why the simulation result is differenct between fork join and begin end?

The difference between "begin-end" and "fork-join" is that in initial block "begin-end" operations work step-by-step, but in "fork-join" they work simultaneously. You can rewrite the testbench using "begin-end". For example,

begin

arst <= 1'b0; // resetting the contoller initially

iderst <= 1'b0;

ideen <= 1'b1;

pioiordyen <= 1'b1;

piorqst <= 1'b1;

pioaddr <= 4'b0111;

piodatain <= 16'b1111111100000000;

piowe <= 1'b1;

iordy <= 1'b1;

intrq <= 1'b1;

flag <= 1'b0; //if error occured in the test procedure,then asserted

ddi <= 16'b0000000011111111;

# 250 arst <= 1'b1;

# 350 piowe <= 1'b0;

# 4400 $stop;

end

I believe, it will work like original "fork-join"