user-visible CSR needed is VL. This is ignoring all the state for
context-switching and exception handling.
+> the consequence of that would be that P48/64 would need
+> its own CSR State to track the subelement index.
+> or that any exceptions would need to occur on a group
+> basis, which is less than ideal,
+> and interrupts would have to be stalled.
+> interacting with SUBVL and requiring P48/64 to save the
+> STATE CSR if needed is a workable compromise that
+> does not result in huge CSR proliferation
+
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What are the interaction rules when a 48/64 prefix opcode has a rd/rs