LOADER.SYSTEM

An Apple][ ProDOS 8 loader for cc65 programs

Background

Apple][ ProDOS 8 system programs (filetype SYS) are always loaded into memory starting at location $2000. This poses the problem of how to make efficient use of the memory in the range $0800-$2000. The usual approach of relocation has two downsides:

• Relocating e.g. 30 kB from $2000-$9800 to $0800-$8000 takes a considerable amount of time.
• Really large programs just don't fit into memory when loaded starting at location $2000.

The relocation can be eliminated by loading the major part(s) of the program from disk right to the final location by a rather small system program.

LOADER.SYSTEM is such a small program. In fact it's so small that it fits into a single block in the ProDOS 8 file system making it a so-called seedling file, which are loaded really fast. LOADER.SYSTEM can load cc65 programs into memory anywhere in the range $0800-$BB00 (44,75 kB).

Usage

Link the cc65 program to the start address $0803 (or any other address) and store it as binary program (filetype BIN). This is in fact no different from a binary program to be run by BASIC.SYSTEM's BRUN command in the usual way.

If however the cc65 program isn't run by BASIC.SYSTEM but is rather run by LOADER.SYSTEM then it behaves like a system program which means:

• It uses memory up to the ProDOS 8 system global page located at $BF00.
• It supports the ProDOS 8 startup file mechanism (mapped to argv[1]).
• It quits to the ProDOS 8 dispatcher.

Obviously LOADER.SYSTEM has to be told which cc65 program to run. Unfortunately the ProDOS 8 dispatcher has no notion of system program parameters so the usual approach would have been to make LOADER.SYSTEM bring up yet another menu to select the cc65 program to run.

But to allow to select the cc65 program directly from the ProDOS 8 dispatcher anyway LOADER.SYSTEM detects the path to the cc65 program from its own path by just removing the '.SYSTEM' from its name. So if you want to run the cc65 program MYPROGRAM you'll need a copy of LOADER.SYSTEM in the same directory being renamed to MYPROGRAM.SYSTEM.

This means you will end up with a copy of LOADER.SYSTEM for every cc65 program to be run by it. But as LOADER.SYSTEM is a ProDOS 8 seedling file using up only a single block in the ProDOS 8 file system this should be no issue.