The ProDOS operating system for the Apple II executes the first `.SYSTEM` file found in the boot directory on startup. A common pattern is to have the boot directory contain several "driver" files that customize ProDOS by installing drivers for hardware or modify specific parts of the operating system. These include:
Early versions of these drivers would often invoke a specific file on completion, sometimes user-configurable. The best versions of these drivers simply execute the following `.SYSTEM` file, although this is non-trivial code and often did not work with network drives.
In addition, `QUIT.SYSTEM` is present which isn't a driver but which immediately invokes the QUIT handler (a.k.a. program selector). This will happen automatically if the last driver can't find another `.SYSTEM` file, but `QUIT.SYSTEM` can be used to stop the chain.
There's also `PAUSE.SYSTEM` which just waits for a fraction of a second before invoking the next driver file. (Why? In case the log messages from the other installers goes by too fast!)
The intent is that you use a tool like Copy II Plus or [Apple II DeskTop](https://github.com/a2stuff/a2d) to copy and arrange the SYSTEM files on your boot disk as you see fit. A boot disk image catalog that is used on multiple different hardware configurations might include:
Alternately, you might want to install some drivers then immediately launch into BASIC. In that case, put `BASIC.SYSTEM` after the drivers in place of `QUIT.SYSTEM`.
