decoding single instruction

I want to make a function that decodes
any single instruction.

Opcode Instruction Description
89 /r MOV r/m16,r16 Move r16 to r/m16

this is from intels INSTRUCTION SET REFERENCE

So I am wondering is this opcode same for
evry procesor from 8086 to P4
or does for example 386 have diferent opcode
even if the instuction has the same mnemonic
and does the same thing.
In other words do same instructions have diferent opcodes
depending on the processor generation.
I am thinking like there could be an old and a new
version with new processor supporting both.

What other documentation do you recommend.

Thanks.