vhdl- array, record and access types

VHDL:ARRAY, RECORD and ACCESS

TYPES

INSTRUMENTATION TECHNOLOGY DEPT.RVCE

A SEMINAR ON

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Type declaration

• a hardware description language at various level of abstraction should not be limited to Bit or Boolean types

• VHDL is a highly typed language

• VHDL allows the use of integers, floating point, and enumerate data types as well as user defined type• It has already arithmetic operators defined for these types

• VHDL allows bit and boolean types▫ It has logical operators defined for these

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TYPES

SCALAR

INTEGER

REAL

ENUMERATED

PHYSICAL

FILE

ACCESS

COMPOSITE

ARRAY

RECORD

VECTOR

VHDL Data Types 04/11/2023

DEPARTMENT OF INSTRUMENTATION TECHNOLOGY

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Composite Types

• A composite type object is one having multiple elements

• VHDL composite types consists of arrays and records

• The elements can be of any scalar, composite or access type

• SYNTAX: composite_type_definition ::= type_definition

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Array Type• Used to collect one or more elements of a similar type in

a single construct

• Elements can be any VHDL data type

• ARRAY is formally defined as follows;

A type, the value of which consists of elements that are all of the same subtype (and hence, of the same type).

Each element is uniquely distinguished by an index (for a one-dimensional array) or by a sequence of indexes (for a multidimensional array).

Each index must be a value of a discrete type and must lie in the correct index range.

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Array Type

• Each object of this data type can hold more than one value.

• Arrays consist of many similar elements of any data type, including arrays.

• The array is declared in a TYPE statement.

• The first item is the name of the array.

• Second, the range of the array is declared

• The third item in the array declaration is the specification of the data type for each element of the array.

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Array Type

•Simplified Syntax:

type type_name is array (range) of element_type

Or

type type_name is array (type range) of element_type

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Array Type• constrained or unconstrained.

• constrained if the size of the array is constrained.

• The size of the array can be constrained using a discrete type mark or a range.

• In both cases, the number of the elements in the array is known during the compilation.

• unconstrained if its size is unconstrained

• Number of elements of unconstrained array type is unknown.

• The size of a particular object is specified only when it is declared.

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Array Type

• Constrained Array ▫ type Real_Matrix is array (1 to 10) of REAL;

type BYTE is array (0 to 7) of BIT;type DATA_BUS is array (0 to 7) of BIT;

• Unonstrained Array ▫ type Real_Matrix is array (POSITIVE range <>) of Real;

variable Real_Matrix_Object : Real_Matrix (1 to 8);

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Differences between vectors and arrays

VECTOR ARRAY

• A vector is a dynamic array (edit the contents)

• Only 1 dimension• A vector will expand its size as

necessary to accommodate new elements

• Can reserve spaces in between

• Syntax:▫ Type_Vector(range);

• An array is a static array (just to store values)

• Multiple dimensions• An array is fixed

• Can’t reserve spaces in between

• Syntax:▫ type type_name is

array (type range ) of element type

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Array Type: example to compare elements

Library IEEE;Use STD_LOGIC_1164.all;

package arr_pkg isconstant N: integer := 4;constant M: integer := 3;subtype wordN is STD_LOGIC_VECTOR (M

downto 0);

type string is array (N downto 0) of wordN;

END arr_pkg;

--building a package for array-- no. of elements of array is

N+1-- no of bits of each element is

M+1

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Array Type: example to compare elementsLibrary IEEE;

Use STD_LOGIC_1164.all;Use work.arr_pkg.all;

Entity large_array isgeneric (N: integer :=4;

M:integer:=3);

Port( a: inout strng; z: out std_logic_vector(M downto 0));

End large_array;

-- using this statement, we can make the

--package arr_pkg visible in this module

-- no. of elements of array is N+1-- no of bits of each element is M+1

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Array Type: example to compare elementsArchitecture arry of large_array is

Begin

Process( a)Variable great: wordN;

Begin;

a <= (“0110”, “0111”, “0010”, “0011”, “0001”);great := “0000”;Loopx : for i in 0 to N loopif (great <= a(i)) then

great := a(i); report(“great is less than or equal to a”);Else

report(“great is greater than a”);end if;end loop loopx;z <= great;

End process;

End arry;

-- data of the array

-- reports that the number is not

-- the largest

-- reports that the number is not

-- the largest

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Record Type• The second VHDL composite type is the record.

• An object of type record may contain elements of different types.

• A record type declaration is used to define a record, which is a collection of one or more elements.

• A TYPE declaration is used to define a record.

• Note that the types of a record's elements must be defined before the record is defined.

• Also notice that there is no semi-colon after the word RECORD.

• The RECORD and END RECORD keywords bracket the field names.

• After the RECORD keyword, the record's field names are assigned and their data types are specified.

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Record Type• Used to collect one or more elements of a different types in single

construct

• Elements can be any VHDL data type

• Elements are accessed through field name

• There are no predefined records in VHDL, but user-defined records can be very useful.

• A record holds several units within a ”group” and the code gets easier to read.

• A record may contain an unlimited amount of elements.

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Record Type

• A record element may be of any data type, including another record.

• The elements need not be of the same type.

• This construct allows users to create data structures which are useful for the system being represented.

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Record Type•Syntax:

type identifier is recordelement1 : type 1element2 : type 2...elementN : type N

end record [ record-type-identifier ] ;

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Record Type: examples

type DATE is record MONTH : STRING (0 to 20); DAY : INTEGER range 1 to 31;

YEAR : INTEGER range 1800 to 2050;

end record DATE;

-------------------------------------------- type ADDRESS is record STREET : STRING (0 to 50);

PO_BOX : INTEGER range 0 to 100000;

CITY : STRING (0 to 50);

STATE : STRING (0 to 40);

ZIP : INTEGER range 0 to 100000000;

end record ADDRESS;

-- to declare a date-- with a string for the month-- and an int for the day-- int for year

--------------------------------------------- a record for an address-- line for street-- possible PO box-- line for city-- state-- keep zip code as int

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Record Type: examples

type EMPLOYEE is record NAME : STRING (0 to 50); HIRE_DATE : DATE; HOME_ADR : ADDRESS; PHONE_NO : STRING(0 to 20); JOB_TITLE : STRING(0 to 20); JOB_CLASS : INTEGER range 1 to 1000;

end record EMPLOYEE;

• -- declare an employee type• -- space for name• -- date when hired• -- where • -- can keep as string or int• -- what job is• -- comp has 1000 job classes

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Access Type• Access are just like pointers in C programming language

• Only variables can be an access type; that is, it is not permissible for a signal to be utilized as an access type.

• If the access type is instantiated in such a way that it is not assigned an initial value, then the initial value is the literal, null.

• The access type declaration begins with the keyword "type", followed by an identifier .

• The identifier is then followed by the keywords "is access", which is then followed by a subtype indication .

• The subtype indication may be as simple as a simple type, or it could be as complex as types can become, including a resolution function and/or index or range constraints.

• After the subtype indication the statement is concluded with a semicolon.

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Access Type• Syntax

type identifier is access subtype-indication ;

• Example:

--side module

type DATE is recordMONTH : STRING (0 to 20);DAY : INTEGER range 1 to 31; YEAR : INTEGER range 1800 to 2050;end record DATE;

-- main module

type calendar is access DATE -- calendar is a pointer to DATE in

-- the side module

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Conclusion• Arrays contain a number of elements of the same type or

subtypes • Arrays are Used to collect one or more elements of a

similar type in a single construct

• Records may contain a number of elements of different types or subtypes

• Record are used to collect one or more elements of a different types in a single construct

• Access types are pointers• Access is used for the implementing queues, fifos, etc.

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References

• http://www.csee.umbc.edu/portal/help/VHDL/VHDL-Handbook.pdf

• http://vhdl.renerta.com/mobile/index.html

• http://www.ece.unm.edu/faculty/pollard/types.html#AccessTypeDec

• http://tams-www.informatik.uni-hamburg.de/vhdl/doc/cookbook/VHDL-Cookbook.pdf

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QUESTIONS04/11/2023


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THANK YOU04/11/2023


vhdl- array, record and access types

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