WWW.TESTBENCH.IN - SystemVerilog Constructs

Tutorials

Specman E
Interview Questions

FORK CONTROL

Wait Fork Statement

The wait fork statement is used to ensure that all immediate child subprocesses (processes created by the current process, excluding their descendants) have completed their execution.

EXAMPLE
program main();

initial begin
#(10);
$display(" BEFORE fork time = %0d ",$time );
fork
begin
# (20);
$display(" time = %0d # 20 ",$time );
end
begin
#(10);
$display(" time = %0d # 10 ",$time );
end
begin
#(5);
$display(" time = %0d # 5 ",$time );
end
join_any
$display(" time = %0d Outside the main fork ",$time );
end
endprogram

RESULTS

BEFORE fork time = 10
time = 15 # 5
time = 15 Outside the main fork

In the above example, Simulation ends before the #10 and #20 gets executed. In some situations, we need to wait until all the threads got finished to start the next task. Using wait fork, will block the till all the child processes complete.

EXAMPLE:
program main();

initial begin
#(10);
$display(" BEFORE fork time = %0d ",$time );
fork
begin
# (20);
$display(" time = %0d # 20 ",$time );
end
begin
#(10);
$display(" time = %0d # 10 ",$time );
end
begin
#(5);
$display(" time = %0d # 5 ",$time );
end
join_any
$display(" time = %0d Outside the main fork ",$time );
wait fork ;
$display(" time = %0d After wait fork ",$time );

end
endprogram
RESULTS

BEFORE fork time = 10
time = 15 # 5
time = 15 Outside the main fork
time = 20 # 10
time = 30 # 20
time = 30 After wait fork

Disable Fork Statement

The disable fork statement terminates all active descendants (subprocesses) of the calling process.

In other words, if any of the child processes have descendants of their own, the disable fork statement shall terminate them as well. Sometimes, it is required to kill the child processes after certain condition.

EXAMPLE
program main();

initial begin
#(10);
$display(" BEFORE fork time = %0d ",$time );
fork
begin
# (20);
$display(" time = %0d # 20 ",$time );
end
begin
#(10);
$display(" time = %0d # 10 ",$time );
end
begin
#(5);
$display(" time = %0d # 5 ",$time );
end
join_any
$display(" time = %0d Outside the main fork ",$time );
end

initial
#100 $finish;
endprogram
RESULTS

BEFORE fork time = 10
time = 15 # 5
time = 15 Outside the main fork
time = 20 # 10
time = 30 # 20

In the following example, disable for kills the threads #10 and #20.

EXAMPLE
program main();

initial begin
#(10);
$display(" BEFORE fork time = %0d ",$time );
fork
begin
# (20);
$display(" time = %0d # 20 ",$time );
end
begin
#(10);
$display(" time = %0d # 10 ",$time );
end
begin
#(5);
$display(" time = %0d # 5 ",$time );
end
join_any
$display(" time = %0d Outside the main fork ",$time );
disable fork;
$display(" Killed the child processes");
end

initial
#100 $finish;
endprogram
RESULTS

BEFORE fork time = 10
time = 15 # 5
time = 15 Outside the main fork
Killed the child processes

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