One of the most important features of a motherboard is its physical form factor, or its size and shape and the locations of key features. Many manufacturers, particularly major brands, use proprietary form factors, which should be avoided. If you buy a machine that has a proprietary motherboard and you need to replace it due to a repair or upgrade, you will find your selection limited (or non-existent) and overpriced. Some manufacturers undoubtedly use these proprietary designs to lower their manufacturing cost by eliminating cables for serial, parallel, and other I/O ports; others may have more sinister motives.
The older AT (or baby AT) form factor motherboards are interchangeable, but have very little printed circuit board real estate along the back edge of the machine on which to
The Linux Programming Toolkit Part i mount connectors. The case only has holes to accommodate the keyboard and maybe a mouse connector. The newer ATX standard has many advantages. Although an ATX motherboard is approximately the same size and shape as a baby AT motherboard (both are about the same size as a sheet of 8-1/2"x11" writing paper), the ATX design rotates the dimensions so the long edge is against the back of the machine. An ATX case has a standard rectangular cutout that accommodates metal inserts, which have cutouts that match the connectors on a particular motherboard. The large cutout is large enough to easily accommodate the following using stacked connectors:
• keyboard port
• VGA connector
• audio connectors
Also, ATX moves the CPU and memory where they will not interfere with full-length I/O cards, although some manufacturers still mount some internal connectors where they will interfere. Many case manufacturers have retooled. More information about the ATX form factor can be found at http://www.teleport.com/-atx/. Figure 2.1 illustrates the physical difference between AT and ATX form factors.
AT versus ATX motherboard form factors.
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