Computer addresses are almost always positive unsigned numbers. An example if this might be the cluster address in a FAT file system. Today, many CPU functions are controlled the same way. For an example look at the Pentium processor System Architecture Overview. 5.1.11 Structures Structures are a combination of the above constructs.
Some fields can be of variable length.
The structure can be dealt with as a single construct. It is also easy to think of instructions as a packed structure. In the structure you will find various fields. An enumerated type This is likely to be a variable length item. Some instructions will use more bits than others.