L8_Subprograms and Packages

8/6/2019 L8_Subprograms and Packages

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Packages The primary purpose of a package is to

encapsulate elements that can be shared (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.


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Package (cont..) A package consists of two parts:

A package declaration section

A package body


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Package Declaration The package declaration section can contain

the following declaration:

Subprogram declaration

Type, Subtype declaration

Constant, deferred constant

Component declaration Attribute declaration

File declaration


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Package declaration (example)Package example is

type nineval is (z0,z1,z2,r0,r1,r2,f0,f1,f2);

type t_cluster is arary (0 to 15) or nineval;type t_clus_vec is array (natural range ) oft_cluster;

function resolve (s : t_clus_vec)

return t_cluster;subtype t_wclus is resolve_cluster t_cluster;

constant undriven : t_wclus;

End example;


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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


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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 slength = 0 then return undriven

end if;


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(cont..)

For I in srange loop

if s(I) /= undriven then

drive_count := drive_count + 1;

if drive_count = 1 thenresult := a (I);

else

result : = undriven

end if;End loop

Return result; -- return value

End resolve_cluster; -- end function

End cluspack; -- end package


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Subprograms Subprograms consist of

Procedure

Function


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Subprogram properties Similar to PROCESS statements

IF, CASE, and LOOP, WAIT is not allowed.

Fundamental difference at the level of application. Process is intended for immediate use in the main

code, the others are intended mainly for Libraryallocation, I.e. their purpose is to store commonly

used pieces of code, so they can be reused orshared by other projects.

If, desired, a Function or Procedure can also beinstalled in the main code itself.


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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 returnsjust one.

All parameters are inputparameters

E.g. data type conversions,logical operations,arithmetic computations,

new operator andattributes

Signal declaration andcomponent instantiation isnot allowed.


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Function syntax To construct and use of a function, two

parts are necessary:

The function body and

A call to the function


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Function bodyFUNCTION function_name [] RETURN data_type is

[declaration]

Begin

[sequential statements]

END function_name;


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Function body (cont..) = [CONSTANT] constant_name: constant_type; or

= SIGNAL signal_name:signal_type;

VARIABLES are not allowed. There can be any type of synthesizable data types.

No range specification should be included (e.g. do not

enter range when using INTEGER or TO/Down to when

using std_logic_vector) There is only one return value, whose type is specified by

data_type.


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Example of function bodyFUNCTION f1

(a,b : integer;

signal c : std_logic_vector)

return BOOLEAN is

Begin

(sequential statements)

END f1;


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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 orsequential).

Example of function calls:

X


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FunctionUse 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;


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Use library ieee;

Use ieee.std_logic_1164.all;Use work.num_type.all;

Entity convert is

port (i1 : in log8;

o1 : out integer);

End convert;


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Architecture behave of convert is

function vector_to_int (s : log8)

return integer isvariable result : integer := 0;

begin

for I in 0 to 7 loop

result : = result * 2:

if s(I) = 1 then

result := result + 1;

end if;

end loop;

return result;

end vector_to_int;

Begin

o1


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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.


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Conversion function (example)Function conversion4val (s: fourval)

return fourvalue is

Begincase s is

when x => return x;

when L => return 0;

when H => return 1

when z => return z;

end case;

End convert4val;


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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.


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Operator Overloading When a standard operator symbol is made to

behave differently based on the type of its

operands, the operator is said to be overloaded. Function bodies are written to define the behavior

of overloaded operators.

The number of parameters in such a function must

match in cardinality with that of the predefined

operator.


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Procedure A procedure is very similar to a function.

A procedure can return more than one value

Two parts are required to construct and use

a procedure

Procedure body

Procedure call


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Procedure bodyProcedure procedure_name [] is

[declarations]

Begin

(sequential statements)

End procedure_name;


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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;


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= [CONSTANT] constant_name: mode type

= SIGNAL signal_name: mode type; or

= VARIABLE variable_name : mode type

A procedure can have any number of IN,OUT or INOUT parameters,

Which can be SIGNALS, VARIABLES, or CONSTANTS.


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Procedure call Procedure call is a statement on its own.

It can appear by itself or associated to a

statement (either concurrent or sequential)

Ex.

Compute_min_max(in1,in2,out1,out2)

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