Ada Programming/Pragmas/Pack:3

Exact data representation
It is important to realize that pragma Pack must not be used to specify the exact representation of a data type, but to help the compiler to improve the efficiency of the generated code. The compiler is free to ignore the pragma, therefore if a specific representation of a type is required, representation clauses should be used instead (record representation clauses, and/or attributes 'Size or 'Component_Size).

Bit-wise operations
Although in Ada 83 packed boolean arrays were used for bit-wise operations, since Ada 95 modular types are more adequate for these operations. The argument may be weighed against the advantages of named Boolean array indexes such as Traffic_LightsRed True   False, depending on use case. Pack