Microsoft Office Tutorials and References
In Depth Information
System Unit
System Unit
The system unit is a case that contains electronic components of the computer used to process
data (Figure 12). System units are available in a variety of shapes and sizes. The case of the system
unit, also called the chassis, is made of metal or plastic and protects the internal components from
damage. The motherboard , sometimes called a system board, is the main circuit board of the
system unit. Many electronic components attach to the motherboard; others are built into it. The
sound card and video card shown in Figure 12 are examples of adapter cards, which are circuit
boards that provide connections and functions not built into the motherboard or expand on the
capability of features integrated into the motherboard.
The processor (bottom of Figure 12), also called the central processing unit ( CPU ), interprets
and carries out the basic instructions that operate a computer. Processors contain a control unit and
an arithmetic/logic unit. The control unit directs and coordinates most of the operations in the
computer. The arithmetic/logic unit ( ALU ) performs arithmetic, comparison, and other operations.
On a personal computer, all functions of the processor usually are on a single chip. A computer
chip is a small piece of semiconducting material that contains many microscopic pathways capable
of carrying electrical current. Today’s processors can perform some operations in less than the time
it takes to blink your eye.
For more information, visit and
then click Processor.
Memory consists of electronic components that store instructions waiting to be executed and
data needed by those instructions. Most memory keeps data and instructions temporarily, which
means its contents are erased when the computer is shut off. When discussing computer memory,
users typically are referring to RAM. Also called main memory, RAM (random access memory)
consists of memory chips that can be read from and written to by the processor and other devices.
These chips are placed on a memory module (lower left of Figure 12) that fits in a slot on the
motherboard in the system unit.
The amount of memory in computers is measured in kilobytes, megabytes, gigabytes, or tera-
bytes. A byte usually stores one character, such as the letter A. One kilobyte ( KB or K ) equals
exactly 1,024 bytes, and one megabyte ( MB ) equals approximately one million bytes. One
gigabyte ( GB ) equals approximately one billion bytes, and one terabyte ( TB ) equals approximately
For more information, visit and
then click Memory.
drive bay
power supply
sound card
system unit
Figure 12 The system unit
on a typical personal computer
consists of numerous electronic
components, some of which are
shown in this figure. The sound
card and video card are two
types of adapter cards.
video card
memory module
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