ARM64 vs ARM32 -- What's different for Linux programmers?

DebugN writes: When ARM introduced 64-bit support to its architecture, it aimed for Linux application compatibility with prior 32-bit software on its architecture. But for Linux programmers, there remain some significant differences that can affect code behaviour. If you are a Linux programmer working with — or will soon be working with — 64-bit, you might want to know what those differences are, and this useful EDN article says it all.