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I had a "Blackbook", it was definitely a 32 bit CPU (T2500).

Software-wise 64 bit userland processes were supported since Tiger (CLI and Cocoa only, Carbon UI stayed 32 bit) before the kernel itself moved to 64 bits in Snow Leopard (although IIRC not the default except on Xserve and Mac Pro, the two that could have loads of RAM. You could tell it to boot the 64 bit kernel through nvram boot args or something though)

A few Macs had a 64 bit CPU but a 32 bit only bootloader/firmware. That made them disqualified for one of the major OS version updates when it required a full 64 bit boot chain.

There are definitely Mac 32-bit Intel games on Steam, like Portal, that won't run on anything recent.

Apple switched to Intel with first generation Intel Core Solo/Duo which were 32 bit CPUs in mid 2006. Then later that year they released "late 2006" models using Core 2 CPUs which were 64 bit. However, they only has support for 32-bit EFI so they could not boot 64-bit OS.

https://everymac.com/mac-answers/snow-leopard-mac-os-x-faq/m...