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VHDL Tutorial |
Functions in VHDL | ||
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Introduction Fundamental concepts Modelling concepts Elements of behaviour Elements of structure Analysis elaboration Lexical elements Identifiers Numbers Characters and strings Syntax descriptions Constants and variables Scalar type Integer types Floating point types Time type Enumeration types Character types Boolean type Bits type Standard logic Sequential statements Case statements Loop and exit statements Assertion statements Array types & array operations Architecture bodies Entity declarations Behavioral descriptions Wait statements Delta delays Process statements Conditional signal assignment Selected signal assigment Structural descriptions Library and library clauses Procedures Procedure parameters Signal parameters Default values Unconstrained array parameter Functions Package declarations and bodies Subprograms in package Use clauses Resolved signals and subtypes Resolved signals and ports Parameterizing behavior Parameterizing structure
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Functions
Let us now turn our attention to the second kind of subprogram in VHDL: functions. The syntax rule for a function declaration is very similar to that for a procedure dec- laration:
subprogram_body ⇐ function identifier [ ( parameter_interface_list ) ] return type_mark is { subprogram_declarative_item } begin { sequential_statement } end [ function ] [ identifier ] ;
The identifier in the declaration names the function. It may be repeated at the end of the declaration. Unlike a procedure subprogram, a function calculates and re- turns a result that can be used in an expression. The function declaration specifies the type of the result after the keyword return. The parameter list of a function takes the same form as that for a procedure, with two restrictions. First, the parameters of a function may not be of the class variable. If the class is not explicitly mentioned, it is assumed to be constant. Second, the mode of each parameter must be in. If the mode is not explicitly specified, it is assumed to be in. Like a procedure, a function can declare local items in its declarative part for use in the statements in the function body. A function passes the result of its computation back to its caller using a return statement, given by the syntax rule
return_statement ⇐ return expression ;
A function must include at least one return statement. The first to be executed causes the function to complete and return its result to the caller. A function cannot simply run into the end of the function body, since to do so would not provide a way of specifying a result to pass back to the caller. A function call looks exactly like a procedure call. The syntax rule is function_call ⇐ function_name [ ( parameter_association_list ) ] The difference is that a function call is part of an expression, rather than being a se- quential statement on its own, like a procedure call.
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Contact : webmaster@freehost7com The function in Figure 5-7 determines the number represented in binary by a bit-vector value.
function bv_to_natural ( bv : in bit_vector ) return natural is variable result : natural := 0; begin for index in bv'range loop result := result * 2 + bit'pos(bv(index)); end loop; return result;
A function that converts the binary representation of an unsigned number to a numeric value.
As an example of using this function, consider a model for a read-only mem- ory, which represents the stored data as an array of bit vectors, as follows:
type rom_array is array (natural range 0 to rom_size–1) of bit_vector(0 to word_size–1); variable rom_data : rom_array;
If the model has an address port that is a bit vector, we can use the function to convert the address to a natural value to index the ROM data array, as follows:
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