As for inkey$ - A quick google search
As mentioned earlier, the version of BASIC for Atari's eight-bit computers has no built-in statement designed specifically to retrieve a single keypress. However, you can achieve the same effect by first opening a channel to the keyboard device with the statement OPEN #1, 4, 0, "K:", then using GET#1 to retrieve character values. (Any unused file number in the range 17 can be substituted for the 1 in these statements). The main difference between the Atari's GET#, and the INPUT$ and GET commands described above, is that GET# returns character code values rather than string characters, so GET# must always be followed by a numeric variable. If characters are desired, the CHR$ function can be used. Also, note that GET# waits for a key to be pressed.
To modify the first example program segment in this article for eight-bit Ataris, you must first add lines to open the channel for input and set up the testing variable:
100 OPEN #1, 4, 0, "K"
110 DIM K$(1)
This needs to be done only once in the program, but it must be done before the first GET# command. Then replace line 210 with:
210 GET #1, K : K$ = CHR$(K)
You may find it easier to simply check for character code values rather than characters. Refer to your BASIC manual for a complete list of Atari ASCII codes. For example, you could test for a RETURN keypress with
300 PRINT "PRESS RETURN TO CONTINUE."
310 GET#1, R:IF R< >155 THEN 310