Subprograms and Packages - Computer Science - Lecture Slides, Slides of Computer Science

Download Subprograms and Packages - Computer Science - Lecture Slides and more Slides Computer Science in PDF only on Docsity! Packages • The primary purpose of a package is to encapsulate elements that can be shred (globally) among two or more design units. • A package is a common storage area used to hold data to be shared among a number of entities • Declaring data inside of a package allows the data to be referenced by other entities; thus data can be shared. Docsity.com Package (cont..) • A package consists of two parts: – A package declaration section – A package body Docsity.com Package body • The package body can also contain the following declaration: – Subprogram declaration/body – Type, subtype declaration – Constant declaration – File declaration – Alias declaration – Use clause Docsity.com Package body (example) Package body cluspack is constant undriven : t_wclus:= (zx,zx); function resolve_cluster (s: t_clus_vec) return t_cluster is variable result : t_cluster; variable drive_const: integer; begin if s’length = 0 then return undriven end if; Docsity.com (cont..) For I in s’range loop if s(I) /= undriven then drive_count := drive_count + 1; if drive_count = 1 then result := a (I); else result : = undriven end if; End loop Return result; -- return value End resolve_cluster; -- end function End cluspack; -- end package Docsity.com Procedure and Function • A procedure can return more than one argument. • A procedure can have input parameters, output parameters and inout parameters • A function always returns just one. • All parameters are input parameters • E.g. data type conversions, logical operations, arithmetic computations, new operator and attributes • Signal declaration and component instantiation is not allowed. Docsity.com Function syntax • To construct and use of a function, two parts are necessary: – The function body and – A call to the function Docsity.com Function body FUNCTION function_name [<parameter list>] RETURN data_type is [declaration] Begin [sequential statements] END function_name; Docsity.com Function call • A function is called as par of an expression. The expression can obviously appear by itself or associated to a statement (either concurrent or sequential). • Example of function calls: X <= conv_integer (a); -- converts a to an integer Y <= maximum (a,b); --returns the largest of a and b; If x > maximum (a,b); --compares x to the largest of a,b Docsity.com Function Location Function/Procedure location Package Main code (+Package body) Architecture Entity Library Docsity.com Function Use library ieee; Use ieee.std_logic_1164.all; package num_type is type log8 is array (0 to 7) of std_logic; end num_type; Docsity.com Conversion Function • Conversion Function are used to convert an object of one type to another. • They are used in component instantiation statement to allow mapping of signals and ports of different types. Docsity.com Conversion function (example) Function conversion4val (s: fourval) return fourvalue is Begin case s is when x => return ‘x’; when L => return ‘0’; when H => return ‘1’ when z => return ‘z’; end case; End convert4val; Docsity.com Resolution Function • A resolution function is used to return the value of a signal when the signal is driven by multiple drivers. • A resolution function consists of a function that is called whenever one of the drivers for the signal has an event occur on it. • The resolution function is executed and returns a single value from all of the driver values; this value is the new value of the signal. Docsity.com Procedure body Procedure procedure_name [<parameter list>] is [declarations] Begin (sequential statements) End procedure_name; Docsity.com Procedure (example) • Procedure my_procedure( a : in std_logic; signal b,c : in std_logic; signal x : out std_logic_vector(7 downto 0); signal y : inout integer range 0 to 99) is Begin ------ End my_procedure; Docsity.com <parameter list> • <parameter list> = [CONSTANT] constant_name: mode type • <parameter list> = SIGNAL signal_name: mode type; or • <parameter list> = VARIABLE variable_name : mode type • A procedure can have any number of IN,OUT or INOUT parameters, Which can be SIGNALS, VARIABLES, or CONSTANTS. Docsity.com