How to use control-D character?

[ali hagigat]

My question is about as, GNU assembler, Version 2.14 and I have copied
the related extract from the manual:
-----------------------------------------
1.5 Input Files
If you give 'as' no file names it attempts to read one input file from
the as standard input,
which is normally your terminal. You may have to type ctl-D to tell as
there is no more
program to assemble.
----------------------------------------

How can i use <ctl-D> key combination? Will it be typed at the end of
'as' command line?
When i type:
as <ctl-D>
'as' will wait for user to enter some lines of instruction!!

If I use 'as' alone and then enter some assembly lines , how can i
terminate the input terminal text and make 'as' assemble the lines I
have written?

Thank you to read my message.