Verilog Loop Statements

Loop statements are used to control repeated execution of one or more statements. There are 4 types of looping stetements in Verilog:

forever statement;

repeat(expression) statement;

while(expression) statement;

for(initial_assignment; expression; step_assignment) statement;

We can combine more than one statements using begin -- end block in an looping instruction. Looping statements should be used within a procedural block.

forever Loop:

The forever instruction continuously repeats the statement that follows it. Therefore, it should be used with procedural timing controls (otherwise it hangs the simulation). Consider this example:

initial 
begin 
   clk = 0; 
   forever #5 clk = ~clk; 
end 

repeat Loop:

Repeats the following instruction for specified times. The number of executions is set by the expression or constant value. If expression evaluates to high impedance or un-known, then statement will not be executed.

initial 
begin 
  x = 0; 
  repeat( 16 ) 
  begin 
     #2 $display("y= ", y); 
     x = x + 1; 
  end 
end 

while Loop:

while loop repeats the statement until the expression returns true. If starts with false value, high impedance or unknown value, statement will not be executed.

initial 
begin 
  x = 0; 
  while( x <= 10 ) 
  begin 
     #2 $display("y= ", y); 
     x = x + 1; 
  end 
end 

for Loop:

Executes initial_assignment once when the loop starts, Executes the statement or statement group as long as the expression evaluates as true and executes the step_assignment at the end of each pass through the loop.

for(initial_assignment; expression; step_assignment) statement;

Syntax is similar to C language except that begin--end is used instead of {--} to combine more than one statements. Remember that we don't have ++ operator in Verilog.

for( i = 0; i <= 10; i++ )
   mem[i] = 0;

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