Industry Standard Architecture (in practice almost always shortened to ISA) is a bus standard for IBM compatibles introduced in 1984 that extends the XT bus architecture to 16 bits. It is designed to connect peripheral cards to the motherboard. The protocols also allows for bus mastering although only the first 16 MB of main memory is available for direct access. In reference to the XT bus architecture it is sometimes referred to as the AT bus architecture.
Upon the end of the 1990s, ISA's popularity started to wane, and most IBM PC motherboards began to be designed with several PCI slots but with few if any ISA slots. Although some motherboards with ISA slots are still being produced today, they've become quite rare in modern systems. System manufacturers often shield customers from the term 'ISA bus', referring to it instead as the legacy bus (see legacy system).