A for loop in SystemVerilog repeats a given set of statements multiple times until the given expression is not satisfied. Like all other procedural blocks, the for loop requires multiple statements within it to be enclosed by begin and end keywords.
Syntax
For loop controls execution of its statements using a three step approach:
- Initialize the variables that affect how many times the loop is run
- Before executing the loop, check to see if the condition is true
- The modifier is executed at the end of each iteration, and jumps to step 2.
for ( [initialization]; <condition>; [modifier])
// Single statement
for ( [initialization]; <condition>; [modifier]) begin
// Multiple statements
end
Example #1 - Array Iteration
In this example, we will iterate through a string array and print out its contents. The array array is initialized with 5 different names of fruits.
The for loop initialization declares a local variable called i that represents index of any element in the array. The conditional expression checks that i is less than size of the array. The modifier increments the value of i so that every iteration of the for loop operates on a different index.
module tb;
string array [5] = '{"apple", "orange", "pear", "blueberry", "lemon"};
initial begin
for (int i = 0; i < $size(array); i++)
$display ("array[%0d] = %s", i, array[i]);
end
endmodule
Simulation Log ncsim> run array[0] = apple array[1] = orange array[2] = pear array[3] = blueberry array[4] = lemon ncsim: *W,RNQUIE: Simulation is complete.
Example #2 - Multiple Initializations
There can be multiple initializations done in the first part of a for loop. In the code shown below, variables i and j are both initialized as soon as the for loop is entered. To keep the example interesting, the index of each string pointed to by j is replaced by 0.
module tb;
string array [5] = '{"apple", "orange", "pear", "blueberry", "lemon"};
initial begin
for (int i = 0, j = 2; i < $size(array); i++) begin
array[i][j] = "0";
$display ("array[%0d] = %s, %0dth index replaced by 0", i, array[i], j);
end
end
endmodule
Simulation Log
ncsim> run
array[0] = ap0le, 2th index replaced by 0
array[1] = or0nge, 2th index replaced by 0
array[2] = pe0r, 2th index replaced by 0
array[3] = bl0eberry, 2th index replaced by 0
array[4] = le0on, 2th index replaced by 0
ncsim: *W,RNQUIE: Simulation is complete.
Example #3 - Adding multiple modifiers
In the code shown below, j is decremented after each iteration of the for loop along with incrementing i.
module tb;
string array [5] = '{"apple", "orange", "pear", "blueberry", "lemon"};
initial begin
for (int i = 0, j = array[i].len() - 1; i < $size(array); i++, j--) begin
array[i][j] = "0";
$display ("array[%0d] = %s, %0dth index replaced by 0", i, array[i], j);
end
end
endmodule
Simulation Log ncsim> run array[0] = appl0, 4th index replaced by 0 array[1] = ora0ge, 3th index replaced by 0 array[2] = pe0r, 2th index replaced by 0 array[3] = b0ueberry, 1th index replaced by 0 array[4] = 0emon, 0th index replaced by 0 ncsim: *W,RNQUIE: Simulation is complete.