Operators and Attributes in VHDL: A Comprehensive Guide, Slides of Computer Science

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Operators

• Assignment Operators
• Logical Operators
• Relational Operators
• Shift Operators
• Adding Operators
• Multiplying Operators
• Miscellaneous Operators

Assignment Operators

• Used to assign values to signals, variables

and constants.

<= Used to assign a value to a Signal := Used to assign a value to a variable, constant, or Generic Must be the same size and type => Used to assign values to individual vector elements or with OTHERS.

Relational Operator

• = Equality
• /= inequality
• <
• <=

• =

• Must be one-dimensional discrete array of the same type.

Shift Operators

• Sll Shift left logical
• Srl Shift right logical
• Sla Shift left arithmetic
• Sra Shift right arithmetic
• Rol Rotate left
• Ror Rotate Right

Adding Operators

• • addition
• • subtraction
• & Concatenation
• Example:
• A = 1010
• B = 1000
• C <= a & b; c = 10101000
• C <= b & a; c := 10001010

Multiplying Operators

• • Multiplication
• / Division
• Mod modulus
• Rem Remainder

Attributes

• An attributes is a value, function, type,

range, signal or a constant that may be

associated with one or more names within a

VHDL description.

• Two types of attributes
  • User defined attributes
  • Predefined attributes

Classes of attributes

1. Value attributes : Return a constant value
2. Function attributes : Call a function that return a value
3. Signal Attributes : Creates a new implicit signal
4. Type Attributes : Return a type
5. Range attributes : Returns a range

To use a attribute the “’” (apostrophe) construct must be employed

• T’right Returns right value specified in

type declaration

Example: Integer’right is 2147483647 bit’right is ‘1’

• T’high Returns largest value specified in

declaration

Example: Type bit8 is 255 downto 0 bit8’high is 255

• T’low Returns smallest value specified in

declaration

Example: Type bit8 is 255 downto 0; bit8’low is 0;

• T’pos(x) Returns position number of

argument in type (first position is 0)

Example: Type color is (red,green, blue, orange); color’pos(green) is 1;

• T’pred(x) Returns the predecessor to

the value passed in

Example: Type color is (red,green, blue, orange); color’pred(blue) is green;

• T’leftof(x) Returns the value to the left

of the value passed in

Example: Type color is (red,green, blue, orange); color’leftof(blue) is green

• T’rightof(x) Returns the value to the right

of the value passed in

Example: Type color is (red,green,blue,orange); color’rightof(blue) is orange;

Type s is (1,0,0,1,0,1) S’rightof(1)

Type a_type is array(0 to 3, 7 downto 0) of bit;

• A’high(n) Returns largest array bound value of selected index range Example: a_type’high(1) is 3 a_type’high(2) is 7
• A’low(n) Returns smallest array bound value of selected index range Example: a_type’low(1) is 0 a_type’low(2) is 0

Type a_type is array(0 to 3, 7 downto 0) of bit;

• A’range Returns selected index range Example: a_type’range(1) is 0 to 3 a_type’range(2) is 7 downto 0
• A’reverse_range(n) Returns selected index

range reversed

Example: a_type’reverse_range(1) 3 downto 0 a_type’reverse_range(2) 0 to 7;