Hardware Overview 33
2.3.1.2 PCI Features and Benefits
The PCI bus architecture has many advantages including the following:
• High data transfer speed
• Processor independence
• Cross-platform compatibility
• Multi-bus support
• Plug and Play
• Investment protection
2.3.2 The ISA Bus Architecture
The Industry Standard Architecture (ISA) is the most widely used system bus
in the PC industry. Initially, it had no official definitions or standards. However,
its specifications have since been defined by the Institute of Electrical and
Electronics Engineers (IEEE) standards group.
The ISA bus allows a transfer rate of up to 8.3 MB/s. Transfers over the ISA
bus are synchronized around 8 MHz, and they usually take a minimum of two
cycles of the bus clock to perform a data transfer. Since the data path of an
ISA bus is 16 bits wide, up to 2 bytes may be transferred during each
transaction.
On the RS/6000 43P Series Model 150 and Model 260 computers, the ISA
bus is used to attach internal devices such as the diskette drive. No ISA slots
are provided for the addition of non-native ISA devices.
There is a disadvantage in trying to connect the processor to the ISA bus
directly. The processor's speed is reduced to match the slow ISA bus speed.
Thus, the systems cannot take advantage of a fast processor.
To avoid this disadvantage, the PCI local bus is used as the primary system
bus and the ISA bus as an expansion bus. This way, the system can take
advantage of the high-speed data transfer provided by the PCI bus when
communicating with the processor and memory. On the other side, through
the PCI-ISA bridge, the bus clock can be reduced to match the ISA bus
requirements.
Table 4 provides the bus specification for different architectures and
compares them to the PowerPC processor's speed.
Table 4. PowerPC and Bus Specification
Specification PowerPC PCI ISA(8) ISA(16) EISA
Procssor
Speed 166, 200
233, 375