Operators for VHDL: Complete Overview and Examples
* Question
What are the commonly used operators for VHDL?
* Answer
In VHDL (VHSIC Hardware Description Language), operators are used to perform various operations on data types such as bit, boolean, std_logic, integer, and others. They are categorized into several types based on functionality.
Below is a detailed classification of the commonly used VHDL operators:
1. Logical Operators
Used for boolean and std_logic types.
Operator | Description | Usage Example |
and | Logical AND | A and B |
or | Logical OR | A or B |
nand | Logical NAND | A nand B |
nor | Logical NOR | A nor B |
xor | Logical XOR | A xor B |
xnor | Logical XNOR | A xnor B |
not | Logical NOT | not A |
Note: These are often used with signals of type std_logic, boolean, or bit.
2. Relational Operators
Used for comparing two values; return a boolean result.
Operator | Description | Usage Example |
= | Equal to | A = B |
/= | Not equal to | A /= B |
< | Less than | A < B |
<= | Less than or equal | A <= B |
> | Greater than | A > B |
>= | Greater or equal | A >= B |
3. Arithmetic Operators
Used for numeric types such as integer, real, or vector types (signed, unsigned from IEEE libraries).
Operator | Description | Usage Example |
+ | Addition | A + B |
– | Subtraction | A – B |
* | Multiplication | A * B |
/ | Division | A / B |
mod | Modulo | A mod B |
rem | Remainder | A rem B |
** | Exponentiation | A ** B (only for real or integer) |
4. Shift and Rotation Operators (defined in IEEE 1076-2008 or numeric_std)
Operator | Description | Usage Example |
sll | Shift left logical | A sll 2 |
srl | Shift right logical | A srl 3 |
sla | Shift left arithmetic | A sla 1 |
sra | Shift right arithmetic | A sra 2 |
rol | Rotate left | A rol 1 |
ror | Rotate right | A ror 1 |
5. Concatenation Operator
Used to join vectors or bits.
Operator | Description | Usage Example |
& | Concatenation | “01” & “10” → “0110” |
6. Miscellaneous
Unary Minus / Plus: -A, +A
Aggregate assignment: (others => ‘0’) is often used for initializing vectors.
Type Conversion Functions (not operators, but commonly used):
to_integer(), to_unsigned(), to_signed() from numeric_std.
Notes:
Many arithmetic and logical operations on std_logic_vector require type conversion to signed or unsigned when using numeric_std.
VHDL is strongly typed. Operator overloading (via packages like numeric_std) enables operations on composite types.
Conclusion:
VHDL offers a wide range of operators for logic, arithmetic, and data manipulation tasks in digital design. Choosing the correct operator, along with proper type handling, is crucial to ensure synthesis-friendly and simulation-accurate HDL code.