VHDL - VHSIC Hardware Description Language

VHDL stands for Very high Speed Integrated Circuit Hardware Description Language. Some of the main points to ponder about it are as following below:

• VHDL is a non-proprietary language (IEEE-1076 1987/1993).

• VHDL is a widely supported type of Hardware Description Language      
   (HDL).

• VHDL is a kind of programming language.

• VHDL is similar to Ada - typing definitions, type checking, and
   overloading.

• VHDL is also a simulation language.

• VHDL is even a kind of documentation language.

• VHDL is usable for register-transfer level and logic synthesis.

• VHDL has algorithmic level which are supported in parts.

Some properties of VHDL are as following:

• One of the main feature of it is it's Openness and
   availability.

• Support of different design methodologies and
   technologies.

• Independence from technologies and production
   processes.

• Large variety of description possibilities.

• Worldwide design process, and project interchangeability      
   and reuse.

Object and Classes:
• VHDL mainly categorizes objects into four classes which are as following:

• Constant - an object whose value may not be changed.

• Signal - an object with a past history.

• Variable - an object with a single current value.

• File - an object used to represent file in the host environment.

• The type of an object represents its structure, composition, and
   storage requirements.

• A class is relevant to the nature of the object and represents HOW the object is used in the model.

Types & packages

• Types and subtypes present the set of values and a set of operations,
   structure, composition, and storage requirement that an object, such  
   as a variable or a constant or a signal, can hold.

• There exists a set of predefined types in package Standard.

• A package is a design unit which allows the specification of groups
  of logically related declarations.

Basic Language Elements of VHDL are:

• Lexical elements, Identifiers

• Syntax

• Delimiters, Literals, Operators and Precedence

• Types and Subtypes

• Scalar type - integer, enumerate (IEEE Std. 1164), physical, real

• Composite type - arrays, records

• Access type

• File

• Attributes

• Aliases