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The Z80 increments the program counter while it is performing an opcode fetch. As a consequence, by the time an opcode byte is fetched, the Z80 will be committed to completing the instruction before any interrupt may occur. If a DD or FD happened to be followed by an opcode byte whose meaning would not be affected by it (including another DD or FD prefix), ...


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The 0xDD does not allow an interrupt to happen after it's been fetched, no matter what comes next in the instruction stream. As Wayne Conrad pointed out, otherwise the CPU would have to time travel into the future to find out what instruction is going to get fetched.


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