left in this type of memory is lost forever. The
CMOS manufacturing technology produces chips with very low power requirements. One
ramification of this fact is that today’s electronic circuitry is more susceptible to damage
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