Originally Posted by A1200
If I remember correctly, the original Amiga did not ship with AmigaBASIC, but with a different BASIC interpreter created by MetaComCo (ABasiC).
AmigaBASIC followed later, this being a port/adaptation of Macintosh BASIC. AmigaBASIC worked well enough on plain 68000 machines but not so much on 68020/68030, etc. machines which suggests to me that it really was Macintosh BASIC after all.
In the original 68000 Macintosh the address space was limited to 24 bits, and application software used the most significant bits of the addresses for "tagging" the memory properties. This seems to have been carried over to AmigaBASIC.