The non-block do construct looks like
integer i
do 100, i=1, 5
100 print *, i
That is, where the labelled termination statement is not a continue statement. There are various restrictions on the statement that can be used as the termination statement and the whole thing is generally very confusing.
Such a non-block construct can be rewritten in block form as
integer i
do 100 i=1,5
print *, i
100 continue
or better, using an end do termination statement,
integer i
do i=1,5
print *, i
end do