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In the 16-bit flags register of the intel 8086 processor there are 9 flags (each 1-bit, bit numbers 0,2,4,6,7,8,9,10,11 which we know them as ZF,OF,…) the bit numbers 1,3,5 are marked as”U” (undefined) and the bit numbers 12,13,14,15 are marked as “R” (reserved).
My question is that what is the difference between reserved bits and the undefined bits in the intel 8086 processor?
Thanks
Reserved bits may be defined. Undefined bits never are. If a bit is “reserved”, it is reserved for some specific purpose. If it’s “undefined”, then at that point no purpose for it was decided.
For example, if a bit sets a particular testing mode that users are never supposed to enable, the bit would be reserved but it wouldn’t be undefined.