FC8 is designed to be as fast as possible to decompress on "legacy" hardware, while still maintaining a decent compression ratio. Generic C code for compression and decompression is provided, as well as an optimized 68K decompressor for the 68020 or later CPUs. The main loop of the 68K decompressor is exactly 256 bytes, so it fits entirely within the instruction cache of the 68020/030. Decompression speed on a 68030 is about 25% as fast as an optimized memcpy of uncompressed data.
The algorithm is based on the classic LZ77 compression scheme, with a sliding history window and duplicated data replaced by (distance,length) markers pointing to previous instances of the same data. No extra RAM is required during decompression, aside from the input and output buffers. The match-finding code and length lookup table were borrowed from liblzg by Marcus Geelnard.
The compressed data is a series of tokens in this format:
- _LIT = 00aaaaaa = next aaaaaa+1 bytes are literals
- BR0 = 01baaaaa = backref to offset aaaaa, length b+3
- EOF = 01x00000 = end of file
- BR1 = 10bbbaaa'aaaaaaaa = backref to offset aaa'aaaaaaaa, length bbb+3
- BR2 = 11bbbbba'aaaaaaaa'aaaaaaaa = backref to offset a'aaaaaaaa'aaaaaaaa, length lookup_table[bbbbb]
The length lookup table enables encoding of backrefs up to 256 bytes in length using only 5 bits, though some longer lengths can't be encoded directly. These are encoded as two successive backrefs, each with a smaller length.
An optional block compression format (-b option in the compressor) can compresses the input as multiple independent blocks of a fixed size. This is useful for on-the-fly decompression where only a portion of the original data is desired.