Industry Standard Architecture - History

History

The ISA bus was developed by a team led by Mark Dean at IBM as part of the IBM PC project in 1981. It originated as an 8-bit system. The newer 16-bit standard, the IBM AT bus, was introduced in 1984. In 1988, the Gang of Nine IBM PC compatible manufacturers put forth the 32-bit EISA standard and in the process retroactively renamed the AT bus to "ISA" to avoid infringing IBM's trademark on its PC/AT computer. IBM designed the 8-bit version as a buffered interface to the external bus of the Intel 8088 (16/8 bit) CPU used in the original IBM PC and PC/XT, and the 16-bit version as an upgrade for the external bus of the Intel 80286 CPU used in the IBM AT. Therefore, the ISA bus was synchronous with the CPU clock, until sophisticated buffering methods were developed and implemented by chipsets to interface ISA to much faster CPUs.

Designed to connect peripheral cards to the motherboard, ISA allows for bus mastering although only the first 16 MB of main memory are available for direct access. The 8-bit bus ran at 4.77 MHz (the clock speed of the IBM PC and IBM PC/XT's 8088 CPU), while the 16-bit bus operated at 6 or 8 MHz (because the 80286 CPUs in IBM PC/AT computers ran at 6 MHz in early models and 8 MHz in later models.) IBM RT/PC also used the 16-bit bus. It was also available on some non-IBM compatible machines such as Motorola 68k-based Apollo (68020) and Amiga 3000 (68030) workstations, the short-lived AT&T Hobbit and later PowerPC based BeBox.

In 1987, IBM moved to replace the AT bus with their proprietary Micro Channel Architecture (MCA) in an effort to regain control of the PC architecture and the PC market. (Note the relationship between the IBM term "I/O Channel" for the AT-bus and the name "Micro Channel" for IBM's intended replacement.) MCA had many features that would later appear in PCI, the successor of ISA, but MCA was a closed standard, unlike ISA (PC-bus and AT-bus) for which IBM had released full specifications and even circuit schematics. The system was far more advanced than the AT bus, and computer manufacturers responded with the Extended Industry Standard Architecture (EISA) and later, the VESA Local Bus (VLB). In fact, VLB used some electronic parts originally intended for MCA because component manufacturers already were equipped to manufacture them. Both EISA and VLB were backwards-compatible expansions of the AT (ISA) bus.

Users of ISA-based machines had to know special information about the hardware they were adding to the system. While a handful of devices were essentially "plug-n-play", this was rare. Users frequently had to configure several parameters when adding a new device, such as the IRQ line, I/O address, or DMA channel. MCA had done away with this complication, and PCI actually incorporated many of the ideas first explored with MCA (though it was more directly descended from EISA).

This trouble with configuration eventually led to the creation of ISA PnP, a plug-n-play system that used a combination of modifications to hardware, the system BIOS, and operating system software to automatically manage resource allocations. In reality, ISA PnP can be troublesome, and did not become well-supported until the architecture was in its final days.

PCI slots were the first physically incompatible expansion ports to directly squeeze ISA off the motherboard. At first, motherboards were largely ISA, including a few PCI slots. By the mid-1990s, the two slot types were roughly balanced, and ISA slots soon were in the minority of consumer systems. Microsoft's PC 99 specification recommended that ISA slots be removed entirely, though the system architecture still required ISA to be present in some vestigial way internally to handle the floppy drive, serial ports, etc., which was why the software compatible LPC bus was created. ISA slots remained for a few more years, and towards the turn of the century it was common to see systems with an Accelerated Graphics Port (AGP) sitting near the central processing unit, an array of PCI slots, and one or two ISA slots near the end. In late 2008, even floppy disk drives and serial ports were disappearing, and the extinction of vestigial ISA (by then the LPC bus) from chipsets was on the horizon.

It is also notable that PCI slots are "rotated" compared to their ISA counterparts—PCI cards were essentially inserted "upside-down," allowing ISA and PCI connectors to squeeze together on the motherboard. Only one of the two connectors can be used in each slot at a time, but this allowed for greater flexibility.

The AT Attachment (ATA) hard disk interface is directly descended from ISA (the AT bus). ATA has its origins in hardcards that integrated a hard disk controller (HDC) — usually with an ST-506/ST-412 interface — and a hard disk drive on the same ISA adapter. This was at best awkward from a mechanical structural standpoint, as ISA slots were not designed to support such heavy devices as hard disks (and the 3.5" form-factor hard disks of the time were about twice as tall and heavy as modern drives), so the next generation of Integrated Drive Electronics drives moved both the drive and controller to a drive bay and used a ribbon cable and a very simple interface board to connect it to an ISA slot. ATA, at its essence, is basically a standardization of this arrangement, combined with a uniform command structure for software to interface with the controller on a drive. ATA has since been separated from the ISA bus, and connected directly to the local bus (usually by integration into the chipset), to be clocked much much faster than ISA could support and with much higher throughput. (Notably when ISA was introduced as the AT bus, there was no distinction between a local and extension bus, and there were no chipsets.) Still, ATA retains details which reveal its relationship to ISA. The 16-bit transfer size is the most obvious example; the signal timing, particularly in the PIO modes, is also highly correlated, and the interrupt and DMA mechanisms are clearly from ISA. (The article about ATA has more detail about this history.)