Its the same on X64, although I think its better documented/understood on that platform.

Yeah that's one of the things Cutler played an important role in -- formalizing x64 prologue and epilogue constructs. The epilogue stuff is interesting, there's a small set of instructions that are allowed because the unwind code literally does opcode analysis to figure out if the current function is in its epilogue or not.

Heh - that was partly me. The kernel team (i.e. mostly DaveC) mostly prescribed which instructions were allowed in prologues/epilogues. I was dev lead of VC's CRT team at the time (2000-ish), and got assigned to work on that and come up with an x64 version of the prologue/epilogue descriptors needed to unwind. I looked at what was done for Itanium, and simplified that waaaay down to what was needed for x64. I remember designing the ability to split prologues into multiple parts, to do something called shrink-wrapping, where you might not want to push all your saved registers all the time - there might be a subset of a function's code where a register needs to be pushed/popped. DaveC needed convincing that was a good idea.