IBM RS/6000 - Manual (Page 38)

The key to cross-platform compatibility is processor independence. Until PCI,
different systems used different buses, such as ISA, EISA, NuBus, and so forth.
Now, different systems can use one bus.
Multi-bus Support
An important aspect to PCI-based system architecture is support for multiple PCI
buses, operating transparently to existing software.
Plug and Play
PCI peripherals, following the PCI standard, load the appropriate set of installation,
configuration and booting information to the host CPU without user intervention.
This provides a greater ease of use for the system integrator or end-user.
Investment protection
The PCI bus is designed for 64-bit addressing support.
2.1.2 The ISA BUS Architecture
The Industry Standard Architecture (ISA) is the most widely used system bus in the
PC industry. Originally, there were no official definition or standards for it. Later
on, its specifications were defined by the Institute of Electrical and Electronics
Engineers (IEEE) standards group.
The ISA bus, if implemented, 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 two bytes may be transferred during each
transaction.
Since ISA is the most widely bus architecture used in the PC industry, it makes
sense to provide users with the possibility to use hardware peripherals they already
use with other systems, rather than having to expend additional money for these
peripherals.
Moreover, supporting the ISA bus architecture, the system-provider has access to a
wide spectrum of adapters and devices already available in the marketplace and
does not have to wait for adapters to be built for a specific system bus. The
provider has to ensure that the device driver for the specific operating system is
also available.
The problem when connecting the processor to the ISA bus directly is that the
processor's speed has to be reduced to match the slow ISA bus speed; thus the
systems cannot take advantage of a fast processor.
Table 1 shows the bus specification for different architectures and compares them
to the PowerPC processor's speed.
14 Introduction to PCI-Based RS/6000 Servers