Added information about commodore formats

Commodore/SDA.TXT

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+
+*** SDA (Self-Dissolving compressed Archive)
+*** Document revision: 1.4
+*** Last updated: March 11, 2004
+*** Compiler/Editor: Peter Schepers
+*** Contributors/sources: Chris Smeets (source code)
+
+  The name stands for "Self-Dissolving Archive", and thats exactly what  it
+does. There is a decompression engine at the beginning of the  file  called
+by a BASIC SYS command. It will decompress all the files contained  in  the
+archive to whatever device you specify, or disk you select.
+
+  I have  found  two  somewhat  different  SDA  files,  one  has  a  longer
+decompression header than the other. They  would  appear  to  be  different
+revisions of the decompression engine,  likely  version  1  and  version  2
+files, but they all seem to be self-extracting ARC files.  There  are  also
+different revisions of the version 2 header, where the newer ones allow for
+the decompression of more compressed ARC formats. The HEX dump below  is  a
+sample of the shorter version of SDA...
+
+      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F        ASCII
+      -----------------------------------------------   ----------------
+0000: 01 08 0D 08 0A 00 9E 28 32 30 36 33 29 00 00 00   ....<2E><><EFBFBD>(2063)<29><><EFBFBD>
+
+  which decodes to...
+
+10 SYS(2063)
+
+  The starting location of  the  first  file  in  an  SDA  archive  can  be
+calculated given the BASIC header from above. Here's the steps...
+
+  1. Get the line number of the BASIC SYS statement (here it's 10,  in  the
+     longer one its 13)
+
+  2. Subtract 6, and multiply by 254 (result 1016, or $03F8)
+
+  3. If the first number of the SYS call is a 7 (in our  case  it's  a  2),
+     then subtract 1 from the previous result. A 7 indicates  it's  a  C128
+     archive, a 2 means it's a C64 archive.
+
+  4. You now have the starting position into the SDA archive  to  find  the
+     first file.
+
+  From here on, the file has the same layout as an ARC. See the  ARC  topic
+for a better description.
+
+  There are some exceptions to the above, expecially with the C128 versions
+of files. If the first numeric value of the SYS call is a 7, we have a C128
+file. If this is the case, you can assume that the line number value is 15,
+rather than using the line number in the BASIC header. Some files I've seen
+don't have the proper value for the line number.
+
+  The easiest way the decompress these files is to  use  64COPY.  There  is
+also C code available on the High Voltage CD #2 to allow you to  decompress
+these files on a PC.
+
+  SDA files can also be decompressed by running it on either a real C64  or
+an emulator window, and let the file undo itself to a disk image. ARC files
+are decompressable using the C64 program called ARC 2.50.
+