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ISA Industry Standard Architecture
Five 16-bit and one 8-bit ISA slots on a motherboard
Year created:	1981
Created by:	IBM
Superseded by:	PCI (1993)

Width:	8 or 16 bits
Number of devices:	1 per slot
Capacity	8 MHz
Style:	Parallel
Hotplugging?	no
External?	no

Industry Standard Architecture (in practice almost always shortened to ISA) was a computer bus standard for IBM compatible computers.

1 History
2 8-bit ISA (XT bus architecture)
3 16-bit ISA (AT bus architecture)
4 Technical data
- 4.1 8/16-bit Incompatibilities
5 Current use
6 Standardization
7 See also
8 References

History

ISA originated as an 8-bit system in the IBM PC in 1981, and was extended in 1983 as the XT bus architecture. 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 method were developed and implemented by chipsets to interface ISA to much faster CPUs. (For example, no Pentium CPU ran nearly slowly enough to directly drive the ISA bus.)

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 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 the nitty-gritty details. In reality, ISA PnP can be a major headache, and didn't become well-supported until the architecture was in its final days. This was a major contributor to the use of the phrase "plug-n-pray."

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 97 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. 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. Now (in late 2008), even floppy disk drives and serial ports are disappearing, and the extinction of vestigial ISA from chipsets may be 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 "hard cards" 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.)

8-bit ISA (XT bus architecture)

The XT bus architecture is an eight-bit ISA bus used by Intel 8086 and Intel 8088 systems in the IBM PC and IBM PC XT in the 1980s.

An 8-bit ISA (XT-bus) mouse adapter

The XT architecture has four DMA channels, of which three are brought out to the XT bus expansion slots. Of these three, two are normally allocated to machine functions:

DMA channel	Expansion	Standard function
0	No	Dynamic RAM refresh
1	Yes	Add-on cards
2	Yes	Floppy disk controller
3	Yes	Hard disk controller

The XT bus architecture has single Intel 8259 PIC and eight interrupt lines.

16-bit ISA (AT bus architecture)

The AT bus architecture is a 16-bit version of the ISA (IBM PC/XT) bus, so named because it was first introduced in the IBM PC/AT. IBM officially called this bus the "I/O Channel" of the IBM Personal Computer AT. The AT bus extends the XT-bus by adding a second, shorter, card edge connector alongside and in-line with the 8-bit ISA (XT-bus) connector, which is unchanged. (The bus is extended both logically in function and physically in connector length.) Therefore, compatibility with all 8-bit cards is retained (unless the bus is clocked faster than they can handle.) The second connector adds additional interrupt lines, control for four 16-bit DMA channels (from the AT architecture's second PIC and DMA controller chips, respectively), four additional address bits (extending the address bus from 20 to 24 bits), 8 additional data bits (for the total of 16), and additional control lines for 8 or 16 bit transfers.

A 16-bit ISA bus (AT bus) slot was originally intended to be implemented with two standard card-edge connector sockets on the motherboard or backplane, and in fact slots were implemented this way in original PC/AT machines manufactured by IBM. However, with the popularity of the AT architecture and the 16-bit ISA bus in general, connector manufacturers introduced specialized connectors that integrated the two aligned card-edge sockets into one unit, with the ISA standard spacing between them. These unified ISA slot connectors can be found in almost all AT-class PCs manufactured after approximately the mid-1980s, including those built by IBM. The ISA slot connector is almost always black, distinguishing it from EISA connectors, which are brown, and PCI connectors, which are white (and shorter, as well as horizontally offset [along the dimension perpendicular to the plane of the card] so as not to align with the edge of an ISA card whose end bracket is aligned with a slot).

Technical data

8-bit ISA or XT bus architecture

Bus width	8-bit
Compatible with	8-bit ISA
Pins	62
Vcc	+5 V, -5 V, +12 V, -12 V
Clock	4.7727266 MHz

16-bit ISA

Bus width	16-bit
Compatible with	8-bit ISA, 16-bit ISA
Pins	98
Vcc	+5 V, -5 V, +12 V, -12 V
Clock	8.333333 MHz
Capacity	3.97 MiB/s (pio) 2.65 MiB/s (dma)

Note: The ISA bus speed was synchronous with the CPU clock speed on the early PCs, resulting in many differing clock speeds, due to the many different speed machines produced by the many 'IBM clone' manufacturers. This led to problems by the late 1980s, where certain ISA cards would suffer compatibility problems and malfunctions when fitted to machines with bus speeds as high as 16 MHz, as those cards (or the software using them) were either not designed or not tested for a bus that fast. This problem was solved with later, highly integrated chipsets that (generally) standardized the 16-bit ISA bus to 8 MHz; this was achieved by using a separate clock generator inside the chipset's south bridge, which controls all peripheral I/O.

8/16-bit Incompatibilities

Memory address decoding for the selection of 8 or 16-bit transfer mode was limited to 128 KiB sections - A000-BFFF,C000-DFFF, E000-FFFF leading to problems when mixing 8 and 16-bit cards, as they could not co-exist in the same 128 KiB area.

Current use

Apart from specialized industrial use, ISA is all but gone today. Even where present, system manufacturers often shield customers from the term "ISA bus", referring to it instead as the "legacy bus" (see legacy system). The PC/104 bus, used in industrial and embedded applications, is a derivative of the ISA bus, utilizing the same signal lines with different connectors. The LPC bus has replaced the ISA bus as the connection to the legacy I/O devices on recent motherboards; while physically quite different, LPC looks just like ISA to software, so that the peculiarities of ISA such as the 16 MiB DMA limit (which corresponds to the full address space of the Intel 80286 CPU used in the original IBM AT) are likely to stick around for a while.

Starting with Windows Vista, Microsoft is phasing out support for ISA cards in Windows. Vista still supports ISA-PnP for the time being, although it's not enabled by default. However, consumer market PCs discontinued the ISA port feature on their motherboards before Windows XP was released.

As explained in the History section, ISA was the basis for development of the ATA interface, used for ATA (a.k.a. IDE) and more recently Serial ATA (SATA) hard disks. Physically, ATA is essentially a simple subset of ISA, with 16 data bits, support for exactly one IRQ and one DMA channel, and 3 address bits plus two IDE address select ("chip select") lines, plus a few unique signal lines specific to ATA/IDE hard disks (such as the Cable Select/Spindle Sync. line.) ATA goes beyond and far outside the scope of ISA by also specifying a set of physical device registers to be implemented on every ATA (IDE) drive and accessed using the address bits and address select signals in the ATA physical interface channel; ATA also specifies a full set of protocols and device commands for controlling fixed disk drives using these registers, through which all operations of ATA hard disks are performed. A further deviation between ISA and ATA is that while the ISA bus remained locked into a single standard clock rate (for backward compatibility), the ATA interface offered many different speed modes, could select among them to match the maximum speed supported by the attached drives, and kept adding faster speeds with later versions of the ATA standard (up to 100 MB/s for ATA-6, the latest.) In most forms, ATA ran much faster than ISA.

Before the 16-bit ATA/IDE interface, there was an 8-bit XT-IDE (also known as XTA) interface for hard disks, though it was not nearly as popular as ATA has become, and XT-IDE hardware is now fairly hard to find (for those vintage computer enthusiasts who may look for it.) Some XT-IDE adapters were available as 8-bit ISA cards, and XTA sockets were also present on the motherboards of Amstrad's later XT clones. The XTA pinout was very similar to ATA, but only eight data lines and two address lines were used, and the physical device registers had completely different meanings. A few hard drives (such as the Seagate ST351A/X) could support either type of interface, selected with a jumper.

A derivation of ATA was the PCMCIA specification, merely a wire-adapter away from ATA. This then meant that Compact Flash, based on PCMCIA, were (and are) ATA compliant and can, with a very simple adapter, be used on ATA ports.

Standardization

IEEE started a standardization of the ISA bus in 1985, called the P996 specification. However, despite there even having been books published on the P996 specification, it never officially progressed past draft status.