VHDL Programming

Sequential Processing 49

sequence_of_statements

sequence_of_statements ::=

{sequential_statement}

choices ::=

choice{| choice}

choice ::=

SIMPLE_expression|

discrete_range|

ELEMENT_simple_name|

OTHERS

A CASEstatement consists of the keyword CASEfollowed by an expression

and the keyword IS. The expression either returns a value that matches one

of the CHOICESin a WHENstatement part, or matches an OTHERSclause. If the

expression matches the CHOICEpart of a WHEN choices =>clause, the

sequence_of_statementsfollowing is executed. After these statements are

executed, control is transferred to the statement following the END CASE

clause.

Either the CHOICESclause must enumerate every possible value of

the type returned by the expression, or the last choice must contain an

OTHERSclause.

Let’s look at some examples to reinforce what the BNF states:

CASE instruction IS

WHEN load_accum =>

accum <= data;

WHEN store_accum =>

data_out <= accum;

WHEN load|store =>

process_IO(addr);

WHEN OTHERS =>

process_error(instruction);

END CASE;

The CASEstatement executes the proper statement depending on the

value of input instruction. If the value of instruction is one of the choices

listed in the WHENclauses, then the statement following the WHENclause

is executed. Otherwise, the statement following the OTHERSclause is ex-

ecuted. In this example, when the value of instruction is load_accum, the

first assignment statement is executed. If the value of instruction is load

or store, the process_IOprocedure is called.

If the value of instruction is outside the range of the choices given, then

the OTHERSclause matches the expression and the statement following the