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As I know, nowdays, x86's IO address space is used almost only for legacy devices. E.g ARM and RISC-V have only one address space, so, what is the idea behind multiple address spaces?

Peter Cordes
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ser
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  • I'd say to have all the memory space dedicated to - that is, memory. For the ancient 16 bit wide address bus (we are talking legacy here, think 8085) it could only address 64KB of memory, which is not that big. – Eugene Sh. Oct 15 '21 at 19:45
  • So, second address space makes sense only in systems with relatively small memory address space? – ser Oct 15 '21 at 20:01
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    https://stackoverflow.com/questions/3215878/what-are-in-out-instructions-in-x86-used-for has some answers that discuss the rationale and history. – Nate Eldredge Oct 15 '21 at 20:11

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