What’s the Industry Standard Architecture?

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Industry standard architecture (ISA) is a type of computer bus that connects add-in expansion cards to a computer 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 expansion buses of its type developed for personal computers (PCs), the technology of which eventually 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 called the AT bus. Since the AT bus was owned by IBM®, the industry standard architecture name only came around 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 EISA (Extended Industry Standard Architecture), which was developed to compete with IBM®’s next generation bus. The ISA bus name remained, and earlier versions got the ISA name retroactively to avoid infringing on IBM® ownership.

The first eight-bit ISA bus ran at 4.77 megahertz, which matched IBM® central processing units (CPUs) of the time. With the 16-bit version, it increased the speed to six and then eight megahertz to compensate for 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 support expansion, and two are 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.

Managing industry standard architecture expansion cards could sometimes be challenging as the user had to configure the input and output (I/O) addresses, DMA channel and IRQ by activating the dip-switches and positioning the jumpers on the expansion board. In the last few years of the ISA bus, there was an attempt to create a plug-and-play implementation to alleviate some of the configuration problems. Plug-and-play implementations have suffered from problems and have been given the nickname “plug-and-pray” in jest.

The first death knell sounded in the late 1990s with a specification called PC 97, published by Microsoft®, which required the total removal of industry-standard architecture from motherboards. The ISA bus persisted for a few more years, until the beginning of the 21st century, particularly in specific industrial and military systems. However, many offspring of ISA technology have 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.

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