PUBLIC MARKS from sylvainulg with tags dos & 6502

Applesoft, like Integer BASIC before it, did not come with any built-in commands for dealing with files or disks, other than a feature to save programs to, and load programs from, audiotape. The Apple II disk operating system, known simply as DOS, thus intercepted all input typed at the BASIC command prompt to determine whether it was a DOS command. Similarly, all output was scrutinized for a Control-D character (ASCII 4), which BASIC programs would send before seemingly PRINTing a disk command to get DOS's attention (the disk commands would not really get PRINTed but were intercepted by DOS and prevented from making it to the screen output).[3] ProDOS followed this lead, although the BASIC command interpreter was placed in a separate program called BASIC.SYSTEM and the hook worked in a different manner.[4]