How do you even use all those registers? Serious question. I've toyed with a couple of 256-register ISAs, and the moment you hit function calls/parameter passing you realize that to utilize those efficiently, you really need some way to indirectly refer to registers, be it register windows, or MMIX's register slide, or Am29k's IPA/IPB/IPC registers; the only other option seems to be to perform global register allocation but that hardly works in scenarios with separate compilation/dynamic code loading.