Operators are means for constructing expressions. this are the different type of operator in VHDL
VHDL has a wide set of different operators, which can be divided into groups of the same precedence level (priority). The table below lists operators grouped according to priority level, highest priority first.
| Operator type | and | or | nand | nor | xor | xnor |
|---|---|---|---|---|---|---|
| logical | and | or | nand | nor | xor | xnor |
| relational | = | /= | < | <= | > | >= |
| shift | sll | srl | sla | sra | rol | ror |
| adding | + | - | & | |||
| sign | + | - | ||||
| multiplying | * | / | mod | rem | ||
| miscellaneous | ** | abs | not |
The expressions are evaluated form left to right, operations with
higher precedence are evaluated first. If the order should be
different from the one resulting from this rule, parentheses can be
used (Example 1).
The operands, connected with each other by an operator, are evaluated
before the operation described by that operator is carried out. For
some operators the right operand is evaluated only when the left
operand has a certain value assigned to it.
The logical operators
such as and, or , nand , nor
defined for the BIT and BOOLEAN operands belong to those operators.
The operators for the predefined types are defined in the STANDARD
package in the STD library.
These operators are functions, which
always return the same value when they are called with the same
values of the actual parameters. These functions are called the pure
function (see function for details)
The logical operators and, or , nand , nor, xor, xnor, not
are defined for BIT and BOOLEAN types, as well as for one-dimensional arrays containing
the elements of BIT and BOOLEAN.
All these operators have the lowest
priority, except for the operator not
which has the highest priority. The results of the logical operators
for the predefined types are presented in the tables 2 through 8.
The
BIT type is represented by the values '0' and '1', while the Boolean
type by True and False.
| A | not(A) |
| '0' | '1' |
| '1' | '0' |
| A | B | A and B |
| '0' | '0' | '0' |
| '0' | '1' | '0' |
| '1' | '0' | '0' |
| '1' | '1' | '1' |
| A | B | A or B |
| '0' | '0' | '0' |
| '0' | '1' | '0' |
| '1' | '0' | '0' |
| '1' | '1' | '1' |
| A | B | A xor B |
| '0' | '0' | '0' |
| '0' | '1' | '1' |
| '1' | '0' | '1' |
| '1' | '1' | '0' |
| A | B | A nand B |
| '0' | '0' | '1' |
| '0' | '1' | '1' |
| '1' | '0' | '1' |
| '1' | '1' | '0' |
| A | B | A nor B |
| '0' | '0' | '1' |
| '0' | '1' | '1' |
| '1' | '0' | '1' |
| '1' | '1' | '0' |
The relational operators allow checking relation between operands,
i.e. to state whether they are equal, not equal or are ordered in a
way defined by operator (Table 2).
Both operands must be of the same
type, and the result received is always of the Boolean type.
| = | Equality |
| /= | Inequality |
| < | less than |
| <= | less than or equal |
| > | greater than |
| >= | greater than or equal |
The operators: equality and inequality are predefined for all types
available in the language except the file type. For other relations
the operands must be of a scalar type or one-dimensional array types.
The equality operator returns the value TRUE only when both operands
have the same values, and FALSE when the values are different.
The
inequality operator returns the value TRUE when the operators are
different and FALSE when they are equal.
There are certain rules that
are used to compare operands depending on their type: in case of the
scalar type, the operand values are equal only when the values are
the same.
Two values of the composite type are equal only when each
value of the left operand corresponds to the value of the right
operand and vice versa.
In the record the corresponding elements have
identical identifiers, and in the array the corresponding elements
are those which appear at the same positions of arrays. In particular
two null arrays of the same type are always equal.
The operators: <, <=, >, and >= return the TRUE logical
value only when the condition in the given relation is met, otherwise
the FALSE value is returned.
The shift operators are defined for the one-dimensional array with
the elements of the type BIT or BOOLEAN. For the shift operator an
array is the left operand L and integer is the right operand R.
The
right operand represents the number of positions the left operand
should be shifted. As the result of shifting, the value of the same
type as the left operand is returned.
(Table 2) below shows predefined
shift operators.
| sll | Shift left logical |
| srl | Shift right logical |
| sla | Shift left arithmetic |
| sra | Shift right arithmetic |
| rol | Rotate left logical |
| ror | Rotate right logical |
| + | Addition |
| - | Subtraction |
| & | Concatenation |
The adding and subtraction operators perform mathematical operations,
and their operands can be of any numeric type.
The concatenation (&) operator is defined for elements of
one-dimensional arrays. In the concatenation, the following
situations can take place:
Sign operators are unary operators, i.e. have only one, right
operand, which must be of a numeric type. The result of the
expression evaluation is of the same type as the operand.
There are
two sign operators.
| + | Identity |
| - | Negation |
When ( + ) sign operator is used, the operand is returned unchanged,
but In case of ( - ) sign operator the value of operand with the
negated sign is returned. Because of the lower priority,
the sign
operator in the expression cannot be directly preceded by the
multiplication operator, the exponentiation operator (**) or the abs
and not operators. When
these operators are used then sign operator and its operand should be
enclosed in parentheses.
The multiplication and division operators are predefined for all
integers, floating point numbers. Under certain conditions, they may
be used for operations on physical type objects as well.
The mod
and rem operators, on the
other hand, are defined only for the integers. When mod
and rem operators are used,
then both the operands and the result are of the same integer type.
The multiplying operators are shown in the table 5.
| * | Multiplication |
| / | Division |
| mod | Modulus |
| rem | Remainder |
The two miscellaneous operators are shown in the Table 6.
Table 6. Miscellaneous operators| ** | Exponentiation |
| abs | Absolute value |
The abs operator has only
one operand. It allows defining the operand's absolute value. The
result is of the same type as the operand.
The exponentiation operator has two operands. This operator is
defined for any integer or floating point number. The right operand
(exponent) must be of integer type.
When the exponent is the positive
integer, then the left operand is repeatedly multiplied by itself.
When the exponent is the negative number, then the result is a
reverse of exponentiation with the exponent equal to the absolute
value of the right operand.
If the exponent is equal to 0
the result will be 1.
variable a : integer := 5;
variable b : integer := 2;
variable y : integer := -3;
variable result : integer ;
result := a + y * b; -- ( 5 + ((-3) * 2)) = -1
signal a, b, c, d : std_logic ;
s0 <= a and b;
s1 <= c or d;
s2 <= not(a) xor b;
s3 <= a and b xnor c nor d;
variable data : BIT_VECTOR(3 downto 0) := ('1','0','1','1');
data sll 1 -- ('0', '1', '1', '0')
data sll 3 -- ('1', '0', '0', '0')
data sll -3 -- data srl 3
data srl 1 -- ('0', '1', '0', '1')
data srl 3 -- ('0', '0', '0', '1')
data srl -3 -- data sll 3
data sla 1 -- ('0', '1', '1', '1')
data sla 3 -- ('1', '1', '1', '1')
data sla -3 -- data sra 3
data sra 1 -- ('1', '1', '0', '1')
data sra 3 -- ('1', '1', '1', '1')
data sra -3 -- data sla 3
data rol 1 -- ('0', '1', '1', '1')
data rol 3 -- ('1', '1', '0', '1')
data rol -3 -- data ror 3
data ror 1 -- ('1', '1', '0', '1')
data ror 3 -- ('0', '1', '1', '1')
data ror -3 -- data rol 3