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Explains that the Z80 processor supports undocumented 8-bit operations on the IX and IY index registers using the DD and FD prefix bytes. Any standard H or L register instruction preceded by DDh operates on the high/low bytes of IX (XH, XL), and FDh for IY (YH, YL), providing 4 additional 8-bit registers. Lists usable instructions (LD, ADC, ADD, AND, CP, DEC, INC, OR, SBC, SUB) and notes BIT, IN, and OUT instructions do not work with these registers.