/sys$common/syshlp/helplib.hlb Lexicals, F$CVSI, Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 start-bit

    Specifies the offset of the first bit to be extracted. The
    low-order (rightmost) bit of a string is position number 0
    for determining the offset. Specify the offset as an integer
    expression.

    If you specify an expression with a negative value, or with a
    value that exceeds the number of bits in the string, then DCL
    displays the INVRANGE error message.

 number-of-bits

    Specifies the length of the bit string to be extracted, which
    must be less than or equal to the number of bits in the string.

    If you specify an expression with a negative value, or with a
    value that is invalid when added to the bit position offset, then
    DCL displays the INVRANGE error message.

 string

    Specifies the string from which the bits are taken. Specify the
    string as a character string expression.