Reading VHDL BNF

Appendix C Reading VHDL BNF After the basic concepts of VHDL are understood, the designer might want to try to write VHDL in a more elegant manner. To fully understand how to apply all of the syntactic constructs available in VHDL, it is helpful to know how to read the VHDL Bachus-Naur format (BNF) of the lan- guage. This format is in Appendix A of the IEEE Std 1076-1987 VHDL Language Reference Manual (LRM), pages AϪ1 to AϪ17. This format specifies which constructs are necessary versus optional, or repeatable versus singular, and how constructs can be associated. BNF is basically a hierarchical description method, where complex constructs are made of successive specifications of lower-level constructs. Our purpose for examining BNF is not to understand every nuance of the BNF but to put the basics to use to help build complex VHDL constructs. To this end, let us examine some BNF and discuss what it means. Following is the BNF for the IF statement: if_statement ::= IF condition THEN sequence_of_statements {ELSIF condition THEN sequence_of_statements} [ELSE sequence_of_statements] END IF; The first line of the BNF description specifies the name of the construct being described. This line is read as follows: “The IF statement consists of,” or “The IF statement is constructed from.” The rest of the description represents the rules for constructing an IF statement. The second line of the description specifies that the IF statement starts with the keyword IF , is followed by a condition construct, and ends the clause with the keyword THEN . The next line contains the construct SEQUENCE_OF_STATEMENTS (which is discussed later in this appendix). All of the constructs discussed so far are required for the IF statement because the constructs are not enclosed in any kind of punctuation. Statements enclosed in brackets [ ] , as in lines 6 and 7, are optional constructs. An optional construct can be specified or left out depending on the functionality required. The ELSE clause of the IF statement is an example of an optional construct. A legal IF statement may or may not have an ELSE clause. Statements enclosed in curly braces { } , as in lines 4 and 5, are optional and repeatable constructs. An optional and repeatable construct can either be left out or have one or more of the construct exist. The ELSIF clause is an example of an optional and repeatable construct. The IF statement can be constructed without an ELSIF clause, or have one or more ELSIF clauses, depending on the desired behavior. The last line of the IF_STATEMENT description contains the END IF clause. This is a required clause because it is not optional [ ] and is not optional and repeatable { } . The IF statement contains two other constructs that need more description: the SEQUENCE_OF_STATEMENTS and the CONDITION . The SEQUENCE_OF_STATEMENTS construct is described by the BNF shown here: sequence_of_statements ::= {sequential_statement} The SEQUENCE_OF_STATEMENTS construct is described by one or more sequential statements, where a sequential statement is described in the following: sequential_statement ::= wait_statement | assertion_statement | signal_assignment_statement | variable_assignment_statement | procedure_call_statement | if_statement | case_statement | loop_statement | next_statement | exit_statement | return_statement | null_statement The | character means OR , such that a sequential statement can be a WAIT statement, or an ASSERT statement, or a SIGNAL ASSIGNMENT state- ment, and so on. From this description, we can see that the statement part of the IF statement can contain one or more sequential statements, such as WAIT statements, ASSERT statements, and so on. The CONDITION construct is specified with the BNF description shown here: condition ::= boolean_expression Notice that the keyword boolean is italic. The italic indicates the type of the expression required for the CONDITION . If a designer looks for a Appendix C: Reading VHDL BNF 446 boolean expression construct to describe the syntax required, none will be found. The reason is that all expressions share the same syntax description. For our purposes, the boolean type of the expression is ignored, and the construct description can be found under the following description: expression ::= relation {and relation} |relation {or relation} |relation {xor relation} | relation [nand relation] |relation [nor relation] To summarize, curly braces { } are optional and repeatable constructs, square brackets [ ] are optional constructs, and italic pieces of a con- struct can be ignored for purposes of finding descriptions. 447 Appendix C: Reading VHDL BNF This page intentionally left blank. . Appendix C Reading VHDL BNF After the basic concepts of VHDL are understood, the designer might want to try to write VHDL in a more elegant. in VHDL, it is helpful to know how to read the VHDL Bachus-Naur format (BNF) of the lan- guage. This format is in Appendix A of the IEEE Std 1076-1987 VHDL