miselect
Machine Indirect Register Select
The CSRs listed in the table above provide a window for accessing register state indirectly.
The value of miselect determines which register is accessed upon read or write of each of
the machine indirect alias CSRs (mireg*). miselect value ranges are allocated to dependent
extensions, which specify the register state accessible via each miregi register, for each
miselect value. miselect is a WARL register.
The miselect register implements at least enough bits to support all implemented miselect
values (corresponding to the implemented extensions that utilize miselect/mireg* to
indirectly access register state). The miselect register may be read-only zero if there are
no extensions implemented that utilize it.
Values of miselect with the most-significant bit set (bit XLEN - 1 = 1) are designated only for custom use, presumably for accessing custom registers through the alias CSRs. Values of miselect with the most-significant bit clear are designated only for standard use and are reserved until allocated to a standard architecture extension.
If XLEN is changed, the most-significant bit of miselect moves to the new position, retaining its value from before.
An implementation is not required to support any custom values for miselect.