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Reading Decoding Z80 Opcodes, There's a section about the 0xDD prefix. This instruction prefix causes the next instruction to use the IX register instead of the HL register. That makes sense. HL and IX (also IY) are designed for holding pointers. That's why there's such an efficient encoding for (HL) (called M in Intel's parlance). This question is also about the 0xFD instruction prefix which does the same but for IY.

Anyway, there's this comment about prefix 0xDD in particular:

If the next opcode makes use of HL, H, L, but not (HL), any occurrence of these will be replaced by IX, IXH, IXL respectively. An exception of this is EX DE, HL which is unaffected.

I think it's weird that EX DE, HL is unaffected by this instruction prefix. I can see the utility of being able to efficiently switch pointers around. So has this been "special-cased" somehow to remove the possibility of exchange DE with the index registers?

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Simply because these registers are not exchanged, but renamed (*1).

All EX DE,HL does is toggle a flip-flop which decides which is HL and which is DE.

Much the same way the exchange for alternate AF (EX AF,AF') or alternate register (EXX) set is done. It's also the reason why all of them can be done in just 4 cycles — toggling either flip-flop does not need any data transfer.

*1 - I have a vague feeling I did already explain that as part of some question about the register set, but can't find it right now.

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  • Your answer implies there is no hardware provision for renaming DE to IX
    – Omar and Lorraine
    Oct 13, 2020 at 9:36
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    Of course not, that would be overly complex. each EX variation controls a single FF - including a double rename would require 3 of them just for DE/HL/IX and result in an undefined state at power up. Way overkill for a function just to be added to make some instruction set symmetry - if that would be one intended at all, since the prefix is about replacing a source/destination register in a transfer, but EX does not do any transfer.
    – Raffzahn
    Oct 13, 2020 at 10:01
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    Hmmm... does EXX bring back the previous state of the DE/HL flip-flop as well then?
    – TeaRex
    Oct 13, 2020 at 10:50
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    @TeaRex Each set has it's own FF
    – Raffzahn
    Oct 13, 2020 at 10:56
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    You may consult Ken Shirrif's blog entry on the implementation of the Z80 register. It goes down to the level of the transistor but at the beginning he explains it in a overview mode which might clarify how the flip-flops Raffzahn mentioned function. One thing to get from that article is that the physical layout of the registers does not reflect the register overview one gets in assembly tutorials. The physical layout explains what operation are possible and why. righto.com/2014/10/how-z80s-registers-are-implemented-down.html
    – Patrick Schlüter
    Oct 13, 2020 at 10:59

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