A stack is register renaming. Instead of "Register #n" you refer to it as "SP - n". The difference is that it's rebased with every op, and in ops the registers

A stack is register renaming. Instead of "Register #n" you refer to it as "SP - n". The difference is that it's rebased with every op, and in ops the registers are almost always implied: "ADD" always loads reg #0 and #1, always puts results back in #1, then renames #1 .. n to #0 .. n-1. OoO should handle the continuous renaming OK, I think. ADD + PUSH would add (orig #0, orig #1), place result in orig #1, put new data in new #0, alias new #0 as "SP-0". Only the aliasing would need to be ordered. Maybe it could be part of the decode. As I noted elsewhere, some HP 3000 models had multiple ALUs so some form of OoO.