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| Industry Standard Architecture (ISA) Bus Definition |
Five 16-bit and one 8-bit ISA slots shown on a motherboard |
The Industry Standard Architecture or ISA bus began as part of IBM's revolutionary PC/XT released in 1981. However, it was officially recognized as "ISA" in 1987 when the IEEE (Institute of Electrical and Electronics Engineers) formally documented standards governing its 16-bit implementation.
This first XT bus was intended to allow the addition of system options which could not be fit onto the motherboard. This XT bus was completely under the microprocessor's direct control, and its addressing width was limited to the 8-bit level of the processor. To make the bus useful, control lines were added to signal interrupts for input/output ports. Bus speed was also limited to match the processor.
The PC/XT's 8088 was a one-byte wide 4.77 MHz processor. Thus the XT bus, which required two clock cycles for data transfer, was limited to an excruciatingly slow (by today's standards) 2.38 Mbps, that could be curtailed even further if the system was busy with other tasks.
The choices made in defining the main characteristics of the ISA bus--its width and speed--can be seen by looking at the processors with which it was paired on early machines. The original ISA bus on the IBM PC was 8 bits wide, reflecting the 8 bit data width of the Intel 8088 processor's system bus, and ran at 4.77 MHz, again, the speed of the first 8088s. In 1984 the IBM AT was introduced using the Intel 80286; at this time the bus was doubled to 16 bits (the 80286's data bus width) and increased to 8 MHz (the maximum speed of the original AT, which came in 6 MHz and 8 MHz versions).
Later, the AT processors of course got faster, and eventually data buses got wider, but by this time the desire for compatibility with existing devices led manufacturers to resist change to the standard, and it has remained pretty much identical since that time. The ISA bus provides reasonable throughput for low-bandwidth devices and virtually assures compatibility with almost every PC on the market.
Bus Comparison
Chart
| Bus Type |
MB/sec |
| 8-Bit ISA |
4.77 MBps |
| 16-Bit ISA |
16.6 MBps |
| EISA |
32 MBps |
| VL-bus |
100 MBps |
| VL-bus |
132 MBps |
| 32-Bit PCI |
132 MBps |
| PCI-X
66 |
512 MBps |
| PCI-X
133 |
1 GBps |
| AGP x1 |
264 MB/s |
| AGP x2 |
528 MB/s |
| AGP x4 |
1056 MB/s |
| AGP x8 |
2112 MB/s |
| PCI
Express x1 |
500
MB/s |
| PCI
Express x2 |
1000
MB/s |
| PCI
Express x4 |
2000
MB/s |
| PCI
Express x8 |
4000
MB/s |
| PCI
Express x12 |
6000
MB/s |
| PCI
Express x16 |
8000
MB/s |
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Many expansion cards, even modern ones, are still only 8-bit cards (you can tell by looking at the edge connector on the card; 8-bit cards use only the first part of the ISA slot, while 16-bit cards use both parts). Generally, these are cards for which the lower performance of the ISA bus is not a concern. However, access to IRQs 9 through 15 is provided through wires in the 16-bit portion of the bus slots. This is why most modems, for example, cannot be set to the higher-number IRQs. IRQs cannot be shared among ISA devices.
Techincal Data
8 bit ISA or XT bus architecture view pinout diagram
| bus width |
8 Bit |
| compatible with |
8 bit ISA |
| pins |
62 |
| Vcc |
+5 V, -5 V, +12 V, -12 V |
| clock |
4.77 MHz |
16 bit ISA view pinout diagram
| bus width |
16 Bit |
| compatible with |
8 bit ISA, 16 bit ISA |
| pins |
98 |
| Vcc |
+5 V, -5 V, +12 V, -12 V |
| clock |
4.77 MHz |
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