VHDL Tutorial

Integer types in VHDL
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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Integer Types

 

In  VHDL,  integer  types  have  values  that  are  whole  numbers.   The  predefined  type integer includes all the whole numbers representable on a particular host computer. The  language  standard  requires  that  the  type  integer  include  at  least  the  numbers  

2,147,483,647  to  +2,147,483,647  (231  + 1  to  +231   1),  but  VHDL  implementations

may extend the range.

There are also two predefined integer subtypes

 

natural, containing the integers from 0 to the largest integer, and

 

positive, containing the integers from 1 to the largest integer.

 

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Where the logic of a design indicates that a number should not be negative, it is good style to use one of these subtypes rather than the base type integer In this way, we can detect any design errors that incorrectly cause negative numbers to be produced. The operations that can be performed on values of integer types include the fa-

miliar arithmetic operations:

 

+          addition, or unary identity

 

          subtraction, or unary negation

 

*          multiplication

 

/           division (with truncation)

 

mod     modulo (same sign as right operand) rem       remainder (same sign as left operand) abs          absolute value

**        exponentiation (right operand must be non-negative)

 

EXAMPLE

 

Here is a declaration that defines a subtype of integer:

 

subtype small_int is integer range 128 to 127;

 

Values of small_int are constrained to be within the range 128 to 127.   If we de- clare some variables:

 

variable deviation : small_int;

variable adjustment : integer;

 

we can use them in calculations:

 

deviation := deviation + adjustment;

 

 


 

 

 

 

 

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