Undocumented instructions for LD IX
IX and IY registers
If an opcode works with the Registers HL, H or L then if that opcode is prefixed by 0xDD then it will also work on the appropriate IX, IXh or IXl registers, with some exceptions.
The 0xFD prefix would also work but for the IY, IYh or IYl registers
The exceptions are instructions like
LD H,IXh or
LD L,IYh where it
isn't clear from the opcode which register the 0xFD or 0xDD prefix should operate against.
Undocumented instructions for LD IY
Undocumented math instructions for IX register
Undocumented math instructions for IY register