VHDL Operators

VHDL OPERATORS

The predefined operators in the VHDL language are classified into the following five categories:

1. Logical operators

2. Relational operators

3. Adding operators

4. Multiplying operators

5. Miscellaneous operators

·         The operators have increasing precedence going from category (1) to (5).

·         Operators in the same category have the same precedence and evaluation is done left to right.

·         Parentheses may be used to override the left to right evaluation.

1. Logical Operators

The six logical operators are

and or nand nor xor not

These operators are defined for (one-dimensional arrays) the predefined types BIT and BOOLEAN.

During evaluation, bit values '0' and 1' are treated as FALSE and TRUE values of the BOOLEAN type, respectively. The result of a logical operation has the same type as its operands.

The not operator is a unary logical operator which has the same precedence as that of miscellaneous operators.

2. Relational Operators

These are

= /= < <= > >=

The result types for all relational operations is always BOOLEAN.

The = (equality) and the /= (inequality) operators are permitted on any type except file types.

The remaining four relational operators are permitted on any scalar type (e.g., integer or enumerated types) or discrete array type (i.e., arrays in which element values belong to a discrete type).

When operands are discrete array types, comparison is performed one element at a time from left to right.

Eg.,

BIT_VECTOR'('0', '1', '1') < BIT_VECTOR'('1', '0', '1')

is true, since the first element in the first array aggregate is less than the first element in the second aggregate.

Also, if

type MVL is ('U', '0', '1', 'Z' );

then,

MVL'( 'U' ) < MVL'( 'Z' )

is true since 'U' occurs to the left of 'Z'.

3. Adding Operators

These are

+ - &

The operands for the + (addition) and - (subtraction) operators must be of the same numeric type with the result being of the same numeric type.

The addition and subtraction operators may also be used as unary operators then the operand and the result type must be the same.

The operands for the & (concatenation) operator can be either a I-dirnensional array type or an element type. The result is always an array type.

Eg.,

'0' & '1'

results in an array of characters "01".

'C' & 'A' & 'T'

results in the value "CAT".

"BA" & "LL"

creates an array of characters "BALL".

4. Multiplying Operators

These are

* / mod rem

The * (multiplication) and / (division) operators are predefined for both operands being of the same integer or floating point type. The result is also of the same type.

The multiplication operator is also defined for the case when one of the operands is of a physical type and the second operand is of integer or real type. The result is of physical type.

For the division operator, division of an object of a physical type by either an integer or a real type object is allowed and the result type is of the physical type.

Division of an object of a physical type by another object of the same physical type is also defined and it yields an integer value as a result.

The rem (remainder) and mod (modulus) operators operate on operands of integer types and the result is also of the same type.

The result of a rem operation has the sign of its first operand and is defined as

A rem B = A - ( A / B ) * B

The result of a mod operator has the sign of the second operand and is defined as

A mod B = A – B * N -For some integer N.

5. Miscellaneous Operators

The miscellaneous operators are

abs **

The abs (absolute) operator is defined for any numeric type.

The ** (exponentiation) operator is defined for the left operand to be of integer or floating point type and the right operand (i.e., the exponent) to be of integer type only.

The not logical operator has the same precedence as the above two operators.