In quite a few programs on Atari, I found the self-programming trick:
10 GRAPHICS 0 : REM clear the screen, enter standard text mode 20 POSITION 2,4 : REM place cursor 4 lines from top of screen 30 PRINT "1000 SOUND 0, 200, 10, 15 : REM Beep" 40 PRINT "1010 FOR I=0 TO 500 : NEXT I : REM wait a bit" 50 PRINT "1020 SOUND 0, 0, 0, 0 : REM Silence" 60 PRINT "1030 RETURN" 70 PRINT "CONT" 80 POSITION 2,0 : REM place cursor on top of screen 90 POKE 842,13 : REM The Magical POKE 100 STOP : REM Abort program execution 110 POKE 842,12 : REM Program resumes from here. This stops the effect of the prior POKE 120 GOSUB 1000 : REM Emit the Beep 130 END
The above program appends to itself a subroutine that emits a beep, then executes it. The uses in practice were much more creative.
The key point is
POKE 842,13 which causes effect similar to holding down the Return key - the cursor starts riding down the screen, executing whatever's printed out - including
CONT which resumes the program.
I found memory location 842 is ICAX1 not very informatively described as The auxiliary information bytes are used to give CIO or the device any special information needed. Bits 2 and 3 are responsible for Read and Write flags when opening a file... but I didn't find anything about bit 0, plus no file is being accessed here.
So, 'from the guts side' what did that POKE do?