IBM PC 100, Technical Manual

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Chapter 5. Bus Architecture
PCI Bus
The PCI bus connects to the microprocessor local bus through a buffered bridge controller. A bridge
translates signals from one bus architecture to another. PCI and ISA devices receive all their data and
control information through the PCI controller. The PCI controller looks at all signals from the
microprocessor local bus and then passes them to the ISA controller, or to peripheral devices connected
to the PCI bus. However, the PCI bus is not governed by the speed of the microprocessor bus. PCI can
operate at speeds as fast as 33 MHz, slow down, or even stop if there is no activity on the bus, all
independent of the microprocessor’s operations. This independence is a distinguishing feature of PCI that
allows the microprocessor to do other work while the I/O bus is busy. Microprocessor independence also
makes PCI adaptable to various microprocessor speeds and families and allows consistency in the design
and use of PCI peripheral devices across multiple computer families.
PCI Performance
One of the most significant features of PCI is its 32-bit data path, which is twice the width of the ISA data
path. With a 32-bit data path, the PCI bus can transfer more information per second than the ISA bus
with its 16-bit data path. Also, PCI operates at higher speeds of up to 33 MHz. Depending on the mode
of operation and computer components used, the PCI bus can transfer data at speeds up to 132 MB per
second. While many factors can reduce practical performance, achieving just half or a third of the PCI
maximum theoretical throughput far exceeds the practical performance of the ISA bus at 4 MB to 8 MB
per second.
PCI Peripheral Devices
The wider data path and higher throughput make PCI a more suitable bus for today's high-speed
microprocessors and I/O devices. Higher throughput translates into higher performance of peripheral
devices, such as higher video resolutions, more colors, and quicker screen refreshes. The use of PCI
architecture enhances the performance of the monitor and the storage devices. Both the video controller
and the EIDE drive controller are connected to the PCI bus on the system board. Thus, the peripheral
devices that have the greatest demand for high performance are supported by the benefits of PCI
architecture.
Expansion-Bus Features
The bit-width of the I/O bus determines the type of adapters the computer supports. The shared slots
handle 16-bit, ISA adapters and 32-bit, PCI adapters. The dedicated ISA slots handle 16-bit, ISA adapters
only. The width of the I/O bus does not affect software compatibility.
The PC 100 and PC 300 riser card has one shared PCI and ISA slot, one dedicated PCI slot, and two
dedicated ISA slots to support a maximum of four adapters at a given time.
(On other side)
PCI
(Shared
Slot 3)
ISA
(Shared
Slot 3)
(On other side)
PCI
(Slot 4)
Riser Card
ISA
(Slot 2)
ISA
(Slot 1)
3.3 Volt PCI
Connector
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