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LC_LZ2 Compression Format.md

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LC_LZ2 Format

This format is identical to LZ1, except the repeat command's address is big endian.

The compressed stream consists of chunks, each starting with a 1 byte header.

bits
76543210
CCCLLLLL

CCC:   Command bits  
LLLLL: Length

The header byte $FF marks the end of the data stream.

List of commands

  • %000 - "Direct Copy" - followed by (L+1) bytes of data
  • %001 - "Byte Fill" - followed by 1 byte to be repeated (L+1) times
  • %010 - "Word Fill" - Followed by two bytes. Output first byte, then second, then first, then second, etc. until (L+1) bytes has been outputted
  • %011 - "Increasing Fill" - Followed by one byte to be repeated (L+1) times, but the byte is increased by 1 after each write
  • %100 - "Repeat" - Followed by two bytes (big endian) containing address (in the output buffer) to copy (L+1) bytes from
  • %101 - Unused
  • %110 - Unused
  • %111 - "Long length" - This command has got a two-byte header:
111CCCLL LLLLLLLL
CCC:        Real command
LLLLLLLLLL: Length

Normally you have 5 bits for the length, so the maximum value you can represent is 31 (which outputs 32 bytes). With the long length, you get 5 more bits for the length, so the maximum value you can represent becomes 1023, outputting 1024 bytes at a time.