VHDL Tutorial

Boolean typein VHDL
Fundamental concepts
Modelling concepts
Elements of behaviour
Elements of structure
Analysis elaboration
Lexical elements
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
Procedure parameters
Signal parameters
Default values
Unconstrained array parameter
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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The predefined type boolean is defined as


type boolean is (false, true);


This type is used to represent condition values, which can control execution of a be- havioral model.   There are a number of operators that we can apply to values of dif- ferent types to yield Boolean values, namely, the relational and logical operators.  The  relational operators equality (=) and inequality (/=) can be applied to operands of any type, provided both are of the same type.   For example, the expressions


123 = 123,    'A' = 'A',    7 ns = 7 ns


all yield the value true, and the expressions


123 = 456,    'A' = 'z',    7 ns = 2 us


yield the value false.

The  relational  operators  that  test  ordering  are  the  less-than  (<),  less-than-or- equal-to   (<=),   greater-than   (> and   greater-than-or-equal-to   (>= operators. These can only be applied to values of types that are ordered, including all of the sca- lar types described in this chapter.

The  logical  operators  and,  or,  nand,  nor,  xor,  xnor  and  not  take  operands  that must be Boolean values, and they produce Boolean results.