VHDL Tutorial

Boolean typein VHDL
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Booleans

 

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.

 

 

 

 

 

 

 

 

 

 

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