Early versions of Mac OS ran on the 68000 and 68010, which had a 24-bit address bus. The upper 8 bits of the address were completely ignored, so some parts of the Memory Manager (documented in my link above!) would use those bits to store flags for handles, like "locked", "purgeable", and "resource". Seeing that Apple used these bits, some third-party developers followed suit in their own applications (or accessed those bits directly as a questionable optimization).
When Apple released computers which used 68030 processors, which had a 32-bit address bus, this trick stopped working. There was an awkward period where "32-bit addressing" was an option which could be turned on and off in the Memory control panel for compatibility with older software (and hardware!) which wasn't yet "32-bit clean".
When Apple released computers which used 68030 processors, which had a 32-bit address bus, this trick stopped working. There was an awkward period where "32-bit addressing" was an option which could be turned on and off in the Memory control panel for compatibility with older software (and hardware!) which wasn't yet "32-bit clean".