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GET previous track,sector and change
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link to 0,255
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PRINT "archive has been repaired."
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ENDIF
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ENDIF
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DONE fixing archive.
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ARC VERSION 2.20 PAGE - 48
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ARC TRICKS AND TIPS.
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1) Manipulating Archives.
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If you were to use a disk doctor to look at an archive,
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you would notice that every file in the archive begins at
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the beginning of a CBM disk sector and ends at the end of a
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sector. This fact can be used to advantage, and will be in
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future ARCs, in a number of ways.
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First of all, it makes it possible to break an archive
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up into a number of smaller archives simply by manipulating
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the directory and a few track,sector links. Since it is not
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necessary to move any data, this should be a relatively fast
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proceedure. You could split an archive in two with a disk
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doctor as follows:
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1) create a dummy file on the same disk as the archive.
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2) scratch the dummy file.
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3) locate the archive and follow the sector links until
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you see the filename of the archive entry which is
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to be the first filename in the second archive.
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4) jot down the track and sector you are looking at.
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5) backup to the previous sector and change the link
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to 0,255
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6) get back to the directory and locate the dummy
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file from (1)
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7) change the filetype from (6) to program (130)
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8) change the track,sector to the ones you jotted
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down in (4)
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You now have two separate archives. The block count for
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these files will not be correct in the directory, and should
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be fixed, but no errors should arise if they are not.
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A similar proceedure could be used to delete an archive
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entry. Simply manipulate the sector links to point around
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the entry to be deleted and then validate the disk.
|
You could also re-arrange the order of the archive
|
entries in an archive, append one archive to another,
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transfer an archive entry to another archive and so on and
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so forth... All of this could be done without having to move
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a single byte of data!
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ARC VERSION 2.20 PAGE - 49
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2) Random Access Archives.
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An archive could be made random access in each of two
|
ways.
|
The first method would be to follow the track,sector
|
links and read the number of CBM disk blocks in each archive
|
entry. This information could then be used to calculate
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where each archive entry begins.
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A second method would be to convert the archive to a
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relative file. Simply create the archive by normal
|
proceedures, and then use ARC to archive the archive you
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just created. Now load and run your favorite disk sector
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editor and view the first sector of the archive of the
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archive. Change the file's type to "R" for RELative in the
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archive header. (You will see the filetype as a "P" two
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bytes before the filename) Now all you have to do is
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unarchive it and it will become a relative file with record
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length 254 and can be accessed randomly.
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But why would you want a random access archive
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anyway???
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We plan to write a simple un-squash routine that can be
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accessed as a subroutine by a BASIC or Machine Language
|
programmer. Unfortuneately the CRUNCH routines require a
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great deal of workspace (about 28K) and would not be of much
|
use to most applications. However, a small routine could
|
reside inside about 2K bytes which would un-pack, un-
|
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