Just to add my two pence, the 16 bit complementary register to A is the flag register F... that's why there's no such 16 bits operation involving A anyway, since the MSB would send quite an unpredictable value to the address bus during an I/O operation and it could potentially lead to catastrophic results.
Hence the reason why Zilog limited the use of 16 bits addressing to the contents of regular 16 bits registers.
It still leaves a lot of possibilities anyway, as even the historical undocumented op codes allowed to divert the regular use of HL to IX and IY in many cases by adding a second prefix, even though not mentionned in the original manufacturer's documentation...
Do you live and breathe hexadecimal? Do you like speaking to hardware directly?
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