System-defined objects consist of either a single access segment, or an access segment and a data segment. System-defined segments contain data or access descriptors for system-defined data at designated offsets, though the operating system or user software may extend these with additional data. Each system object has a type field which is checked by microcode, such that a Port Object cannot be used where a Carrier Object is needed. User programs can define new object types which will get the full benefit of the hardware type checking, through the use of Type Control Objects (TCO).