It's a bit more inconvenient in terms of register protection (BC
is much more generally useful than IY), but it makes tight spots
such as next and execute much faster, so I think it's worth it.

My intention is to reuse the SPI relay to program AVR MCUs from a
RC2014.

Initially, I used the same letters as those used in the z80 ref
docs, but it makes the different assemblers harder to use than they
should. Having consistent "argtype" rules across assemblers should
help.