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FORK JOIN

A Verilog fork...join block always causes the process executing the fork statement to block until the termination of all forked processes. With the addition of the join_any and join_none keywords, SystemVerilog provides three choices for specifying when the parent (forking) process resumes execution.

Fork Join None

The parent process continues to execute concurrently with all the processes spawned by the fork. The spawned processes do not start executing until the parent thread executes a blocking statement.

EXAMPLE : fork/join none
program main ;
initial
begin
#10;
$display(" BEFORE fork time = %d ",$time );
fork
begin
# (20);
$display("time = %d # 20 ",$time );
end
begin
#(10);
$display("time = %d # 10 ",$time );
end
begin
#(5);
$display("time = %d # 5 ",$time );
end
join_none
$display(" time = %d Outside the main fork ",$time );
#(40);
end
endprogram
RESULTS

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

Fork Join Any

The parent process blocks until any one of the processes spawned by this fork completes.

EXAMPLE : fork/join any
program main;
initial begin
#(10);
$display(" BEFORE fork time = %d ",$time );
fork
begin
# (20);
$display("time = %d # 20 ",$time );
end
begin
#(10);
$display("time = %d # 10 ",$time );
end
begin
#(5);
$display("time = %d # 5 ",$time );
end
join_any
$display(" time = %d Outside the main fork ",$time );
#(40);
end
endprogram
RESULTS

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

For Join All

The parent process blocks until all the processes spawned by this fork complete.

EXAMPLE : fork/join all
program main ;
initial
begin
#(10);
$display(" BEFORE fork time = %d ",$time );
fork
begin
# (20);
$display("time = %d # 20 ",$time );
end
begin
#(10);
$display("time = %d # 10 ",$time );
end
begin
#(5);
$display("time = %d # 5 ",$time );
end
join
$display(" time = %d Outside the main fork ",$time );
#(40);
end
endprogram
RESULTS

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

When defining a fork/join block, encapsulating the entire fork inside begin..end, results in the entire block being treated as a single thread, and the code executes consecutively.

EXAMPLE : sequential statement in fork/join
program main ;
initial begin
#(10);
$display(" First fork time = %d ",$time );
fork
begin
# (20);
$display("time = %d # 20 ",$time);
end
begin
#(10);
$display("time = %d # 10 ",$time);
end
begin
#(5);
$display("time = %d # 5 ",$time);
#(2);
$display("time = %d # 2 ",$time);
end
join_any
$display(" time = %d Outside the main fork ",$time );
#(40);
end
endprogram
RESULTS:

First fork time = 10
time = 15 # 5
time = 17 # 2
time = 17 Outside the main fork
time = 20 # 10
time = 30 # 20

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