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ARC calculates a two byte checksum for each file in an
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archive. The checksum is calculated using the bytes of
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the original file before they are passed on to the
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compression routines. When you verify an archive, ARC
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actually de-compresses each archive entry and
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calculates a new checksum using the bytes passed to it
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by the decompressor. Both checksums should be the same.
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If they do not match, an error message is displayed
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indicating that the archive may have been corrupted due
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to a disk error or a transmission error during upload
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or downloading.
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Again the syntax is the same as that for ARC/X.
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By now you must have noticed that ARC/X ARC/P and
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ARC/V are all minor variations of the same thing.
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ARC VERSION 2.20 PAGE - 27
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4) MEMORY MAP
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$033c-$03ff - cassette buffer. used by ARC
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$0801-$0fff - not used
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$1000-$4fff - work space for ARC/C and ARC/X
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CRUNCH string table is stored here
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$5000-$7fff - workspace for ARC and MOVE
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commands only. All other commands
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leave this area alone.
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$8000-$8fff - used in 80 column version. (ROM)
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$9000-$97ff - not used
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$9800-$9fff - 80 column screen.
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Not used in 40 column ARC
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$a000-$bfff - program area. ARC, MOVE, and DIR
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$c000-$cfff - program area. Editor commands.
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(sys 12*4096 to enable ARC after
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a KILL)
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$d000-$ffff - work space for ARC/C and ARC/X
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You may notice that there is a rather significant
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jump in the amount of workspace that ARC needs to do
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its job from previous versions of ARC (28K to be
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exact). This is due to the CRUNCH routines, which are
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rather demanding in terms of memory. If you have a
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program in memory when archiving or dearchiving a file,
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then it will almost certainly be clobbered by ARC.
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When invoking the ARC command, BASICs pointers are
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not changed in any way by ARC. You may have to type NEW
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before running a program after using the ARC command.
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(otherwise you may get an ?out of memory error.)
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ARC VERSION 2.20 PAGE - 28
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THEORY OF OPERATION
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All methods of data compression take advantage of
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redundancy of one form or another. Run-length coding is one
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of the simplest, and often the most effective techniques.
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Graphics files often contain long sequences of the same
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byte. Zeros for blank space, or 255's for filled in space.
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Run-length coding recodes these long sequences as shorter
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control sequences. For example, a graphical image stored in
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RAM may look something like the following if viewed with
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the machine language monitor:
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.:2000 00 00 00 00 00 00 00 00
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.:2008 00 00 ff ff ff ff ff 00
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.:2010 00 00 00 00 00 00 00 00
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.:2018 a0 0b ff ff ff ff ff ff and so on....
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This could be stored on disk as the sequence:
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fe 00 0a fe ff 05 fe 00 09 a0 0b fe ff 06
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The first byte ($fe) is a control character. When the
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unsqueeze routine encounters a one of these it gets the next
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two characters and interprets them as a character
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identifier and a count. Thus the first 3 byte sequence is
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interpreted as 10 zeros, the next 3 byte sequence as 5 ff's
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and so on. When a character is not repeated, it is simply
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coded directly to the output file. (the $a0 at $2018 above)
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And so the above is squeezed from 32 bytes down to 14.
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Database programs often sacrifice disk space in order
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to gain speed. Relative files, for instance, store their
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data at the beginning of each record, and pad the record
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with zeros. Since every record is the same length, the DOS
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