Every assembly language source file must end with END on
a line by itself.
If the source file has been included in a parent file by a GET directive,
the assembler returns to the parent file and continues assembly
at the first line following the GET directive.
If END is reached in the top-level source file
during the first pass without any errors, the second pass begins.
If END is reached in the top-level source file
during the second pass, the assembler finishes the assembly and
writes the appropriate output.