arc-shrinkler

An Acorn Archimedes port of the Shrinkler decompressor in (26-bit) ARM assembly language by Kieran Connell.

Compilation

Use vasm with standard syntax.

There a few compilation options:

• _DEBUG=1 to enable additional run-time checks and throw OS_GenerateError SWI calls in the event of "assert" failures etc.
• _PARITY_MASK=0 is the "no parity context" variant that works better with byte-oriented data. Initial testing suggest this is also better for 32-bit word ARM code.
• _PARITY_MASK=1 is the original variant for Amiga 16-bit words.
• _ENDIAN_SWAP=0 assumes that all 32-bit long-word data has been converted to ARM little-endian byte order at compression time. This is faster but requires a custom version of the Shrinkler.exe tool, see Compression section for more info.
• _ENDIAN_SWAP=1 will swap the byte order of 32-bit long-words at run-time to match ARM little-endian format. This is slower but can be used with the original Shrinkler.exe tool for compatibility.
• _PARSE_HEADER=1 adds an additional function ShrinklerParseHeader to parse and decompress data with a Shrinkler header.

Usage

Call ShrinklerDecompress with the following registers:

R0 = compressed data address (source)
R1 = decompressed data address (dest)
R2 = callback function address (or 0 if not required)
R3 = callback argument
R9 = context (scratch memory) buffer (NUM_CONTEXTS * 4 = 6144 bytes)

Returns R0 = number of bytes written
No registers are preserved!

Alternatively call ShrinklerParseHeader with the same register arguments.

Compression

Either use the original compressor or my fork if you want to endian-swap the data at compression time for additional speed and smaller code.

Use Shrinkler raw data compression:

Shrinkler -p -d INPUT_FILE OUTPUT_FILE

With the following additional options:

-b => disable "parity context" for byte-oriented data (recommended).
-z => endian-swap 32-bit long-words in the output file (recommended).
-w => add the Shrinker header to the output file (optional).

Adjust compression options from -1 to -9 according to your patience and requirements. ;)

Future Work

This code is certainly not size optimal, so there are likely to be improvements to be had. (Although a lack of Cinter or AmigaKlang port to the Archimedes hampers size-coded demo production more accutely.)

It is possible that investigating a 4 byte offset parity mask might well perform better compression on old ARM code that has fixed 32-bit instruction size. This would be at the expense of doubling the context (scratch) memory required during decompression.

License

This code is released under the license terms of the original author Aske Simon Christensen. See LICENSE.txt for details.

Contact

Find me via the Bitshifters website.