[color=#000000] (Perhaps stating the obvious,) a second data stack could use non-ZP space for situations where you don't need the (ZP,X) addressing mode (meaning that stack cells won't be addresses needed for indirection without having to copy them to N first), and abs,X is enough. For that matter, since you don't need the bytes of a cell to be together for addresses for indirects, you can split high and low bytes so that even with an 8-bit index register, you can get 256 cells of 16 bits (or even 32 bits), giving each byte of precision its own page, and EDROP ("extension-stack DROP"?) consists of only a single INX, even for 32-bit.