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81 lines
3.3 KiB
Plaintext
81 lines
3.3 KiB
Plaintext
LOADER.SYSTEM - an Apple][ ProDOS 8 loader for cc65 programs (Oliver Schmidt)
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=============================================================================
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Background
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----------
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Apple][ ProDOS 8 system programs (filetype SYS) are always loaded into memory
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starting at location $2000. This poses the problem of how to make efficient
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use of the memory in the range $0800-$2000. The usual approach of relocation
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has two downsides:
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- Relocating e.g. 30 kB from $2000-$9800 to $0800-$8000 takes a considerable
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amount of time.
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- Really large programs just don't fit into memory when loaded starting at
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location $2000.
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The relocation can be eliminated by loading the major part(s) of the program
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from disk right to the final location by a rather small system program.
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LOADER.SYSTEM is such a small program. In fact it's so small that it fits into
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a single block in the ProDOS 8 file system making it a so-called seedling file,
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which are loaded really fast. LOADER.SYSTEM can load cc65 programs into memory
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anywhere in the range $0800-$BB00 (44,75 kB).
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Usage
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-----
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Link the cc65 program to the start address $0803 (or any other address) and
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store it as binary program (filetype BIN). This is in fact no different from
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a binary program to be run by BASIC.SYSTEM's BRUN command in the usual way.
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If however the cc65 program isn't run by BASIC.SYSTEM but is rather run by
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LOADER.SYSTEM then it behaves like a system program which means:
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- It uses memory up to the ProDOS 8 system global page located at $BF00.
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- It supports the ProDOS 8 startup file mechanism (mapped to argv[1]).
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- It quits to the ProDOS 8 dispatcher.
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Obviously LOADER.SYSTEM has to be told which cc65 program to run. Unfortunately
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the ProDOS 8 dispatcher has no notion of system program parameters so the usual
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approach would have been to make LOADER.SYSTEM bring up yet another menu to
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select the cc65 program to run.
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But to allow to select the cc65 program directly from the ProDOS 8 dispatcher
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anyway LOADER.SYSTEM detects the path to the cc65 program from its own path by
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just removing the '.SYSTEM' from its name. So if you want to run the cc65
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program MYPROGRAM you'll need a copy of LOADER.SYSTEM in the same directory
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being renamed to MYPROGRAM.SYSTEM.
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This means you will end up with a copy of LOADER.SYSTEM for every cc65 program
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to be run by it. But as LOADER.SYSTEM is a ProDOS 8 seedling file using up only
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a single block in the ProDOS 8 file system this should be no issue.
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Build
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-----
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In case you want to build 'loader.system' from the source code yourself you can
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do so using the following commands:
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ca65 loader.s
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ld65 -C loader.cfg -o loader.system loader.o
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Installation
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------------
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The file 'loader.system' as generated by the cc65 linker with the command above
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does NOT include the 4-byte address/length header that is generated for Apple][
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programs by default. This is because ProDOS 8 system programs are always loaded
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into memory starting at location $2000.
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The recommended way to transfer 'loader.system' from your native file system to
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a ProDOS 8 file system disk image is to use AppleCommander which is available at
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http://applecommander.sourceforge.net/
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If you want to put the file 'loader.system' onto a disk image 'mydisk.dsk' as
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system program MYPROGRAM.SYSTEM you can do so using the following command:
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java -jar ac.jar -p mydisk.dsk MYPROGRAM.SYSTEM sys < loader.system
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