62. Nibble Swap
Swap the high and low nibbles of an 8-bit input.
Requirements
- Module:
nibble_swap_task - Inputs:
x[7:0] - Outputs:
y[7:0] - Write a task
swap_nibbles(input [7:0] xin, output [3:0] hi, output [3:0] lo)that extracts the high and low nibbles fromxin. - Keep it fully combinational.
Behaviour
- Input
x[7:0] - Output
y[7:0] = {x[3:0], x[7:4]}
Need Help? Refer to the Quick Guide below
Function
- Purpose: Pure combinational calculations.
- Return: Exactly one value.
- Timing controls: Not allowed (
#,@,wait). - Inputs: Passed by value only (cannot be
outputorinout). - Side effects: No global variable updates, only local/internal.
Syntax
function [WIDTH-1:0] func_name;
input [WIDTH-1:0] a, b; // inputs only
begin
func_name = a + b; // return value
end
endfunction
Usage
assign y = func_name(x, z); // can be used inside expressions
Task
- Purpose: Flexible procedure — useful for testbenches or complex modeling.
- Return: Can modify multiple outputs.
- Timing controls: Allowed (
#,@,wait). - Inputs/Outputs:
input,output,inoutsupported. - Side effects: Can update global variables and interact with simulation.
Syntax
task task_name;
input [7:0] a, b;
output [7:0] sum, diff;
begin
sum = a + b;
diff = a - b;
end
endtask
Usage
task_name(x, y, s, d); // call by name
Function vs Task
| Feature | Function | Task |
|---|---|---|
| Return values | One (function name) | Many (via output/inout) |
| Timing controls | ❌ Not allowed | ✅ Allowed |
| Global side effects | ❌ Not allowed | ✅ Allowed |
| Usage in expressions | ✅ Yes (part of continuous/procedural) | ❌ No (must be called as a statement) |
| Synthesis use | Combinational logic | Testbenches, complex procedures |
Quick mnemonic: Use a function when you need a “calculator” (pure math, single return). Use a task when you need a “procedure” (multi-output, delays, events, or side effects).