+ comb += extra2_lsb.eq(extra[EXTRA2.IDX3_MSB])
+ with m.If(spec[SPEC.VEC]): # vector mode
+ # can express reg numbers range(0, 127, 2)
+ comb += spec[SPEC.MSB].eq(extra2_lsb)
+ with m.Else(): # scalar mode: can express r0-63
+ comb += spec[SPEC.LSB].eq(extra2_lsb)