The biggest hack in the known universe. Some idiots back in 1992 or so noticed that the ISA bus was too slow, so they decided to extend the 486 processor bus so that cards could interface with it. Unfortunately, it was unstable, you couldn't put more than 2 or 3 cards in, it didn't work at all over 33Mhz, and since it was tied so closely to the 486 bus, it didn't scale well to newer processors. It was replaced by the PCI bus thankfully.