Still, with VM enabled (the 68851 PMMU was only available some time after the 68020 came out and an expensive addition, only the 68030 included an on-chip MMU a

Still, with VM enabled (the 68851 PMMU was only available some time after the 68020 came out and an expensive addition, only the 68030 included an on-chip MMU as you probably know - so Apple was unable to use it in the Mac II initially) all 256 possible aliases for software storing arbitrary data in the 8 MSB would have to be mapped in the worst case. This would certainly be possible, but depending on the application might imply quite some overhead due to the limited number of TLB entries available. Of course, on 68k Macs also the OS parts stored in ROM were not 32 bit "clean". MODE32, a tool developed by Connectix, patched the ROMs (traditional MacOS stores entry points for system functions in RAM which can be modified) to get rid of 24 bit addressing (https://en.wikipedia.org/wiki/MODE32 https://en.wikipedia.org/wiki/MODE32). What would be nice, of course, is to use the ARMv8-a top byte ignore feature to implement tagged data types for Lisp and Smalltalk implementations... [and I should read the whole article since this is a large part of it... oops] "65,536 terabytes ought to be enough for anybody." :)