Industry Standard Architecture

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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Superseded by:	PCI (1993)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
5 Current use
6 Standardization
7 References
8 See also

[edit] 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 be "ISA" to avoid infringing IBM's trademark on its PC/AT computer.

Designed to connect peripheral cards to the motherboard, ISA allows for bus mastering although only the first 16 MB of main memory is available for direct access. The 8-bit bus ran at 4.77 MHz, while the 16-bit bus operated at 6 or 8 MHz. 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. 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 parts originally intended for MCA because component manufacturers already had the ability to manufacture it. Both were backwards-compatible expansions of the AT 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 of 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.

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.

[edit] 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 bus has four DMA channels, of which three are brought out to the 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.

[edit] 16-bit ISA (AT bus architecture)

The AT bus architecture is an 16-bit version of the ISA bus first in the IBM PC/AT.

[edit] 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 is actually synchronous with the CPU clock speed, 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 16MHz. This problem was solved with later, highly integrated chipsets that (generally) standardized the 16-bit ISA bus to 8MHz; this was achieved by under-clocking the CPU to 8MHz only during ISA bus accesses.

[edit] 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 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.

ISA was extended to become the ATA bus, used for ATA and more recently Serial ATA hard disks. 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 in ATA ports.

[edit] Standardization

IEEE started a standardization of the ISA bus in 1985, called the P996 specification. However, despite even books being published on the P996 specification, it has never officially gotten past draft status.