DOS 3.3: The I/O vectors

The Apple monitor sends all input and output, including Applesoft and Integer BASIC's I/O, to a pair of two byte vectors. DOS takes the contents of these vectors and saves them to memory, then puts pointers to itself in the monitor's I/O vectors.
DOS is no longer able to intercept commands when the I/O vectors are changed. A DOS routine starting at $3EA, accessable from BASIC with call 1002, checks the monitor I/O vectors to see if they point to DOS. This routine stores the current values in the I/O vectors and points to DOS in the vector if DOS isn't already there. It's most useful when the I/O vectors need to be modified to a custom I/O routine like the High-Res Character Generator routine in Contributed Software, volume 3. Here's how to use the Character Generator and DOS:

10 poke 54,0
20 poke 55,96
30 call 1002
40 print chr$(4);"catalog"
50 print chr$(4);"PR#0"
Published Date: Feb 18, 2012