Editing Apple DOS (section)

===Boot loader===
The process of loading Apple DOS involves a series of very tiny programs, each of which carries the loading process forward a few steps before passing control to the next program in the chain.

*Originally, the Apple II [[read-only memory|ROM]] did not support disk booting at all. At power-up it would display the System Monitor prompt. Both the Monitor and Integer BASIC have commands to redirect printing to a printer [[device driver|driver]] in a designated slot, so the conventional way to boot from disk then was to command the computer to start "printing" to the disk interface card, typically installed in slot 6, using the command {{keypress|6 Control-P}} (from the ML monitor) or {{code|PR#6}} (from BASIC). When the monitor or BASIC issued the next prompt character, the computer would call the ROM routines on the disk card to "print" to it, which would then proceed with the boot sequence. (One could use input redirection to similar ends.) Alternatively, from the ML monitor, the user could invoke the controller's boot code directly with a command like {{code|C600G}}. 
*When the Apple II Plus was introduced, it included the ability to scan each expansion slot (working downward from slot 7 to slot 1) for a bootable expansion card ROM, and automatically call it.
*The expansion card ROM boot code attempts to boot from drive 1 of the controller by moving the read/write arm to track zero and attempting to read 256 bytes from sector zero of that track. (If no readable disk is available, the drive spins indefinitely until one is provided and the drive door is closed.)
*Sector zero contains a small program which instructs the computer to read sectors 0 through 9 of track zero into memory using part of the ROM boot code (rereading sector 0 in the process).
*The program in sectors 1–9 of track 0, including the complete RWTS code, then proceeds to load tracks 1 and 2, which contain the rest of DOS. On a system master disk, code is also included to determine the computer's RAM configuration and relocate DOS as high into system memory as possible, up to the 48&nbsp;KB limit of the Apple II's main memory ($BFFF{{sub|16}}).
*Once DOS is loaded into memory, it attempts to load and execute a startup program as indicated in the DOS program code. This is commonly a BASIC language program named ''HELLO'' (or some other name) but DOS can be modified to run other types of programs at startup, such as an executable binary file. The appearance of the right-hand bracket (''']''') on the screen is an indication to the user that an Applesoft BASIC startup program is loading, while a greater-than symbol ('''>''') indicates that an Integer BASIC program is loading. (These are the prompts for the respective versions of BASIC, which are being initialized at this point.)
*The startup program then begins executing.