but I gather that the IX and IY index registers were added to it to address similar limitations in the 8080 architecture The 8080 was much better than the 6502

but I gather that the IX and IY index registers were added to it to address similar limitations in the 8080 architecture The 8080 was much better than the 6502 in terms of 16-bit registers, even without IX and IY. The architecture was: A an 8 bit accumulator B C two 8 bit regs or combined 16 bit D E two 8 bit regs or combined 16 bit H L two 8 bit regs or combined 16 bit I did a lot of Z80 programming and I usually ignored IX and IY. They were useful for certain things. But they were slower than the 8080's registers, because IIRC they needed an extra opcode byte. The Z80 did, however, add lots of neat stuff that used the existing 8080 architecture registers. E.g., here's a single instruction equivalent of today's memmove() Unix library function. Do this: BC <-- stuff a 16 bit count into it DE <-- stuff a 16 bit destination address into it HL <-- stuff a 16 bit source address into it then do the LDIR instruction and you get a memory copy in a single instruction. Up to 64 K bytes, which was the entire address space. IIRC the instruction was interruptible on a byte-by-byte basis, so it's not like you ruined your interrupt response time.