bits

#bits

##Syntax bits (expn, subrange)

##Description The bits operator is used to extract a sequence of bits from a natural (non-negative) number expression. The bits are numbered from right to left as 0, 1, 2 �

##Example Set bits 2 and 1 (third and second from the right) in the variable d to be 01. We first set b to be the bit string 1100.

    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

##Example Set bit 7 in variable n to be 1. As a result, n will equal 2#10000000.

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

##Details The form of subrange must be one of:

(a) typeSpec % Subrange type (b) compileTimeIntegerExpression

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).

##See also explicitintegerconstant.html (for description of constants such as 16#FFFF) and the following functions that convert one type to another in a machine-independent manner: ord.html, chr.html, intstr.html, strint.html, natstr.html, and strnat.html. See also shr.html and shl.html (shift right and left).