    These take a pointer to a bit field.
    Bt:     Bit Test
    Bts:    Bit Test and Set to one
    Btr:    Bit Test and Rst to zero
    Btc:    Bit Test and Compliment (toggle)
    BEqu:   Set bit to value.

    Bit operations are "atomic", no interrupt between the reading and writing the
    bit, important when multitasking.  For multicore use "locked" forms.

    These don't take a pointer, but the actual field.
    Bsf:    Bit Scan Fwd (Pos of first low one bit or -1)
    Bsr:    Bit Scan Rev (Pos of first high one bit or -1)
    BCnt:   Bit Cnt (Cnt of set bits)