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7. Operators

VHDL supports different classes of operators that operate on signals, variables and constants. The different classes of operators are summarized below.

 

Class            
1. Logical operators and or nand nor xor xnor
2. Relational operators = /= < <= > >=
3. Shift operators sll srl sla sra rol ror
4.Addition operators + = &      
5. Unary operators +        
6. Multiplying op. * / mod rem    
7. Miscellaneous op. ** abs not      

 

The order of precedence is the highest for the operators of class 7, followed by class 6 with the lowest precedence for class 1. Unless parentheses are used, the operators with the highest precedence are applied first. Operators of the same class have the same precedence and are applied from left to right in an expression. As an example, consider the following std_ulogic_vectors, X (=’010’), Y(=’10’), and Z (‘10101’). The expression

 

not X & Y xor Z rol 1

 

is equivalent to ((not X) & Y) xor (Z rol 1) = ((101) & 10) xor (01011) =(10110) xor (01011) = 11101. The xor is executed on a bit-per-bit basis.

 

 

 

 

  1. Logic operators

 

The logic operators (and, or, nand, nor, xor and xnor) are defined for the “bit”, “boolean”, “std_logic” and “std_ulogic” types and their vectors. They are used to define Boolean logic expression or to perform bit-per-bit operations on arrays of bits. They give a result of the same type as the operand (Bit or Boolean). These operators can be applied to signals, variables and constants.

 

Notice that the nand and nor operators are not associative. One should use parentheses in a sequence of nand or nor operators to prevent a syntax error:

 

X nand Y nand Z will give a syntax error and should be written as (X nand Y) nand Z.

 

  1. Relational operators

 

The relational operators test the relative values of two scalar types and give as result a Boolean output of “TRUE” or “FALSE”.

 

Operator Description Operand Types Result Type
= Equality any type Boolean
/= Inequality any type Boolean
< Smaller than scalar or discrete array types Boolean
<= Smaller than or equal scalar or discrete array types Boolean
> Greater than scalar or discrete array types Boolean
>= Greater than or equal scalar or discrete array types Boolean

 

Notice that symbol of the operator “<=” (smaller or equal to) is the same one as the assignment operator used to assign a value to a signal or variable. In the following examples the first “<=” symbol is the assignment operator. Some examples of relational operations are:

 

variable STS               : Boolean;

constant A                  : integer :=24;

constant B_COUNT   : integer :=32;

constant C                  : integer :=14;

STS <= (A < B_COUNT) ; — will assign the value “TRUE” to STS

STS <= ((A >= B_COUNT) or (A > C));   — will result in “TRUE”

STS <= (std_logic (‘1’, ‘0’, ‘1’) < std_logic(‘0’, ‘1’,’1’));–makes STS “FALSE”

 

type new_std_logic is (‘0’, ‘1’, ‘Z’, ‘-‘);

variable A1: new_std_logic :=’1’;

variable A2: new_std_logic :=’Z’;

STS <= (A1 < A2); will result in “TRUE” since ‘1’ occurs to the left of ‘Z’.

 

For discrete array types, the comparison is done on an element-per-element basis, starting from the left towards the right, as illustrated by the last two examples.

 

  1. Shift operators

 

These operators perform a bit-wise shift or rotate operation on a one-dimensional array of elements of the type bit (or std_logic) or Boolean.

 

Operator Description Operand Type Result Type
sll Shift left logical (fill right vacated bits with the 0) Left: Any one-dimensional array type with elements of type bit or Boolean; Right: integer Same as left type
srl Shift right logical (fill left vacated bits with 0) same as above Same as left type
sla Shift left arithmetic (fill right vacated bits with rightmost bit) same as above Same as left type
sra Shift right arithmetic (fill left vacated bits with leftmost bit) same as above Same as left type
rol Rotate left (circular) same as above Same as left type
ror Rotate right (circular) same as above Same as left type

 

The operand is on the left of the operator and the number (integer) of shifts is on the right side of the operator. As an example,

 

variable NUM1          :bit_vector := “10010110”;

NUM1 srl 2;

 

will result in the number “00100101”.

 

When a negative integer is given, the opposite action occurs, i.e. a shift to the left will be a shift to the right. As an example

 

NUM1 srl –2 would be equivalent to NUM1 sll 2 and give the result “01011000”.

 

Other examples of shift operations are for the bit_vector A = “101001”

 

variable A: bit_vector :=”101001”;
 

A sll 2   results   in      “100100”

A srl 2   results in                  “001010”

A sla 2   results in                  “100111”

A sra 2   results in                 “111010”

A rol 2   results in                  “100110”

A ror 2   results in                 “011010”

 

 

  1. Addition operators

 

The addition operators are used to perform arithmetic operation (addition and subtraction) on operands of any numeric type. The concatenation (&) operator is used to concatenate two vectors together to make a longer one. In order to use these operators one has to specify the ieee.std_logic_unsigned.all or std_logic_arith package package in addition to the ieee.std_logic_1164 package.

 

Operator Description Left Operand Type Right Operand Type Result Type
+ Addition Numeric type Same as left operand Same type
Subtraction Numeric type Same as left operand Same type
& Concatenation Array or element type Same as left operand Same array type

 

An example of concatenation is the grouping of signals into a single bus [4].

 

signal MYBUS                        :std_logic_vector (15 downto 0);

signal STATUS                     :std_logic_vector (2 downto 0);

signal RW, CS1, CS2             :std_logic;

signal MDATA                                  :std_logic_vector ( 0 to 9);

MYBUS <= STATUS & RW & CS1 & SC2 & MDATA;

 

Other examples are

 

MYARRAY (15 downto 0) <= “1111_1111” & MDATA (2 to 9);

NEWWORD <= “VHDL” & “93”;

 

The first example results in filling up the first 8 leftmost bits of MYARRAY with 1’s and the rest with the 8 rightmost bits of MDATA. The last example results in an array of characters “VHDL93”.

 

  1. Unary operators

 

The unary operators “+” and “-“ are used to specify the sign of a numeric type.

 

Operator Description Operand Type Result Type
+ Identity Any numeric type Same type
Negation Any numeric type Same type

 

  1. Multiplying operators

 

The multiplying operators are used to perform mathematical functions on numeric types (integer or floating point).

 

 

Operator Description Left Operand Type Right Operand Type Result Type
 

*

 

Multiplication

Any integer or floating point Same type Same type
Any physical type Integer or real type Same as left
Any integer or real type Any physical type Same as right
/ Division Any integer or floating point Any integer or floating point Same type
Any physical type Any integer or real t ype Same as left
Any physical type Same type Integer
mod Modulus Any integer type Same type
rem Remainder Any integer type Same type

 

The multiplication operator is also defined when one of the operands is a physical type and the other an integer or real type.

 

The remainder (rem) and modulus (mod) are defined as follows:

 

A rem B = A –(A/B)*B                        (in which A/B in an integer)

A mod B = A – B * N              (in which N is an integer)

 

The result of the rem operator has the sign of its first operand while the result of the mod operators has the sign of the second operand.

 

Some examples of these operators are given below.

 

11 rem 4                      results in 3

(-11) rem 4                   results in -3

9 mod 4                        results in 1

7 mod (-4)                    results in –1 (7 – 4*2 = -1).

 

  1. Miscellaneous operators

 

These are the absolute value and exponentation operators that can be applied to numeric types. The logical negation (not) results in the inverse polarity but the same type.

 

Operator Description Left Operand Type Right Operand Type Result Type
** Exponentiation Integer type Integer type Same as left
Floating point Integer type Same as left
abs Absolute value Any numeric type Same type
not Logical negation Any bit or Boolean type Same type

 

Delays or timing information

Packages (list standard, 1164 packages).

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