EE295 - ASIC Design With VHDL Class - Fall 1999

Access Types and Abstract Data Types

Assignment: Read Ch 17

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Outline:

• Access Types
• Type Declarations
• Using an Access Type
• procedure READLINE
• Linked Lists
• Type Declarations:
• Building a Linked List

Access Types

Access Types are More Commonly Known as Pointers.

They are a Variable Construct That 'Points To' or Stores the Memory Address of Another Object.

Particularly Useful When Modelling Space Requirements Vary

 e.g. Executing a Few Hundred Instructions Stored Randomly in a 16 MB RAM
Ordinary Techniques Require a Costly 16 MB RAM Worth Variables or Signals
 
Using Pointers We Impliment a 16 MB RAM That Only Requires Storage For Usefull Addresses.

STD TEXTIO Implimetation is an Example.

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Type Declarations

type ptr_type_name : access subtype_indication ;

Example:
 

Type Line is access String ;
 

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Using an Access Type

variable var_name : ptr_type_name ;

Example:
 

procedure ReadLine ( F : in Text; L : out Line ); -- creates an Access Variable L

L Initially Contains the Value null Indicating It's Not Pointing to A Valid Location

Accessing an Invalid Location is an Error
new subtype_indication | qualified_expression
Creates or Allocates a New Storage Location For an Object of Type Specified
L := new string ; --allocates storage for a string and initializes L
 
var_name.all
Access the Value Pointed Too
L.all := "boofar" ; --Stores the String In The Location Pointed To by L
 
new subtype_indication | qualified_expression
Second Form Allocates a New Storage Location and Stores Value In It
L := new string'("foobar") ; --equivalant to Last Two Commands
 

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procedure READLINE(file f: TEXT; L: out LINE)
 
    is
 variable buf : string(1 to MAX_LINE);
 variable len : integer := 0;
 variable c : character;

    begin

 --if L /= NULL then
 --    Deallocate(L);
 --end if;

 if not Endfile(f) then
     READ(f, buf, len);
     assert len <= MAX_LINE
  report "Textio: Truncated input line greater than "
         & Int_to_string(MAX_LINE)
         & " characters."
  severity ERROR;
 end if;
 if (len > 0) and (buf(len) = LF) then
     len := len - 1;
 end if;
 L := new string'(buf(1 to len));
    end;

Linked Lists

VHDL Permits Us to Describe Advanced Data Structures Such as Linked Lists
 

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Type Declarations:

type cell ; -- Incomplete Type Declaration

type cell_ptr is access cell ;

type cell is record

name : string ;
...
next : cell_ptr ;
end record cell ;

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Building a Linked List


variable cell_list : cell_ptr ;
 
Initially Set to null
 
cell_list := new cell'( "AND", ... , cell_list )
 
Now cell_list Points to a New Location