Loop Statements
Formal Definition
Loop statements provide a means of modeling blocks of procedural statements.
Simplified Syntax
forever statement;
repeat (expression) statement;
while (expression) statement;
for (assignment; expression;
assignment) statement;
Description
There are four types of loop statements: forever, repeat, while, and
for statements.
The forever instruction
(Example 1) continuously repeats the statement that follows it.
Therefore, it should be used with procedural timing controls
(otherwise it hangs the simulation).
The repeat instruction
(Example 2) executes a given statement a fixed number of times. The
number of executions is set by the expression, which follows the
repeat keyword. If the expression evaluates to unknown,
high-impedance, or a zero value, then no statement will be executed.
The while instruction
(Example 3) executes a given statement until the expression is true.
If a while statement starts
with a false value, then no statement will be executed.
The for instruction (Example
4) executes a given statement until the expression is true. At the
initial step, the first assignment will be executed. At the second
step, the expression will be evaluated. If the expression evaluates
to an unknown, high-impedance, or zero value, then the for
statement will be terminated. Otherwise, the statement and second
assignment will be executed. After that, the second step is repeated.
Examples
Example 1
always begin
counter = 0;
forever #10 counter =
counter + 1;
end
Example 2
initial begin
repeat (10) a = a + ~b;
end
Example 3
module test;
parameter
MSB = 8;
reg [MSB-1:0] Vector;
integer t;
initial
begin
t = 0;
while (t < MSB)
begin
//Initializes vector elelments
Vector[t] = 1'b0;
t = t + 1;
end
end
endmodule
Example 4
initial begin
for (index=0; index
< 10; index = index + 2)
mem[index] = index;
end
Important Notes
|
