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X Index Register
From SnesLab
The X Index Register exists on 65x processors and often holds the current index when iterating over things.
On the S-SMP it is always 8 bits wide.
On the 65c816, it may be 8 or 16 bits wide. Indexing may cross bank boundaries.[1]
Unlike the Y index register, the value of the stack pointer can be transferred to/from X. See TXS and TSX.