Running programs on an Apple IIgs

When an Apple IIgs attempts to boot from a disk, it looks for a file called
PRODOS in the disk's root directory. If no such file by that name is there,
the boot aborts with an error message which says:

*** UNABLE TO LOAD PRODOS ***

The PRODOS file has a file type of SYS (hexadecimal FF), which is the usual
format used for executable ProDOS files.

There really isn't much substitute for the requirement of a boot file called
PRODOS being on the disk. You can use the GUI of the IIgs' system disk to run
executable files that are not called PRODOS, but even the system disk itself
has a file called PRODOS in its root directory which is needed to boot from
that disk.

If you are at a BASIC prompt on the IIgs and you wish to ask the machine to
try booting off a disk, the command to use is:

PR#X

...where X is the slot you want to boot from. Note that this simply tells the
computer to start running code from that slot's boot memory address, which is
$CX00, where X is, once again, the slot number. You can verify this by
jumping directly into code execution at that address with the Apple II
monitor. For example, to boot from slot 5, you could type PR#5 at the BASIC
prompt, or you could type the following two lines:

CALL -151
C500G

The first command is the classic command to enter the Apple II monitor (which
is always in ROM at memory location $FF69, which is accessed using decimal
number -151 since $FFFF corresponds to decimal -1, $FFFE is -2, and so on).
Once you are in the monitor, the second command tells the monitor to start
executing code at $C500 (the G stands for "go" and is the monitor command to
immediately start running code at a direct hexadecimal memory address).

Once you have actually booted a ProDOS program, most programs will place you
in a rudimentary ProDOS environment after you quit the application. The
oldest of these is a simple prompt which says:

ENTER PREFIX (PRESS "RETURN" TO ACCEPT)

Once you press Enter ("Return") at this prompt, the following prompt appears:

ENTER PATHNAME OF NEXT APPLICATION

These prompts are the earliest form of ProDOS QUIT environment, which allow
you to run programs other than the basic "PRODOS" program by typing the full
pathname and filename of the executable you want to run. This allows you to
run any program on the disk, but you cannot do this directly from the
Applesoft BASIC prompt; you must first quit a ProDOS application to get to a
prompt like this. So, once again, short of some direct-to-the-machine tricks
that might permit you to do things like start code execution from an
arbitrary disk sector, there is no convenient provision in the Applesoft
BASIC user interface to run any program other than the one called "PRODOS".
The Apple II was not designed to have several programs stored on a single
disk; in instances where several programs were stored on a single floppy, the
floppy usually came with its own bootable loader program which allowed you to
select which program to run. Apple IIgs owners who had hard disks attached to
their machines typically used the GUI of the system disk to navigate to the
folders where their programs were stored and run those programs. Indeed, on
the IIgs, installing programs to a hard drive is a remarkably straightforward
process, consisting entirely of copying the contents of the program's floppy
disks into a folder on the hard drive (typically also using the GUI of the
system disk) and double-clicking on the program's executable. Note, however,
that some programs (typically games) cannot run from a hard drive and MUST
actually be run from a floppy. In some cases, this is because the programs
expect all their files to be in the root directory of a disk; in such cases,
you might be able to install the program to a hard disk if you copy all its
files to the hard disk's root directory and have it as the only program on
the disk, but this obviously defeats the purpose of having a hard disk.
Additionally, most games that do support hard drive installation actually
require you to keep the first physical floppy disk in a floppy drive as a
"key disk" for copy-protection purposes, which, again, defeats the purpose of
having a hard disk. So while having a hard disk for an Apple IIgs is useful
as a storage archive, most software is designed for the floppy world, and you
really can't get away from needing a floppy drive for the IIgs.