Haha The Z80 alternate registers...an "almost" good idea. I used teach Z80 assembler language classes back in the late '70s and early '80s. This issue of "which set is active" came up all the time. My reply always had to be "it is based upon the program context that has occurred since reset. The most important thing these alternate sets were useful for was in interrupts to save piles of pushing in ISRs. As long as the Exchange register instructions were always coded in pairs everything worked OK. The alternate sets were indeed useless as general working change overs in the main execution thread of the program.
Michael Karas