Early Macs used the upper byte of 32-bit pointers for other data: https://en.wikipedia.org/wiki/Classic_Mac_OS_memory_management#32-bit_clean https://en.wikipe

Early Macs used the upper byte of 32-bit pointers for other data: https://en.wikipedia.org/wiki/Classic_Mac_OS_memory_management#32-bit_clean https://en.wikipedia.org/wiki/Classic_Mac_OS_memory_manageme... ...and resulted in some models having a backwards-compatibility mode not too dissimilar to the PC's A20 gate, although for only a short period of time.