IBM PC 100, Technical Manual

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Chapter 5. Bus Architecture
Bus Architecture Descriptions
This section gives an overview of input/output (I/O) buses and explains how advanced I/O buses can
improve performance.
A computer bus is a pathway of wires and signals that carry (or transfer) information inside the computer.
Information includes data, addresses, instructions, and controls. The microprocessor has an external bus,
called the
microprocessor bus or local bus, that carries information between the microprocessor and main
memory. The local bus has the same bus width (64 bits) as the microprocessor and operates at the same
external speed.
Another computer bus, the I/O bus or expansion bus, carries information between the microprocessor or
memory and the I/O (peripheral) devices. While microprocessor-bus performance has improved rapidly,
improvements in I/O-bus performance have not equalled those of microprocessors and some peripheral
devices, such as video and disk controllers. Regardless of how fast the microprocessor and other
components are, data transfers between them must pass through the I/O bus.
The computer has two I/O buses: the ISA bus and the PCI bus. ISA has been the standard I/O bus used
in IBM and IBM-compatible computers for many years. PCI is one of the advanced I/O bus standards
developed by the computer industry to keep up with performance improvements of microprocessor buses
and advanced peripheral devices. Although advanced designs, such as PCI, cannot match the
performance of the microprocessor bus, they do achieve higher throughput by speeding up the I/O bus
and widening its data path. PCI is intended to add to, but not replace, the capability of the ISA bus. In
fact, most personal computers today need only three PCI connections: one for video, one for the disk
controller, and one for a network adapter or other optional device.
ISA Bus
One of the most widely used and successful bus architectures is the AT bus, also called the industry
standard architecture (ISA) bus, or the I/O channel. 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 to 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. ISA is adequate for users of DOS applications in a stand-alone
environment, or for DOS network requestors 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 for the performance needs of 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 video adapters.
In PC 100 and PC 300 computers, the ISA bus is buffered to provide sufficient power for the 98-pin
connectors, assuming two low-power Schottky (LS) loads per slot. The signal assignments and pin
assignments for the I/O channel connectors are shown in Figure 15 on page 18.
Chapter 5. Bus Architecture
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