Specifications

VHDL Reference Manual B-1
B. Limitations
VHDL is a technology-independent language and has a very large
feature set. Because the VHDL software is specifically targeted toward
logic design, some VHDL constructs are not applicable to synthesis.
Constraints and Unsupported Constructs
This section lists the VHDL constructs that are not supported by the
VHDL synthesizer, or whose use is constrained.
Unsupported Constructs
The following constructs are not supported; using them results in a
constraint error.
• Access types
• File types
• Signal attributes (except 'event and 'stable)
• Textio package
• Impure Functions
• Shared Variables
Constrained Constructs
The following constructs are constrained in their usage. Constrained
constructs fall into two classes:
• Statements constrained in where they may be used.
• Constrained expressions. The use of a constrained construct will
result in a constraint message.
Constrained Statements
• A wait statement may be first statement in a process only.
• Signal attributes 'event and 'stable are valid only when they
specify a clock edge.
• Subprogram calls cannot be recursive.
• The formal part of a named association may not be a function call.