Industry standard architecture (ISA) is a type of computer bus that allows for additional expansion cards to be attached to a computer's motherboard. The bus is capable of direct memory access (DMA), with multiple expansion cards on a memory channel and separate interrupt request (IRQ) assignment for each card. It was one of the first such expansion buses developed for personal computers (PC), whose technology ultimately led to a number of offspring.
The concept behind the industry standard architecture was first developed by IBM® in 1981 and was an eight-bit system bus for the first-generation IBM® PC. Within a few years, it was expanded to a 16-bit bus for the second-generation IBM® PC known as the IBM® AT, and was named the AT bus. Since the AT bus was proprietary to IBM® the industry standard architecture name didn't come until even later, around 1988, when a 32-bit version was developed by a coalition of PC manufacturers collectively referred to as the Gang of Nine. This third iteration was actually called the extended industry standard architecture (EISA), which was developed to compete with IBM®'s next-generation bus. The ISA bus name stuck around, and previous versions got the ISA name retroactively in order to keep from infringing on IBM®'s property.
The first eight-bit ISA bus ran at 4.77 megahertz, which matched the IBM® central processing units (CPU) of the time. With the 16-bit version, it bumped the speed to six and then eight megahertz to compensate for those CPU speed increases. As CPU speeds continued to increase, an additional clock chip was later added to a motherboard that would keep the ISA bus at a particular speed. The bus has four DMA channels, only three of which supported expansion, and two were reserved for hard disk and floppy disks. The last DMA channel could support four expansion cards on an eight-bit channel, or three on a 16-bit channel.
Dealing with industry standard architecture expansion cards could sometimes be tricky, as the user had to configure the input and output (I/O) addresses, DMA channel, and IRQ by toggling dip-switches and positioning jumpers on the expansion board. Toward the latter years of the ISA bus, there was an attempt to create a plug-and-play implementation to help alleviate some of the configuration hassles. The plug-and-play implementations suffered from issues and were given the "plug-and-pray" moniker in jest.
The first death knell sounded in the late 1990s with a specification called PC 97, published by Microsoft®, that asked for the industry standard architecture to be removed from motherboards altogether. The ISA bus held on for a few more years, up into the early 21st century, particularly in specific industrial and military systems. Still, numerous offspring of ISA technology found use in computing. The advanced technology attachment (ATA) was developed from ISA technology to manage storage devices and further evolved into the more advanced serial ATA (SATA) bus.
