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combinations of operator order are covered with three functions, because the function for two inputs of type BIT are built in
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An alias creates a new name for all or part of the range of an array type It is very useful for naming parts of a range as if they were subfields For example, in a CPU model, an instruction is fetched from memory The instruction may be an array of 32 bits that is interpreted as a number of smaller fields to represent the instruction opcode, source register 1, source register 2, and so on Aliases provide a mechanism to name each of the subfields of the instruction and to reference these fields directly by the alias names This is illustrated by the following example:
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SIGNAL instruction : BIT_VECTOR(31 DOWNTO 0); ALIAS opcode : BIT_VECTOR(3 DOWNTO 0) IS instruction(31 DOWNTO 28); ALIAS src_reg : BIT_VECTOR(4 DOWNTO 0) IS instruction(27 DOWNTO 23); ALIAS dst_reg : BIT_VECTOR(4 DOWNTO 0) IS instruction(22 DOWNTO 18);
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In this example, the aliases have been created for a signal object Using the alias name in an assignment or referencing operation is the same as using the piece of the instruction object being aliased, but much more convenient Remember that the semantics in place for the object being aliased are applied to the alias as well If an alias is created for a constant object, the alias cannot have an assignment for the same reasons that a constant cannot have an assignment
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One of the side effects of overloading is that multiple functions or procedures may match in a particular instance because the types are ambiguous For the compiler to figure out which subprogram to use, a qualified
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expression may be required A qualified expression states the exact type that the expression should attain For instance, when evaluating an expression containing a mixture of overloaded subprograms and constant values, the designer may need to qualify an expression to produce correct results Following is an example of such a situation:
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PACKAGE p_qual IS TYPE int_vector IS ARRAY(NATURAL RANGE <>) OF INTEGER; FUNCTION average( a : int_vector) RETURN INTEGER; FUNCTION average( a : int_vector) RETURN REAL; END p_qual; USE WORKp_qualALL; ENTITY normalize IS PORT( factor : IN REAL; PORT( points : IN int_vector; PORT( result : OUT REAL); END normalize; ARCHITECTURE qual_exp OF normalize IS BEGIN result <= REAL (average(points)) * factor; END qual_exp;
Package p_qual defines two overloaded functions named average and an unconstrained type, int_vector The package body is left as an exercise for the reader Architecture qual_exp has a single concurrent signal assignment statement that calls function average Because there are two functions named average, there are two possible functions that can be used by this call To clarify which function to use, the expression has been qualified to return a REAL type The keyword REAL followed by a specifies that the expression inside the parentheses return a type REAL The expression was qualified to make sure that the average function returning a REAL number was called instead of the average function that returns an INTEGER In this example, the expression required a qualified expression to allow the architecture to compile The compiler does not make any random guesses about which function to use The designer must specify exactly which one to use in cases where more than one function can match; otherwise, an error is generated Another use for a qualified expression is to build the source value for an assignment statement Based on the type of the signal assignment target, the source value can be built Following is an example:
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PACKAGE p_qual_2 IS TYPE vector8 IS ARRAY( 0 TO 7) OF BIT; END p_qual_2; USE WORKp_qual_2ALL; ENTITY latch IS PORT( reset, clock : IN BIT; PORT( data_in : IN vector8; PORT( data_out : OUT vector8); END latch; ARCHITECTURE behave OF latch IS BEGIN PROCESS(clock) BEGIN IF (clock = 1 ) THEN IF (reset = 1 ) THEN data_out <= vector8 (others => 0 ); ELSE data_out <= data_in; END IF; END IF; END PROCESS; END behave;
This example is an 8-bit transparent latch, with a reset line to set the latch to zero When the clock input is a 1 value, the latch is transparent, and input values are reflected on the output When the clock input is 0 , the data_in value is latched When reset is a 1 value while clock input is a 1 , the latch is reset This is accomplished by assigning all 0 s to data_out One method to assign all 0 s to data_out is to use an aggregate assignment Because data_out is 8 bits, the following aggregate assignment sets data_out to all 0 s:
data_out <= ( 0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 );
This aggregate works fine unless the type of data_out changes If the type of output data_out was suddenly changed to 16 bits instead of 8, the aggregate could no longer be used Another method to accomplish the assignment to output data_out is to use a qualified expression The assignment to data_out when reset = 1 in the preceding example shows how this might be done The following expression:
(others => 0 )
can be qualified with the type of the target signal (data_out) This allows the compiler to determine how large the target signal is and how large to
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