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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Monday 31 December 2012

Case Statement

Formal Definition

The case statement selects for execution one of several alternative sequences of statements; the alternative is chosen based on the value of the associated expression.

Simplified Syntax

case expression is

when choice => sequential_statements

when choice => sequential_statements

         . . .

end case;


The case statement evaluates the listed expressions and selects one alternative sequence of statements according to the expression value. The expression can be of a discrete type or a one-dimensional array of characters (example 1).

The case statement contains a list of alternatives starting with the when reserved word, followed by one or more choices and a sequence of statements.

An alternative may contain several choices (example 2), which must be of the same type as the expression appearing in the case statement. For each expression there should be at least one locally static choice. The values of each choice must be unique (no duplication of values is allowed).

A choice can be either a simple name (example 1), a name of a simple element (example 2) or discrete range (a slice, example 3). The choice types can be mixed.

A subtype with a constraint range (example 4) can substitute a slice.

Another option is to use an object name as the choice. The object must be of the same type as the expression in the case statement. Example 5 shows it for a constant.

When all explicitly listed choices do not cover all the alternatives (all the values available for an expression of given type) the others choice must be used because the choicestatements must cover all the alternatives, see example 5).


Example 1

variable x: Integer range 1 to 3;
variable y: BIT_VECTOR (0 to 1);
C1: case x is
      when 1 => Out_1 <= 0;
      when 2 => Out_1 <= 1;
      when 3 => Out_1 <= 2;
  end case C1;
C2: case y is
      when "00" => Out_2 <= 0;
      when "01" => Out_2 <= 1;
      when "10" => Out_2 <= 2;
      when "11" => Out_2 <= 3;
  end case C2;
end process;

Depending on the values of the variable x and y, we assign the values 0, 1, 2 or 3 (in the second case) to the signals Out_1 and Out_2 (both of type Integer).

Example 2

type Codes_Of_Operation is (ADD,SUB,MULT,DIV);
variable Code_Variable: Codes_Of_Operation;
C3: case Code_Variable is
      when ADD | SUB => Operation := 0;
      when MULT | DIV => Operation := 1;
  end case C3;
end process;

When two or more alternatives lead to the same sequence of operations then they can be specified as a multiple choice in one when clause.

Example 3

type Some_Characters is ('a','b','c','d','e');
variable Some_Characters_Variable: Some_Characters;
C4: case Some_Characters_Variable is
      when 'a' to 'c' => Operation := 0;
      when 'd' to 'e' => Operation := 1;
   end case C4;
end process;

Slices can be used as choices. In such a case, the slice name must come from the discrete range of the expression type.

Example 4

variable Code_of_Operation : INTEGER range 0 to 2;
constant Variable_1 : INTEGER := 0;
C6: case Code_of_Operation is
      when Variable_1 | Variable_1 + 1 =>
Operation := 0;
      when Variable_1 + 2 =>
Operation := 1;
  end case C6;
end process;

Constant used as a choice.

Example 5

type Some_Characters is ('a','b','c','d','e');
variable Code_of_Address : Some_Characters;
C7:case Code_of_Address is
      when 'a' | 'c' => Operation := 0;
      when others => Operation := 1;
   end case C7;
end process;

If the Code_of_Address variable is equal to 'a' and 'c', then the assignment Operation:=0; will be chosen. For the 'b', 'd' and 'e' values, the assignment Operation:=1; will be performed.

Important Notes

· The case expression must be of a discrete type or of a one-dimensional array type, whose element type is a character type.

· Every possible value of the case expression must be covered by the specified alternatives; moreover, every value may appear only once (no duplicates or overlapping of ranges is allowed).

· The When others clause may appear only once and only as the very last choice.

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