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2022-08-26
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DISCAN
======
Written by: Joel Ellis Rea
DISCAN is short for DISC SCANner,
and it does just that. It will scan
through every block of all 35 tracks
on a disk, and report any errors. By
measuring the time it takes to get a
correct read, DISCAN also reports mar-
ginal sectors. As it runs, DISCAN
displays the current track and sector
numbers of the block it is currently
scanning. If it finds an error, it
tells the type of error, then checks
to see if the block is allocated.
A block is considered "allocated"
if it is marked as used in the Block
Availability Map, or BAM (located on
Track 18 Sector 0). The Disk Opera-
ting System within the 1541 Disk Drive
(or DOS for short) will not use an al-
located block for saving new files on.
Anyway, if the block is allocated,
that usually means that that block is
currently part of a file.
Therefore, DISCAN says "WARNING - THIS
BLOCK IS PART OF A FILE" and keeps go-
ing. READ ERROR #23 is a special case
(see below).
If the block is NOT allocated, DIS-
CAN will give you an oportunity to al-
locate it so that DOS won't use it.
READ ERROR #23's can usually be
corrected by simply writing the block
back out. This will usually fix the
block itself so that it can be used,
but the contents of the block are vir-
tually always corrupted. DISCAN will
give you the opportunity to "ATTEMPT
FIX?" if it finds a READ ERROR #23.
Don't attempt fix if the block is part
of a file unless you know what you are
doing!
Unlike CHECK DISK on the 1541 TEST/
DEMO disk, DISCAN will not write gar-
bage all over the disk. Therefore,
you can use DISCAN on a disk that has
files already on it. Also, DISCAN is
MUCH faster than CHECK DISK!
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