Hardware Reference
In-Depth Information
The Micro Channel Bus
The introduction of 32-bit chips meant that the ISA bus could not handle the power of
another new generation of CPUs. The chips could transfer 32 bits of data at a time, but
the ISAbuscan handle amaximum ofonly16bits. Rather than extend the ISAbusagain,
IBM decided to build a new bus; the result was the MCA bus. MCA (an abbreviation for
MicroChannel Architecture ) was completely different from the ISA bus and was technic-
ally superior in every way. However, because IBM was the only vendor to use it, MCA
never became a common standard, and was phased out in 1996.
The EISA Bus
TheEISAstandardwasannouncedinSeptember1988asaresponsetoIBM'sintroduction
of the MCA bus—more specifically, to the way IBM wanted to handle licensing of the
MCA bus. Vendors did not feel obligated to pay retroactive royalties on the ISA bus, so
they turned their backs on IBM and created their own buses.
Compaq was the primary developer of the EISA standard, which was intended to be the
company's way of taking over future development of the PC bus from IBM. Compaq
knew that nobody would clone its bus if it was the only company that had it, so it es-
sentially gave the design to other leading manufacturers. Compaq formed the EISA com-
mittee, a nonprofit organization designed specifically to control development of the EISA
bus. Few EISA adapters were developed. Those that were developed centered mainly on
disk array controllers and server-type network cards.
The EISA bus was essentially a 32-bit version of ISA. Unlike the MCA bus from IBM,
you could still use older 8-bit or 16-bit ISA cards in 32-bit EISA slots, providing for full
backward compatibility. As with MCA, EISA also allowed for automatic configuration of
EISA cards via software.
To learn more about the EISA Bus, including pinouts of the EISA slot, see “ The EISA
Bus in Chapter 4 , Motherboards and Buses , ” in Upgrading and Repairing PCs, 19 th
edition , available in its entirety on the DVD packaged with this topic.
Local Buses (VESA, PCI, PCI Express, AGP)
The I/O buses discussed so far (ISA, MCA, and EISA) have one thing in common: relat-
ivelyslowspeed.Thenextthreebustypesthatarediscussedinthefollowingfewsections
use the local bus concept explained in this section to address the speed issue. The main
local buses found in PC systems are
• VL-Bus (VESA local bus)
• PCI
 
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