Industry Standard Architecture

Disposition of pins (pinout16-bit ISA bus.

In computing, Industry Standard Architecture or ISA (in Spanish: Arquitectura Industrial Estándar), is a bus architecture created in 1980 by IBM, in Boca Raton (Florida), to be used in IBM PC computers.

Historical overview

ISA was created in 1980 as an 8-bit system on the IBM PC, and in 1983 it was extended as the XT bus architecture. The new 16-bit standard is introduced in 1984 and is commonly called the AT bus architecture. Designed to connect expansion cards to the motherboard, the protocol also allows for bus mastering, although only the first 16 MiB of main memory is available for direct access. The 8-bit bus runs at 4.77 MHz (the same speed as the Intel 8088 processor used in the IBM PC), while the 16-bit bus runs at 8 MHz (the Intel 80286 used in the IBM AT). It is also available on some machines that are not IBM PC compatible, such as the AT&T Hobbit (of short history), the Commodore Amiga 2000, 3000 and 4000, as well as the PowerPC-based BeBoxes. Physically, the XT slot is a 62-contact (31 per side) and 8.5 cm card edge connector, while the AT adds a second 36-contact (18 per side) connector, with a size of 14 cm.. Both are usually black. Being backward compatible, an XT card can be plugged into an AT slot with no problem, except on poorly physically designed boards.

In 1987, IBM began to replace the ISA bus with its proprietary MCA (Micro Channel Architecture) bus in an attempt to regain control of the PC architecture and with it the PC market. The system is much more advanced than ISA, but physically and logically incompatible, so computer manufacturers respond with Extended Industry Standard Architecture (EISA) and later with VESA Local Bus (VLB). In fact, VLB uses some parts originally designed for MCA because component manufacturers already have the ability to manufacture them. Both are ISA standard compliant extensions.

Users of ISA-based machines had to have special information about the hardware they were adding to the system. Although a handful of cards were essentially plug-and-play, this was not the norm. There were often a number of things to configure when adding a new device, such as the IRQ, input/output addresses, or the direct memory access (DMA) channel. MCA had solved those problems, and today PCI incorporates many of the ideas that were born with MCA (although they descend more directly from EISA).

These configuration issues led to the creation of the ISA PnP, a plug-and-play system that uses a combination of modifications to the hardware, system BIOS, and the operating system software that automatically handles the coarser details. Actually, ISA PnP ended up becoming a chronic headache, and was never well supported except at the very end of the ISA story. This is where the extension of the sarcastic phrase plug-and-pray comes from.

PCI slots were the first physically ISA-incompatible expansion port to force it out of the motherboard. At first, motherboards were largely ISA, including some PCI slots. But by the mid-1990s, the two types of slots were evenly balanced, and soon ISAs became a minority in consumer computers. Microsoft's PC 97 specifications recommended that ISA slots be removed entirely, although the system architecture still requires residual-mode ISA to address floppy drives, RS-232 ports, etc. The ISA slots stick around for a few more years, and it's possible to see boards with an Accelerated Graphics Port (AGP) slot right next to the CPU, a number of PCI slots, and one or two ISA slots near the edge.

It is also notable that the PCI slots are "rotated" compared to ISAs. The external connectors and the main circuitry of ISA are arranged on the left side of the board, while those of PCI are on the right side, always looking from above. In this way both slots could be together, being able to use only one of them, which squeezed the motherboard.

The maximum bandwidth of the 16-bit ISA bus is 16 Mbyte/second. This bandwidth is insufficient for current needs, such as high-resolution video cards, so the ISA bus is not used in modern PCs, where it has been replaced by the PCI bus.

8-bit ISA slot (XT architecture)

8-bit ISA Slot, used by XT PCs.

The XT architecture is an 8-bit bus architecture used in PCs with Intel 8086 and 8088 processors, such as the IBM PC and IBM PC XT in the 1980s. It predates the AT architecture 16-bit used on IBM Personal Computer/AT compatible machines.

The XT bus has four direct memory access (DMA) channels, three of which are in expansion slots. Of those three, two are normally assigned to machine functions: