Industry Standard Architecture
This article needs additional citations for verification. (January 2014) |
Industry Standard Architecture | |
Half-duplex 8 MB/s or 16 MB/s[1] | |
Style | Parallel |
---|---|
Hotplugging interface | No |
External interface | No |
Industry Standard Architecture (ISA) is the
Originally referred to as the PC bus (8-bit) or AT bus (16-bit), it was also termed I/O Channel by IBM. The ISA term was coined as a retronym by IBM PC clone manufacturers in the late 1980s or early 1990s as a reaction to IBM attempts to replace the AT-bus with its new and incompatible Micro Channel architecture.
The 16-bit ISA bus was also used with 32-bit processors for several years. An attempt to extend it to 32 bits, called Extended Industry Standard Architecture (EISA), was not very successful, however. Later buses such as VESA Local Bus and PCI were used instead, often along with ISA slots on the same mainboard. Derivatives of the AT bus structure were and still are used in ATA/IDE, the PCMCIA standard, CompactFlash, the PC/104 bus, and internally within Super I/O chips.
Even though ISA disappeared from consumer desktops many years ago, it is still used in industrial PCs, where certain specialized expansion cards that never transitioned to PCI and PCI Express are used.
History
The original PC bus was developed by a team led by
IBM designed the 8-bit version as a buffered interface to the motherboard buses of the Intel 8088 (16/8 bit) CPU in the IBM PC and PC/XT, augmented with prioritized interrupts and DMA channels. The 16-bit version was an upgrade for the motherboard buses of the Intel 80286 CPU (and expanded interrupt and DMA facilities) used in the IBM AT, with improved support for bus mastering. The ISA bus was therefore synchronous with the CPU clock, until sophisticated buffering methods were implemented by chipsets to interface ISA to much faster CPUs.
ISA was designed to connect peripheral cards to the motherboard and allows for bus mastering. Only the first 16 MB of main memory is addressable. The original 8-bit bus ran from the 4.77 MHz clock of the 8088 CPU in the IBM PC and PC/XT. The original 16-bit bus ran from the CPU clock of the 80286 in IBM PC/AT computers, which was 6 MHz in the first models and 8 MHz in later models. The IBM RT PC also used the 16-bit bus. ISA was also used in some non-IBM compatible machines such as Motorola 68k-based Apollo (68020) and Amiga 3000 (68030) workstations, the short-lived AT&T Hobbit and the later PowerPC-based BeBox.
Companies like
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 "
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 could be troublesome and did not become well-supported until the architecture was in its final days.
A PnP ISA, EISA or VLB device may have a 5-byte EISA ID (3-byte manufacturer ID + 2-byte hex number) to identify the device. For example, CTL0044 corresponds to Creative Sound Blaster 16 / 32 PnP.
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.
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
ISA bus architecture
The PC/XT-bus is an eight-
DMA channel | Expansion | Standard function |
---|---|---|
0 | No | Dynamic random-access memory refresh |
1 | Yes | Add-on cards |
2 | Yes | Floppy disk controller |
3 | Yes | Hard disk controller
|
The PC/AT-bus, a 16-
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 can be found 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).
Number of devices
Motherboard devices have dedicated IRQs (not present in the slots). 16-bit devices can use either PC-bus or PC/AT-bus IRQs. It is therefore possible to connect up to 6 devices that use one 8-bit IRQ each and up to 5 devices that use one 16-bit IRQ each. At the same time, up to 4 devices may use one 8-bit DMA channel each, while up to 3 devices can use one 16-bit DMA channel each.
Varying bus speeds
Originally, the bus clock was synchronous with the CPU clock, resulting in varying bus clock frequencies among the many different IBM "clones" on the market (sometimes as high as 16 or 20 MHz), leading to software or electrical timing problems for certain ISA cards at bus speeds they were not designed for. Later motherboards or integrated chipsets used a separate clock generator, or a clock divider which either fixed the ISA bus frequency at 4, 6, or 8 MHz or allowed the user to adjust the frequency via the BIOS setup. When used at a higher bus frequency, some ISA cards (certain Hercules-compatible video cards, for instance), could show significant performance improvements.
8/16-bit incompatibilities
Memory address decoding for the selection of 8 or 16-bit transfer mode was limited to 128 KiB sections, leading to problems when mixing 8- and 16-bit cards as they could not co-exist in the same 128 KiB area. This is because the MEMCS16 line is required to be set based on the value of LA17-23 only.
Past and current use
This section needs to be updated.(January 2020) |
ISA is still used today for specialized industrial purposes. In 2008 IEI Technologies released a modern motherboard for Intel Core 2 Duo processors which, in addition to other special I/O features, is equipped with two ISA slots. It is marketed to industrial and military users who have invested in expensive specialized ISA bus adaptors, which are not available in
Similarly, ADEK Industrial Computers is releasing a motherboard in early 2013 for Intel Core i3/i5/i7 processors, which contains one (non-DMA) ISA slot.[7]
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.
ATA
As explained in the History section, ISA was the basis for development of the
A further deviation between ISA and ATA is that while the ISA bus remained locked into a single standard clock rate (for backward hardware 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 133 MB/s for ATA-6, the latest.) In most forms, ATA ran much faster than ISA, provided it was connected directly to a local bus (e.g. southbridge-integrated IDE interfaces) faster than the ISA bus.
XT-IDE
Before the 16-bit
Many later AT (and AT successor) motherboards had no integrated hard drive interface but relied on a separate hard drive interface plugged into an ISA/EISA/VLB slot. There were even a few 80486 based units shipped with MFM/RLL interfaces and drives instead of the increasingly common AT-IDE.
Commodore built the XT-IDE based peripheral hard drive / memory expansion unit A590 for their Amiga 500 and 500+ computers that also supported a SCSI drive. Later models – the A600, A1200, and the Amiga 4000 series – use AT-IDE drives.
PCMCIA
The
Emulation by embedded chips
Although most modern computers do not have physical ISA buses, almost all PCs — IA-32, and x86-64 — have ISA buses allocated in physical address space. Some Southbridges and some CPUs themselves provide services such as temperature monitoring and voltage readings through ISA buses as ISA devices.[citation needed]
Standardization
IEEE started a standardization of the ISA bus in 1985, called the P996 specification. However, despite books being published on the P996 specification, it never officially progressed past draft status.[8]
Modern ISA cards
There still is an existing user base with old computers, so some ISA cards are still manufactured, e.g. with USB ports[9] or complete single-board computers based on modern processors, USB 3.0, and SATA.[10]
See also
- PC/104 - Embedded variant of ISA
- Low Pin Count (LPC)
- Extended Industry Standard Architecture (EISA)
- Micro Channel architecture (MCA)
- VESA Local Bus (VLB)
- Peripheral Component Interconnect (PCI)
- Accelerated Graphics Port (AGP)
- PCI-X
- PCI Express (PCI-E or PCIe)
- List of computer bus interfaces
- Amiga Zorro II
- NuBus
- Switched fabric
- List of device bandwidths
- CompactPCI
- PC card
- Universal Serial Bus(USB)
- Legacy port
- Backplane
References
- ^ Kyle Chapman. "The Wonderful World of Buses". Retrieved 2021-06-30.
- OCLC 51258496.
- ^ John Titus (2001-09-15). "Whence Came the IBM PC". edn.com. Retrieved 2020-10-13.
- ^ LaPlante, Alice; Furger, Roberta (1989-01-23). "Compaq Vying To Become the IBM of the '90s". InfoWorld. pp. 1, 8. Retrieved 17 March 2016.
- ^ Lewis, Peter H. (1988-04-24). "Introducing the First PS/2 Clones". The New York Times. Archived from the original on 6 January 2015. Retrieved 6 January 2015.
- ^ IEI Technology Corp: IMBA-9654ISA User Manual, Rev. 1.00, May 2008
- ^ ADEK Industrial Computers: MS-98A9 Product Specifications
- ISBN 1401852300.
- ^ "Lo-tech ISA USB Adapter - lo-tech.co.uk". www.lo-tech.co.uk. Archived from the original on 9 April 2018. Retrieved 3 May 2018.
- ^ "PCA-6763". www.advantech.com. Archived from the original on 24 October 2017. Retrieved 3 May 2018.
Further reading
- Intel ISA Bus Specification and Application Notes - Rev 2.01; Intel; 73 pages; 1989.
External links
- "Connector Bus ISA (Industry Standard Architecture)". Hardware Book.
- Microsoft (1999-06-02). "Removing the ISA Architecture in Windows-Based Platforms". Microsoft. Archived from the original (Microsoft Word) on 2012-03-11. Retrieved 2007-07-14.