bits

        type T12 : 1 .. 2           % Use to specify bit range
        var d : nat2 := 2#1100      % Two byte natural number
        % At this point bits(d, T12) = 2#10
        bits (d, T12) := 2#01
        % At this point d = 2#1010

        var n : nat1 := 0       % A one byte variable set to zero
        bits (n, 7) := 1        % n now contains the pattern 10000000

In form (a) the subrange type specifies a range from L to M (for least and most significant). This is a little confusing because the subrange is written L .. M with L on the left and M on the right, but in a number, the least significant bit is on the right and the most significant is on the left. The subrange type can be either the name of a type, for example T12, or an explicit subrange, for example 3 .. 7. The values in the explicit subrange must be compile time values.

Form (b) represents the range n .. n where n is the non-negative value of the expression. In other words, both L and M equal n. The expression can be any non-negative integer value or natural number value.

If the expression expn is a variable reference, the bits operation can be assigned to, but cannot be passed to, a var parameter. For example, in the above, bits (d, T12) has the value 2#01 assigned to it. For this assignment to be allowed, the expression expn must be a natural number type (nat, nat1, nat2 or nat4).