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was also harmless.
The pack routines have been completely rewritten in ARCs
numbered 2.00 or higher, and the checksum calculation is now
more rigorous and will detect errors that would have gone
un-noticed by ARCs 1.xx
ARC 2.10
In order to overcome the problem of XMODEM padding we
made ARC 2.10 check for and ignore any padding at the end of
an archive. Unfortunately, if the last entry in an archive
happens to be only one block long, then ARC 2.10 is stupid
enough to ignore it as XMODEM padding. This has been fixed
in ARC 2.20.
While we're on the subject of XMODEM padding...ARCs 1.xx
can not handle it properly. The archive will still extract
ok, but ARC may spit out some garbage at the end when it
encounters the XMODEM padding and thinks it is another
archive entry.
Bless us ARC user for we have sinned. It has been six
months since we first released ARC 1.00. For these and any
other bugs we have failed to mention, we are truly sorry.
ARC VERSION 2.20 PAGE - 8
INTRODUCTION
ARC was inspired by the program of the same name which
has been available to MS-DOS users for some time now. It
allows you to take several related files and back them up
into a single archive file. The archive can later be
extracted to obtain exact duplicates of the files that went
into it. Since ARC automatically compresses the files as
they are being archived, the resulting archive file takes up
less disk storage space than the original files. Savings
vary with the types of files being archived, and is
typically 50% for text and 10-30% for program files.
The primary advantage of ARC is as a backup tool. Word
processing files, assembler source code, database files,
graphics and music files compress rather well. In fact, due
to their heavy use of graphics pictures, even games can
squeeze down quit well in some instances. Normally a
1541/4040 diskette has a capacity of 664 blocks of storage.
Using ARC, you can increase this to the equivalent of about
800 blocks for program disks and 1300 or more for text and
graphics disks.
Another big advantage of ARC is the fact that several
files can be combined into a single archive. This makes it
significantly easier (and faster) to transfer information
from one computer to another via the telephone lines. If the
bulletin board you are calling uses ARC files, you will
spend less time reading directories and trying to figure out
which, and how many files you have to download to get the
program you want to work! Not only this, but after you
extract the archive, all of the files will have the correct
name and filetype so renaming files is not nessessary.
ARC is also a useful tool for general file maintenance.
Moving files between drives, renaming files, viewing
directories, and the editing of simple text files is rather
simple with ARC.
ARC VERSION 2.20 PAGE - 9
When Huffman coding is used to squeeze files, the
compression ratio (the original length divided by the
squeezed length) is about 1.11 for programs, 1.6 for text
files, and can be 2.0 or more for graphics screens. Huffman
coded files show up as 'squeezed' on an archive directory.
Run-length squeezing ('packing') is sometimes more
effective, particularly on graphical information.
Version 2.00 of ARC introduced three new data
compression methods. 'Squashing' combines run-length coding
with Huffman squeezing and is generally superior to Huffman
squeezing alone. 'Crunching' is by far the most interesting
method used to date in that ARC will choose crunching as the
most efficient compressor as often as 80 to 90% of the time.
Crunched files are stored using the Lempel-Zev-Welch
algorithm and can result in very impressive compression
ratios. A typically text file will compress to about 45% of
its original length, and word-pro or paperclip files will
crunch down to about one third of their original length.
Crunching combines run-length and Lempel-Zev-Welch
compression and is unique in that it can be performed
without requiring an initial pass through the data file. It