PC Hardware

Industry Standard Architecture (ISA)

The first-generation IBM XT (with the 8088 processor) had an 8-bit external data bus and ran at a speed of 4.77 MHz. These machines were sold with an 8-bit expansion bus (PC bus) that ran at 8.33 MHz (see Figure 10.1).

Figure 10.1 8-bit PC bus slot

IBM took steps that fueled the rapid development of the personal-computer market. Their engineers framed the PC's design as an open system, using standard, off-the-shelf components. That allowed third-party developers to manufacture cards that could snap into the PC bus. IBM also allowed its competitors to copy the PC bus.

With this move, IBM established the Industry Standard Architecture (ISA) interface, thus generating the market for clones. A host of third-party developers worked to create products that enhanced the basic machine's features and to keep prices much lower for add-ons than competing proprietary systems such as those from Apple. Without this push, the PC market would have grown more slowly and probably would have been limited to businesses with the money to pay for the more expensive products.

IBM wanted to include a new expansion bus-one that would be compatible with previously released devices-with the release of its AT (Advanced Technology) PC, featuring Intel's 80286 16-bit processor. To accomplish this, the designers added a bus that allowed insertion of either an 8-bit card or a 16-bit card. This change resulted in the standard 16-bit ISA slot. This new 16-bit bus officially ran at a top speed of 8.33 MHz, but on some Peripheral Component Interconnect (PCI)-based systems the actual rate for ISA slots proved to be as high as about 10 MHz. (PCI is discussed later in this lesson.)

The term "ISA" did not become official until 1990. Therefore, the 8-bit slot is called the XT, and the 16-bit slot is called the AT. When we refer to an ISA slot or an ISA card, we generally mean the 16-bit AT-style interface. The speed of the slots remained at about 7 MHz.

Problems with the ISA Design

The ISA design is one of the most enduring elements of the PC. It can be found on virtually all systems, from the second-generation IBM PC to machines built today. But it suffers from two major shortcomings: lack of speed and compatibility problems stemming from card design.

As CPU performance increased and applications became more powerful, card designers sought an interface that would allow add-on cards to keep up with the need for improved hard drives, display adapters, and similar products.

Expansion cards must make use of system resources in an orderly way, so that they do not conflict with other devices. When demands for these system resources are not coordinated, the system might behave erratically or even fail to boot up. Formerly, ISA cards often used a bewildering array of jumpers and switches to set addresses for memory use or the IRQ locations they would use.

The need to overcome the expansion card's slowness and compatibility problems led to a search for a new, standard expansion card interface-one that everyone could agree on and that would gain user acceptance.