LZSCompress

CzarDragon
Once upon a time, I decided to put Final Fantasy 7 into
my CD-ROM. There wasn't too much of interest, but the DAT
files in the FIELD directory caught my eye. They were the
text to the game, but it was scrambled in areas. After
months of staring at the screen I was able to figure it
out.

More recently, I have been looking at various SNES ROMs
which use compression. Each of these were compressed in a
similar manner as Final Fantasy 7 had been. As such, I
decided to write a document explaining it. For more
information enter "LZW compression" into a search engine.

In this compression, an 8-bit value is used to determine
the nature of the following groups of data. A value of 1
indicated an uncompressed value. This means one byte is
read from the input file and then written to the output
file.

A value of 0 indicated a compressed value. This means
two bytes are read, from which a pointer to previous data
can be generated. The pointer contains the length of the
data to be retrieved as well as the offset for it to be
retrieved from. Since pointers aren't used for 1-2 byte
values, a value of 3 is added to the length.

Here are some of the pointer formats I have seen:
(On is the Offset's nth bit, Ln in the Length's nth bit)

Dragon Ball Z - (One of the RPGs):
O3 O2 O1 O0 L3 L2 L1 L0 | OB OA O9 O8 O7 O6 O5 O4

Final Fantasy 7:
O7 O6 O5 O4 O3 O2 O1 O0 | OB OA O9 O8 L3 L2 L1 L0

Ogre Battle:
L4 L3 L2 L1 L0 OA O9 O8 | O7 O6 O5 O4 O3 O2 O1 O0

Record of Lodoss Wars:
O7 O6 O5 O4 O3 O2 O1 O0 | O8 L6 L5 L4 L3 L2 L1 L0

If you think you know of a ROM that uses this
compression, please e-mail me the title of the ROM.
For quicker results, include the URL to a web site
I can obtain the ROM from.

CzarDragon@Yahoo.com


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