Determines which instructions are legal.

    Option      Function

    GENERIC     All instructions are allowed; equivalent to EV4.
    HOST        Instruction set is that of the host processor.
    EV4	       Instruction set is that of the EV4 base design processors
                (21064, 20164A, 21066, and 21068 chips).
    EV5         Instruction set is that of the EV5 processor
                (some 21164 chips).
    EV56        Instruction set is that of the EV56 processors
                (some 21164 chips).  Includes BWX extensions.
    PCA56       Instruction set is that of the PCA56 processor
                (21164PC chips).  Includes BWX and MAX extensions.
    EV6         Instruction set is that of the EV6 processor
                (21264 chips).  Includes BWX and MAX extensions and
                SQRT instructions.