This command implements a looping construct that loops the code between the line DO WHILE and LOOP util the expression following the keywords on the loop starting line becomes false.
Practically this command is same as the command WHILE with a different syntax to be compatible with different BASIC implementations.
do while
...
loop
You can use the construct that creates more readable code.
See also WHILE, DOUNTIL, DOWHILE, REPEAT, DO and FOR.