TABLE OF CONTENTS In this appendix we summarize the main differences among VHDL-87, VHDL-93 and VHDL-2001, many of which we mentioned throughout the book. Most new models will be written in VHDL-2001, since that is the current "official" version of the language. However, designers must deal with the legacy of models written in the old versions and with VHDL tools that have yet to be updated to handle the current version.
VHDL-87 uses the ASCII character set, rather than the full ISO character set. ASCII is a subset of the ISO character set, consisting of just the first 128 characters. This includes all of the unaccented letters, but excludes letters with diacritical marks.
VHDL-87 only allows basic identifiers, not extended identifiers. The rules for forming basic identifiers are the same as those for VHDL-93 and VHDL-2001.
The following identifiers, introduced in VHDL-93, are not used as reserved words in VHDL-87. They may be used as identifiers for other purposes, although it is not advisable to do so, as this may cause difficulties in porting the models to VHDL-2001.
| group | postponed | ror | sra |
| impure | pure | shared | srl |
| inertial | reject | sla | unaffected |
| literal | rol | sll | xnor |
In addition, the identifier protected was introduced as a reserved word in VHDL-2001. It may be used for other purposes in VHDL-87 and VHDL-93, but again, it is not advisable to do so.
Bit-string literals may only be used in VHDL-87 as literals for array types in which the elements are of type bit. The...
