*** SDA (Self-Dissolving compressed Archive) *** Document revision: 1.4 *** Last updated: March 11, 2004 *** 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 ....???(2063)??? 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.