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There is difference between i386 and i486 and i486 and P5. And it's not only regarding "backword" support. i486 has some features that are not supported by P5+. Also about backword compartability of IA32 - it's partial any way. For example (16)20bit real address mode rounding (when segment*16 + offset >= 2^20 in modulo 2^20 addition for 20 bit address bus) is OK for i386-,
yet causes PF in i386+, also there are difference in NMI handling etc. So for carifull system programmer I'd say - there is only partion backward support.