Ahoy 1986 November/Ahoy_Magazine_86-11_1986_Double_L.d64/new recovery.txt

Text File | 1986-01-01 | 5.8 KB | 211 lines

DISK DIRECTORIES II:
RECOVERING FROM PASS "N:NAME"
by Phyrne Bacon

DISK'EDITOR and DIRECTORY'PROBE can
be used together to recover files
lost by accidentally typing a short
new; that is:

 pass "n:name"  // without any id

Don't save anything on the disk with
the missing files until the error has
been corrected! If you haven't got a
backup disk, now is the time to make
one. Use a whole disk copier such as
four minute backup. Put the original
disk in a safe place, and work on the
backup. We will call the backup "the
damaged disk".

In Disk Directories I: Unscratching
Files, we explained about disk
directory files, directory entries,
program block chains, unscratching
files, and correcting the BAM.

I. Recovering from pass"n:name"

Now suppose that you used
pass"n:name" (without any id) by
accident on both your disk and its
backup. (It has happened to me!) If
you used pass"n:name,id", your files
cannot be recovered.

The sectors in the directory are
chained like those in a program. Look
at track 18, sector 4. If it has any
entries (even scratched ones), the
first two bytes of block 18-1 should
be 18 and 4 (12 and 04 in hex). If
18-4 has directory entries, and the
first two bytes of 18-1 are not 18
and 4, use DISK'EDITOR to change
these two bytes. Remember to save
18-1 after you make each set of
changes.

If you changed the first two bytes of
18-1 to 18 and 4, use pass"v" to
correct the BAM, and you will have
recovered all the files on the disk
except those whose entries were in
18-1.

Sector 18-1 has zeros everywhere
except the second byte (which is
255), and reconstructing the first
eight entries may be extremely
difficult.

II. Restoring file entries.

Begin by running DIRECTORY PROBE with
the damaged disk, and ask for the
fragments and answer no to "complete
block chains?". Unless you have saved
something on the disk since the
pass"n:name", your missing files
should all be listed in the list of
fragments at the end of the printout
along with parts of previously
scratched programs.

You have to find out which ones (if
any) are the missing files.

Take a new disk, and format it by
typing pass"n:new disk,zq". It should
have a different id from the damaged
disk.

Then save the one-line program:
10 print"hi
nine times on the new disk. Use the
filenames "a1" through "a9".

Copy block 18-1 of the new disk onto
block 18-1 of the damaged disk by
using DISK'EDITOR load and save. If
there are no entries in block 18-4
change the first two bytes of 18-1 of
the damaged disk to 0 and 255 (00 ff
in hex).

Now we need to choose the most likely
fragments and to change the
track/sector of each directory entry
in block 18-1 on the damaged disk to
the track/sector of one of the chosen
chain fragments. Label the chosen
fragments as a1-a8 on the printout.

Use DISK'EDITOR to help choose the
fragments. A first block of a listed
COMAL program will be easy to read in
the right column. A COMAL 2.0 program
will begin with the hex bytes: ff ff
02 00 00, but remember that the first
two bytes of the block are not part
of the program; so that it is the
third and fourth bytes which are ff
ff in hex. Almost all BASIC programs
begin with the hex bytes: 01 08. A
COMAL 0.14 program may be
recognizable if its name appears in a
remark near the beginning. The line
number # of the first line of a COMAL
0.14 program appears in the first and
second columns of the second row in
high-byte/low-byte form (# = hi*256 +
lo). The third column contains the
link number: where the next line
starts. If the fourth column of the
second row contains 00 in hex, the
first line is a remark.

Say the fragment begins on 17-0
(track 17 sector 0). Change the
fourth byte of the directory entry to
17 and the fifth to 0. To enter 17,
move the cursor to the correct
position, press n, enter 17, and
press return. Usually the first files
saved begin in tracks near the
directory track, that is, near track
18. If you wonder if a fragment is a
file of a certain type, compare its
first block or so to a known file of
that type. Guess a file type for each
file and enter the guess on the disk
and the printout.

DIRECTORY PROBE printed the chain
length for the fragment (the number
of blocks in the file), say 8. Change
the byte in the seventh column of the
last row of the directory entry to 8
(or whatever).

Give each of the files a temporary
name on the disk and on the printout,
and use a file copying program to
copy the eight files from the damaged
disk to the new disk. Check the
files. You may need to change your
guess as to the file type. Incomplete
COMAL 0.14 files usually crash COMAL.
Be sure this hasn't happened before
testing another COMAL 0.14 fragment
file.

If there were more than eight block
chain fragments, repeat the process
for the next eight fragments if you
haven't found your missing files.

When you are finished, use pass"v" on
the damaged disk to update the BAM,
and use cat to check the disk
directory. Your files may need to be
moved to the same disk.

If you have trouble making this
method work, you may want to practice
by making a copy of disk, say LAKE,
similar to the damaged disk, use
pass"n:sea" on LAKE, and then work on
SEA where the correct answers are on
LAKE.

III. Excessive noise from disk drive.

Frequently disk drives are out of
alignment and the disk drives have
trouble reading and writing near the
outer tracks: 1 and 35. If you asked
for fragments in DIRECTORY'PROBE, and
the the disk drive makes a great deal
of noise when the screen says
"reading track 1", stop the program,
and, in the procedure frag, change
the limits on both of the lines "for
g#:=1 to 35 do" to "for g#:=2 to 35
do". Repeat this process if
necessary.

With luck, all of the disk block
chain fragments will still appear on
the printout, and you will be able to
recover all your files.

IV. More information.

An excellent source of further
information is:

INSIDE COMMODORE DOS by R. Immers and
G. Neufield ($19.95, Datamost)