Definition

ISA (Industry Standard Architecture)

Contributor(s): Corinne Bernstein

ISA (Industry Standard Architecture) is a standard bus (computer interconnection) architecture that was associated with the IBM AT motherboard. The first IBM personal computers (PCs) introduced in 1981 included the 8-bit subset of the ISA bus, and the PC AT, which IBM launched in 1984, was the first full 16-bit implementation of the ISA bus. It allowed 16 bits at a time to flow between the motherboard circuitry and an expansion slot card and its associated device(s).

Pronounced "eye-suh" and called the "AT bus" by some, ISA was one of the first expansion buses for PCs. Providing the hardware interface for connecting peripheral devices in PCs, ISA accepted cards for sound, display, hard drives and other devices. ISA allowed for additional expansion cards to be attached to a computer's motherboard and was capable of direct memory access (DMA), with multiple expansion cards on a memory channel and separate interrupt request (IRQ) assignment for each card. The development and use of ISA led to several later technologies.

Parallel buses such as ISA, Peripheral Component Interconnect (PCI) and Versa Module Europa (VME) once dominated the embedded and PC space. However, most modern PCs have devices integrated into the motherboard rather than expansion cards.

History

The concept for the ISA bus was developed in 1981 by an IBM design team led by inventor and computer engineer Mark Dean. The bus was designed to support the Intel 8088 microprocessor for IBM's first-generation PCs. While ISA started as an 8-bit bus, a 16-bit version, the IBM AT bus, launched in 1984 with the release of the IBM PC AT. In 1988, the 32-bit Extended Industry Standard Architecture (EISA) standard was proposed by a group of companies that manufactured IBM-compatible PCs. The group, dubbed the Gang of Nine, retroactively renamed the AT bus "ISA" to avoid infringing upon IBM's trademark on its PC AT computer. The makers of IBM-compatible PCs improved upon the performance of the AT bus.

In 1987, IBM replaced the ISA bus with the introduction of its Micro Channel Architecture, which was used in the company's PS/2 and later systems until the mid-1990s. The proprietary Micro Channel Architecture was a closed standard, unlike ISA for which IBM had released full specifications and circuit schematics. Makers of IBM-compatible PCs sought to circumvent steep fees for licensing Micro Channel Architecture and focused initially on the 16-bit AT bus.

Manufacturers of IBM compatibles responded to IBM's Micro Channel Architecture by developing the EISA and the later VESA Local Bus (VL-Bus), both compatible expansions of the ISA standard. EISA, which provided 32 bits at 8 MHz, addressed servers' technical needs. Meanwhile, the VL-Bus (named after VESA, the Video Electronics Standards Association, which created the standard) was 32 bits wide and operated at the speed of the local bus, which was normally the speed of the processor itself. Although the VL-Bus tied directly into the CPU, connecting more than two devices to the VL-Bus could interfere with the CPU's performance. So, the VL-Bus was typically used primarily for connecting a graphics card, which could benefit from high-speed access to the CPU.

In the early 1990s, Intel developed PCI, which combined the characteristics of ISA and VL-Bus. PCI provided direct access to system memory for connected devices while employing a bridge to connect to the frontside bus and, thus, to the CPU. As a result, PCI's performance exceeded the VL-Bus while eliminating the potential for interference with the CPU.

By the mid-1990s, new motherboards were manufactured with fewer ISA slots, and PCI became standard for connecting computers and their peripherals. For several years, motherboards had a combination of 8-bit and 16-bit ISA slots. As PCI became popular, motherboards included only 16-bit ISA and PCI. Yet, by the early 2000s, the PCI interface replaced ISA.

Although the PCI bus is still used for specific cards, the USB (Universal Serial Bus) has largely replaced the PCI expansion card. PCI Express (PCIe), PCI's successor, is faster and uses fewer circuit board traces because of its serial nature; in fact, Ethernet has joined PCIe on some backplanes.

ISA bus architecture

The PC bus in early IBM PCs, which was 16 bits wide and operated at 4.77 MHz, remained in use for years because of its compatibility with systems from many hardware makers. This bus design could pass along data at a rate of up to approximately 9 MB (megabytes) per second, fast enough for many of today's applications.

The ISA bus provided direct memory access using multiple expansion cards on a memory channel allowing separate interrupt request transactions for each card. Plus, the ISA bus could support a network card, additional serial ports, a video card, and other processors and architectures, such as the IBM PC with an Intel Core 8088 microprocessor, the IBM AT with an Intel 80286 processor (1984) and Extended Industry Standard Architecture (1988). Additionally, ISA was designed to be backward-compatible with the IBM PC XT and to work with IBM-compatible systems. (The PC XT bus was an 8-bit ISA bus that Intel 8086 and Intel 8088 systems used in the IBM PC and IBM PC XT. The PC AT bus, a 16-bit (or 80286) version of the PC XT bus, was introduced with the IBM PC AT.)

The 16-bit AT bus slot originally used two standard edge connector sockets in early IBM PC AT machines. However, with the popularity of the AT architecture and the 16-bit ISA bus, manufacturers introduced specialized 98-pin connectors that integrated the two sockets into one unit. These were in almost every AT-class PC manufactured after the mid-1980s. The ISA slot connector is typically black, distinguishing it from the brown EISA connectors and white PCI connectors.

The ISA bus, which initially included synchronicity with the CPU clock, was upgraded to high-level buffering, which served as an interface between the chipsets and the CPU. Bus mastering, which the ISA bus employed, directly accessed just the first 16 MB of main memory.

Standardization

Although the PC-AT Technical Reference, which documented the bus architecture included detailed schematics and BIOS (basic input/output system) listings, it did not include the timings, rules and other requirements necessary to make it a good bus specification. So, various implementations of ISA were not always compatible with each other. Although some ISA bus specifications were produced to address the compatibility problems over time, these specifications did not always agree with each other, so no single specification for the ISA bus was developed.

IEEE (Institute of Electrical and Electronics Engineers) did seek to standardize the ISA bus in 1985 with the draft of P996. However, the specification never progressed beyond draft status.

The venerable bus architecture is rarely used today. A small number of ISA cards are still produced -- for example, with USB ports or complete single-board computers based on modern processors, USB 3.0 and Serial ATA (SATA). Even when ISA is still used in specialized industrial applications, system manufacturers often avoid using the term ISA bus, referring to it instead as the legacy bus. The PC/104 bus, used in industrial and embedded applications, is a derivative of the ISA bus and uses the same signal lines with different connectors. The Low Pin Count (LPC) bus has largely replaced the ISA bus as the connection to the legacy I/O (input/output) devices on modern motherboards.

This was last updated in March 2020

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