B The Predefined Package Standard1
The predefined types, subtypes and functions of VHDL are defined in a package called standard, stored in the library std. Each design unit in a design is automatically preceded by the following context clause:
so the predefined items are directly visible in the design. The package standard is listed here. The comments indicate which operators are implicitly defined for each explicitly defined type. These operators are also automatically made visible in design units. The types universal_integer and universal_real are anonymous types. They cannot be referred to explicitly.
VHDL-87
The following items are not included in standard in VHDL-87: the types file_open_kind and file_open_status; the subtype delay_length; the attribute foreign; the operators xnor, sll, srl, sla, sra, rol and ror. The result time of the function now is time. The type character includes only the first 128 values, corresponding to the ASCII character set.
1. From IEEE Std. 1076-2001. Copyright 2001 IEEE. All rights reserved.