Motherboards and buses Intel 80486

the first 80486 system britain on cover of byte, september 1989

early 80486 machines equipped several isa slots (using emulated pc/at-bus) , 1 or 2 8-bit–only slots (compatible pc/xt-bus). many motherboards enabled overclocking of these default 6 or 8 mhz perhaps 16.7 or 20 mhz (half i486 bus clock) in number of steps, within bios setup. older peripheral cards worked @ such speeds used standard msi chips instead of slower (at time) custom vlsi designs. give significant performance gains (such old video cards moved 386 or 286 computer, example). however, operation beyond 8 or 10 mhz lead stability problems, @ least in systems equipped scsi or sound cards.

some motherboards came equipped 32-bit bus called eisa backward compatible isa-standard. eisa offered number of attractive features such increased bandwidth, extended addressing, irq sharing, , card configuration through software (rather through jumpers, dip switches, etc.) however, eisa cards expensive , therefore employed in servers , workstations. consumer desktops used simpler faster vesa local bus (vlb), unfortunately prone electrical , timing-based instability; typical consumer desktops had isa slots combined single vlb slot video card. vlb gradually replaced pci during final years of 80486 period. few pentium class motherboards had vlb support vlb based directly on i486 bus; no trivial matter adapting quite different p5 pentium-bus. isa persisted through p5 pentium generation , not displaced pci until pentium iii era.

late 80486 boards equipped both pci , isa slots, , single vlb slot well. in configuration vlb or pci throughput suffered depending on how buses bridged. initially, vlb slot in these systems compatible video cards (quite fitting vesa stands video electronics standards association); vlb-ide, multi i/o, or scsi cards have problems on motherboards pci slots. vl-bus operated @ same clock speed i486-bus (basically being local 80486-bus) while pci bus depended on i486 clock had divider setting available via bios. set 1/1 or 1/2, 2/3 (for 50 mhz cpu clocks). motherboards limited pci clock specified maximum of 33 mhz , network cards depended on frequency correct bit-rates. isa clock typically generated divider of cpu/vlb/pci clock (as implied above).

one of earliest complete systems use 80486 chip apricot vx ft, produced united kingdom hardware manufacturer apricot computers. overseas in united states popularised world s first 80486 in september 1989 issue of byte magazine (shown right).

later 80486 boards supported plug-and-play, specification designed microsoft began part of windows 95 make component installation easier consumers.

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