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100 lines
6.4 KiB
Markdown
100 lines
6.4 KiB
Markdown
# Multipurpose Chunked File
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## Goals
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- meant for the Commander X16 system, depends on working MACPTR kernal call for fast loading.
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- single file, possibly megabytes in size, that contains various kinds of data to be loaded or streamed into different locations.
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- simple file format with few special cases.
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- no random access, only sequential reading/streaming.
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- simple user code that doesn't have to bother with the I/O at all.
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- a few chunk types that can be handled automatically to load data into system ram, banked ram, or video ram.
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- custom chunk types for flexibility or extensibility.
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Theoretical optimal chunk size is 512 bytes but actual size may be different. (In practice there seems to be no significant speed impact)
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MCF files are meant to be be created using a tool on PC, and only being read on the X16.
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A Python tool is provided to create a demo MCF file.
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A proof of concept Prog8 library module and example program is provided to consume that demo MCF file on the X16.
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## File Format
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This is the MCF file format:
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| Chunk | size (bytes) |
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|------------|--------------|
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| LoadList | 256 |
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| Data | 1 - 65535 |
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| Data | 1 - 65535 |
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| ... | |
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| LoadList | 256 |
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| Data | 1 - 65535 |
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| Data | 1 - 65535 |
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| ... | |
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and so on.
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There is no limit to the number of chunks and the size of the file, as long as it fits on the disk.
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### LoadList chunk
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This chunk is a list of what kinds of chunks occur in the file after it.
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It starts with a small identification header:
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| data | meaning |
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|---------|---------------------|
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| 2 bytes | 'LL' (76, 76) |
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| byte | version (1 for now) |
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| byte | reserved (0) |
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Then a sequence of 1 or more chunk specs (6 bytes each), as long as it still fits in 256 bytes:
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| data | meaning |
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|----------------------|--------------------------------------------|
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| byte | chunk type |
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| word (little-endian) | chunk size |
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| byte | bank number (used for some chunk types) |
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| word (little-endian) | memory address (used for some chunk types) |
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Total 6 bytes per occurrence. Any remaining unused bytes in the 256 bytes LoadList chunk are to be padded with byte 255.
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If there are more chunks in the file than fit in a single loadlist, we simply add another loadlist block and continue there.
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(The file only ends once an End Of File chunk type is encountered in a loadlist.)
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### Chunk types
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| chunk type | meaning |
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|------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| 0 - 239 | custom chunk types. See below. |
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| 240 - 248 | reserved for future system chunk types. |
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| 249 | dummy chunk: read a chunk of the specified number of bytes but don't do anything with it. Useful to realign the file I/O on disk block size. |
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| 250 | bonk ram load: use banknumber + address to set the Cartridge RAM ('bonk' RAM) bank and load address and loads the chunk there, then continue streaming. Note this is slower than other types of ram. Rquires 1 page of user chosen buffer area. |
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| 251 | system ram load: use banknumber + address to set the RAM bank and load address and loads the chunk there, then continue streaming. |
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| 252 | video ram load: use banknumber + address to set the Vera VRAM bank (hi byte) and load address (mid+lo byte) and loads the chunk into video ram there, then continue streaming. |
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| 253 | pause streaming. Returns from stream routine with pause status: Carry=clear. And reg.r0=size. until perhaps the program calls the stream routine again to resume. |
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| 254 | end of file. Closes the file and stops streaming: returns from stream routine with exit status: Carry=set. |
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| 255 | ignore this byte. Used to pad out the loadlist chunk to 256 bytes. |
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### Custom chunk types (0 - 239)
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When such a custom chunk type is encountered, a user routine is called to get the load address for the chunk data,
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then the chunk is loaded into that buffer, and finally a user routine is called to process the chunk data.
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The size can be zero, so that the chunk type acts like a simple notification flag for the main program to do something.
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The first routine has the following signature:
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**get_buffer()**:
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Arguments: reg.A = chunk type, reg.XY = chunksize.
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Returns: Carry flag=success (set = fail, clear = ok), ram bank in reg.A, memory address in reg.XY.
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The second routine has the following signature:
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**process_chunk()**:
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Arguments: none (save them from the get_buffer call if needed).
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Returns: Carry flag=success (set = fail, clear = ok).
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These routines are provided to the streaming routine as callback addresses (ram bank number + address to call).
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If any of these routines returns Carry set (error status) the streaming routine halts, otherwise it keeps on going.
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The streaming continues until a End of File chunk type is encountered in the loadlist.
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