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Fourth Generation (486 Class) Chipsets
There are no major or dominant chipsets for 486 class machines. There were many different types and speeds of 486s and several companies that produced popular chipsets for these machines. Some of the more popular 486 chipset producers are UMC, SiS and OPTi.
The VESA Local Bus was invented at around the time that the 486 was introduced. As a result, the majority of 486-based motherboards are based on VLB designs. There are some older ones that just use an ISA bus. The 486 is essentially obsolete at this time, although it has clung tenaciously to life through AMD and Cyrix's 5x86 chips (which are essentially high-end 486s running on 486 motherboards). The motherboards specifically designed to support these chips are relatively new, and are among the few 486 designs to support the PCI bus.
One interesting chipset design seen on some of these newer 486 motherboards is the so-called "VIP" motherboard, where VIP stands for "VLB, ISA, PCI". These boards actually incorporate all three buses! Usually they have 3 PCI slots, 3 ISA slots and one VESA Local Bus slot. The idea is to provide an upgrade path for 486 users without forcing them to throw away their VLB video cards. In practice, these boards often have trouble using both the VLB and PCI buses at the same time, and with the price of generic PCI video cards headed to the $25 range, the VLB slot is often not used.