How is the phrase "register window" not in this article? https://en.wikipedia.org/wiki/Register_window
"The AMD 29000 improved the design by allowing the windows to be of variable size, which helps utilization in the common case where fewer than eight registers are needed for a call. It also separated the registers into a global set of 64, and an additional 128 for the windows."
Amazing stuff. (Both the 29K and the article).