Largely no, actually; modern z/Architecture is a strict superset of the System/360 ISA from the 70's. Every instruction that accesses memory has the original 24

Largely no, actually; modern z/Architecture is a strict superset of the System/360 ISA from the 70's. Every instruction that accesses memory has the original 24-bit address version, a 32-bit version, and a 64-bit version that also uses 64-bit registers. Thus there's no emulation involved; at least, not that's visible to the OS (but who knows what the microcode is doing underneath)