Lenovo PC 300GL Understanding Your Personal Computer 6267, 6277, 6287 - Page 17

ISA-Bus, PCI Interface, PCI Performance

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PCI Interface The PCI-bus connects to the microprocessor bus through a buffered bridge controller. A bridge translates signals from one bus architecture to another. PCI and ISA devices get all their data and control information through the PCI controller. The PCI controller looks at all signals from the microprocessor 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 MHz1, slow down, or even stop if there is no activity on the bus, all independent of the microprocessor's operations. 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, which operates at 4 MB to 8 MB per second. ISA-Bus One of the most widely used and successful expansion-buses is the industry standard architecture (ISA) bus, also called the AT bus. The ISA-bus is a 16-bit bus that operates at a speed of 8 MHz. It can transfer up to 8 MB of data per second between the microprocessor and an I/O device. Practical performance ranges between 4 MB and 8 MB per second. The ISA-bus continues to be popular because so many adapters, devices, and applications have been designed and marketed for it. Peripheral devices that do not require faster throughput, such as fax modems, can use ISA. Also, ISA is adequate for users of DOS applications in a stand-alone environment, or for DOS network requesters with moderate performance requirements. Although the ISA-bus is widely used and is suitable for many applications, it cannot transfer data fast enough for today's high-speed microprocessors and I/O devices. For example, the ISA-bus might not provide the performance needed by video devices and applications with high resolution and high-color content. Also, ISA might not be capable of handling the throughput required by some fast hard disk drives, network controllers, or full-motion graphics controllers. Expansion-Bus 9

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PCI Interface
The PCI-bus connects to the microprocessor bus through a buffered bridge controller.
A
bridge
translates signals from one bus architecture to another.
PCI and ISA devices get all
their data and control information through the PCI controller.
The PCI controller looks at
all signals from the microprocessor 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
1
, slow down,
or even stop if there is no activity on the bus, all independent of the microprocessor’s
operations.
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, which operates at 4 MB to
8 MB per second.
ISA-Bus
One of the most widely used and successful expansion-buses is the
industry standard
architecture (ISA) bus
, also called the
AT bus
.
The ISA-bus is a 16-bit bus that operates at a
speed of 8 MHz.
It can transfer up to 8 MB of data per second between the microprocessor
and an I/O device.
Practical performance ranges between 4 MB and 8 MB per second.
The ISA-bus continues to be popular because so many adapters, devices, and applications
have been designed and marketed for it.
Peripheral devices that do not require faster
throughput, such as fax modems, can use ISA.
Also, ISA is adequate for users of DOS
applications in a stand-alone environment, or for DOS network requesters with moderate
performance requirements.
Although the ISA-bus is widely used and is suitable for many applications, it cannot
transfer data fast enough for today's high-speed microprocessors and I/O devices.
For
example, the ISA-bus might not provide the performance needed by video devices and
applications with high resolution and high-color content.
Also, ISA might not be capable of
handling the throughput required by some fast hard disk drives, network controllers, or
full-motion graphics controllers.
Expansion-Bus
9