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That sounds like the same ugly hack that caused programs not to be “32 bit clean” back in the day for Macs

One difference is that the Mac OS itself was not initially 32-bit clean, with the top byte being used by the Memory Manager.

Ah yes, these 68000 pointers have a spare 8 bits for me to use! Because nothing will ever need more than 16 MB of memory. Sigh.

It's all good if you have 128kb ram but they should have had a plan to escape it from day one.

This is how pointer authentication codes work on Arm64.