Example 1:
GTM> FOR i=0:1:3 WRITE !,$A("HI",i) -1 72 73 -1 GTM>
This loop displays the result of $ASCII() specifying a character position before, first and second positions, and after the string.
Example 2:
FOR i=1:1:$LENGTH($ZB) WRITE !,$A($ZB,i)
This loop displays the ASCII codes of the terminator in the Intrinsic Special Variable, $ZB. Generally this is 13, for <CR>, or an escape sequence.