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| IR (Instruction Register) || 8-bit || Holds the opcode of the currently executing instruction || Internally used, not accessible by the programmer
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| IRP (Index Register Prefix) || 2-bit || Stores the index register prefix (DD, FD, or none) || Indicates the use of index registers (IX or IY) for memory access
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| EIP (Extended Instruction Prefix) || 2-bit || Stores the prefix for extended instructions (CB, ED, or none) || Used for extended instruction sets like bitwise ops and block instructions
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