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Introduction
CP/M Disk File Structure
Install Disk Doctor
Invoking Disk Doctor
Recovering Disks and Files, Wards A,B, and C
Un-erasing Files, Wards D and E
Common Disk Problems and Recovery Procedures
:Introduction
Disk Doctor is comprehensive diskette recovery package
comprised of an installation program (INSTALL.COM) and five
recovery procedures (DOCTOR.COM), refered to as "Wards,"
each capable of performing a specific recovery task. Three
general kinds of facilities are provided. Ward A can help
restore the usefulness of disks with malfunctions in areas
not assigned to files. Wards B and C can aid in recovering
files affected by disk malfunctions. Wards D and E can aid
in recovering files accidentally erased. As an added
benefit, Ward A can also be used to certify new disks and
eliminate any bad sectors before they can cause problems.
▶04◀
TWO IMPORTANT WARNINGS
If you know or suspect that you have both unwanted
erasures and malfunctions on the same disk, you must deal
with all erasures first. This is necessary because those
procedures used to recover from disk malfunctions may
overwrite erased files, making their recovery impossible.
Before attempting to use any of DISK DOCTOR's
facilities, you must be certain that it is properly
installed for each disk format on which you intend to use
it. DISK DOCTOR comes pre-installed for the type of media
on which it was supplied; but if you wish to use it on other
media or densities, a careful reading of Chapter 3 is
recommended.
: CP/M FILE STRUCTURE
This system is hierarchical in structure. Its levels
are, in order of decreasing size: the disk itself, the
file, the extent, the track, the group, and the sector. It
is one of the most convenient and desirable features of CP/M
that all ordinary data manipulations may be performed in
terms of disks and files alone. However, to better
understand how DISK DOCTOR's Wards perform their tasks and,
in particular to make good use of Ward B, you will need a
good grasp of all levels of the CP/M file structure. In
this chapter we will describe each level of the hierarchy
starting with the smallest sized structure and working up.
The smallest unit (above the byte--or character) in the
hierarchy is the sector. A sector is the minimum unit of
disk data accessible by CP/M. CP/M defines a sector as 128
bytes (characters). It expects these sectors to be
organized into tracks. The track is merely a means of
organizing and locating the individual sectors on a disk.
The unit in which CP/M allocates sectors to files is
the group. A group is composed of a fixed number of
sectors, either 8 or 16 depending on the implementation of
CP/M on your system. Space for the disk directory is also
allocated in groups.
An extent is, in turn, made up of a variable number of
groups with the maximum number of groups per extent being 8
or 16 depending on your CP/M implementation. (Most CP/M
systems have a maximum of 128 sectors per extent: either 16
groups of 8 sectors, or 8 groups of 16 sectors. The
Intertec Superbrain, however, has extents with a maximum of
16 groups of 16 sectors or 256 sectors per extent.) A CP/M
file is composed of one or more extents; therefore,
assigning a variable number of groups to an extent
facilitates the efficient, dynamic allocation of disk
storage to files.
A disk may contain from zero to many files. Each disk
is accessed by the system through its assigned drive name.
A disk retains its hierarchical structure with the exception
of the drive name when it is removed from the drive. Thus
it makes sense to talk about a disk containing a file, even
if the disk is not in a drive (i.e., is not assigned a drive
name).
▶04◀
FORMAT OF A DIRECTORY ENTRY
Thus far, we have described the structure of CP/M disk
files themselves. However, for each extent of each file,
there exists, also on disk, a 32-byte field called the
directory entry. All directory entries together make up the
disk directory which usually resides within the first one or
two groups on the disk. A directory entry fulfills two
important functions in the CP/M disk file system: first, it
provides a means of managing the storage allocated to files
on the disk; and second, it provides the means of locating
those same files.
▶04◀
Knowledge of the format of a directory entry is
potentially useful, since Ward B (in conjunction with CP/M's
DUMP or DDT facilities) can be used to examine directory
entries (see p. 16), and since sophisticated use of Ward B
requires their examination. The diagram appearing below
represents the format of the 32-byte field of a directory
entry.
0 16 31
ø ø ø
øXNøNNøNNøNNøNTøTTøEXøXSøGGøGGøGGøGGøGGøGGøGGøGGø
N -- byte in filename field
T -- byte in file type field
E -- extent number
S -- extent size in sectors
G -- byte in group allocation map field
X -- byte of no interest to us
▶04◀
The group allocation map field contains a list of the
logical group numbers in the order the groups were assigned
to the file. Three formats in use (under different CP/M
implementations) for this 16-byte field are represented in
the three diagrams appearing below (the 'g's stand for group
numbers and the '0's stand for zero bytes --recall that 8080
word format is low order byte, high order byte).
16 31
ø ø
øGGøGGøGGøGGø00ø00ø00ø00ø
8 groups/extent, group number stored per byte
16 31
ø ø
øG0øG0øG0øG0øG0øG0øG0øG0ø
8 groups/extent, group number stored per word
16 31
ø ø
øGGøGGøGGøGGøGGøGGøGGøGGø
16 groups/extent, group number stored per byte
: HOW TO INSTALL DISK DOCTOR
DISK DOCTOR is composed of two files: DOCTOR.COM and
INSTALL.COM. The first few sectors of DOCTOR.COM contain
information about the disk environment in which it expects
to operate. It is this information that INSTALL.COM alters
to tailor DOCTOR.COM to a particular new disk format. To do
this INSTALL.COM requires: that you insert a disk
containing DOCTOR.COM in drive A and a blank formatted disk
in a drive which you may specify when prompted (CR defaults
to B), and that you supply it with the proper values for
three parameters when you are prompted to do so. These
parameters are:
1.) the lowest physical sector number on a track (either 0
or 1, most drives start numbering at 1);
2.) the number of sectors on a track (in CP/M version 2.0
and later STAT <drive name>:DSK: provides the number
of sectors on a track);
3.) the number of tracks on a disk.
: Invoking Disk Doctor
DISK DOCTOR exists as an executable command file
(DOCTOR.COM) on disk. It does not require any command line
arguments when invoked. It currently does require a dual
disk drive system to perform its functions. There should be
a disk containing DOCTOR.COM in drive A before invoking DISK
DOCTOR. Once DISK DOCTOR is loaded into memory and the
first question answered, that disk can be removed, as the
program resides in memory until exited.
First DISK DOCTOR asks you to select one of three disk
operation display options. To enable this display is useful
only if your console can support a carriage return without a
line feed. When the program is performing disk I/O
operations, it will display the track, group, and sector of
each operation by rewriting the same line with updates.
When bad data is encountered a carriage return and a line
feed are executed--leaving the guilty track, group, and
sector displayed above the current line. Choosing the first
option, "S", will result in a full display of all disk I/O
operations by sectors.
: Recovering Disks and Files: Wards A, B, and C
The basic principle underlying Wards A, B, and C is
that most disk malfunctions are simply the result of a
single sector failing to be read or written properly. Such
a malfunction will have different effects and will require
different remedies depending on whether the bad sector is
within a group assigned to the directory or to a file--or
within a group not presently assigned.
▶04◀
BAD SECTORS IN GROUPS ASSIGNED TO THE DIRECTORY
If there is a single bad sector in the directory of a
disk, CP/M will not read the directory past that sector.
Hence, although some files may still be read, none may be
written. In most cases, the bad sector will have nothing to
do with the file or files of interest to the user. To
placate this protective feature of CP/M, Ward C will copy an
entire disk, replacing all data in any damaged sectors in
the directory with the disk formatting character, 0E5h; thus
allowing the rest of the directory to be accessed through
the usual system commands. If the bad sector does affect
the directory entry of a file of interest, then Ward B
provides a means of working around the system to extract
data from the directory and the file.
▶04◀
BAD SECTORS IN GROUPS ASSIGNED TO FILES
If a file contains a single bad sector, CP/M will not
read the file beyond that sector and the entire file is
rendered useless. While a sector of bad data in an object
file might mean that the file is useless, a single bad
sector in a source or text file would be only a minor
annoyance if the rest of the file could be recovered. Ward
C will, as it copies, also replace all data in any bad
sectors that it finds in files with ASCII spaces. This
allows the user to edit the file and easily repair it. It
might seem that a file could be recovered down to a single
bad byte, but this is not possible because many disk control
systems buffer their read operations (read data in blocks)
and most errors cause a loss of sychronization.
▶04◀
BAD SECTORS IN UNASSIGNED GROUPS
If a bad sector appears in an unassigned group, CP/M
will not make use of any of the good sectors above it, and
will refuse to close any file that attempts to write to the
bad sector. Since each disk storage operation will attempt
to use the lowest available group, the disk will be as
useless as if it were full. Ward A circumvents this CP/M
protective feature by means of assigning all groups
containing bad sectors to a special file intended never to
be used again.
▶04◀
WARD A
This Ward is concerned with restoring the usefulness of
a disk rather than recovering data from it. Like the other
DISK DOCTOR programs, it uses the most primitive BIOS hooks
to bypass the automatic CP/M features. Ward A loads the
directory from the disk containing the bad sector(s) and
searches for all unassigned groups. This prevents damage to
the intact files on the disk. Each unassigned group is
written and read to validate its integrity. If the program
locates a bad sector, it will assign the entire group
containing it to a special file called MORGUE; thus
effectively eliminating the bad group from the disk.
Obviously, you should never use the filename MORGUE in any
of your own programs. Also, unless you want the same bad
groups to return to haunt you, you must NEVER ERASE MORGUE
FILES. (Under CP/M Version 2.0 and later, MORGUE files have
the R/O attribute set to prevent such erasures.)
The file type (extension) of a MORGUE file also serves
a purpose. The first time Ward A is run on a disk; if it
finds bad sectors, it will open a file: MORGUE.NO1. Each
time it is rerun on the same disk; if it finds more bad
sectors, it will generate another consecutively numbered
MORGUE file (e.g., MORGUE.NO2). Thus it is possible to see
at a glance if a disk has been prone to trouble, and avoid
selecting it for critical work. Sometimes it is necessary
to run Ward A on a disk several times in order to cause all
faulty sectors to fail.
▶04◀
The three "do's and don'ts" listed below are essential
to the effective use of Ward A.
1.) It cannot be overemphasized that you must never erase
any MORGUE files. Erasing a MORGUE simply undoes
everything Ward A has done.
2.) If there are erased files on a disk which you wish to
recover by using Ward D, you must do so before
executing Ward A on the disk. Once it is executed, no
erased file can be recovered as all of its groups will
be overwritten.
3.) Before running Ward A on a disk, all temporary or
aborted files (such as those with the extension .$$$)
on the disk should be erased. Some text editors may
close such files when bad sectors are encountered, and
then re-open them later. This will cause Ward A to
miss the troublemaker, since the group containing it
appears to be assigned to a file.
▶04◀
If you select Ward A from DISK DOCTOR's menu, you will
first be prompted to place your bad disk in the "patient"
drive you previously designated. You will then be given one
last chance to decide whether you actually want to run Ward
A or not. If not, the program returns you to "Admitting."
▶04◀
WARD B
This Ward allows you to pull arbitrarily specified
groups or sectors of data off of one disk (patient) and
collect them in a specified file on another disk (recovery).
You can then edit this file, discarding the data that is of
no value to you. This is a powerful, though rather
primitive capability. To get the maximum benefit from it
requires an informed and thoughtful approach. You should
note that, through this Ward, the directory is as accessible
as the files. If you are searching for the groups assigned
to a particular file; examining the directory, even if it is
damaged, should provide valuable information.
If you select Ward B from DISK DOCTOR's menu, you will
first be prompted to place the bad disk in the previously
specified "patient" drive and a good disk in the
corresponding "recovery" drive. You will then be prompted
to specify the collection file on your recovery disk. After
you have typed your collection filename and hit return, the
program will ask you whether you wish to recover data by
group or by sector. Then according to your choice, data is
collected into a buffer by specifying either hex group
numbers or hex track and sector numbers when prompted. A
bad disk is not required to have a good directory as it is
never accessed unless specified for collection. In either
collection mode, a carriage return entry will cause the next
logical group or track-sector combination to be collected.
When all desired data has been collected, the file can be
closed by typing a period and carriage return (same as DDT
Substitute). If the buffer becomes full, a message will be
displayed, and the file will be closed automatically. In
the case of very long files, several recovery files may be
specified, and then concatenated using PIP.
Files recovered using Ward B are just like any other
files under CP/M. Files containing text may be viewed using
the TYPE transient. With enough patience, text files with
directory entries damaged beyond recovery can be gleaned
from a disk. Files containing object code may be examined
using DDT dump or DUMP transients. File directories may
also be examined by these means. Much information that is
not otherwise visible can be accessed through this program.
A good example of this is the use of Ward B to determine
whether or not DISK DOCTOR itself has been properly
installed.
▶04◀
WARD C
This Ward is your main resource for dealing with bad
sectors in files and directories. It can best be thought of
as a very tolerant disk copying routine. Ward C copies each
good sector, in sequence, just as it finds it; but each bad
sector is filled with characters which function as blanks.
In the case of directories, the blank disk formatting
character, 0E5h is used. Bad sectors in files are filled
with ASCII spaces. This will solve most problems involving
accessing files. The only access problem it cannot solve is
that of an important file with a damaged directory entry.
Files with directory entries unaffected by bad sectors can
be accessed and manipulated in all the ways CP/M usually
allows. They can thus be examined and any damaged sectors
repaired to the extent possible.
One disadvantage of Ward C is that it is slow. To copy
a disk using this program can take up to an hour depending
on the drive and the number of bad sectors encountered. It
will more commonly take about ten minutes. This routine's
slowness results from its circumvention of the usual CP/M
file handling utilities.
: Un-erasing Files: Wards D and E
Files may be erased in error either directly by user
command (ERA) or indirectly as a result of a program's file
handling. Wards D and E will allow the user to recover
completely from both kinds of errors provided that the
groups originally assigned to the erased files have not been
reallocated to other files. If some groups have been
reallocated, it may still be possible to salvage the
remaining groups in the file through skillful use of Ward B.
If no disk write operations have been performed since the
erasure, then the erased file will always be recoverable.
Wards D and E rely on the fact that CP/M, in its file
management, does the minimum amount of processing necessary.
Hence, CP/M's erasing process involves a few flag-setting
operations rather than any overwriting of a file's assigned
groups or directory entries. This allows files with groups
not yet reallocated (and thus overwritten) to be fully
recovered by re-setting those flags to their original
values.
If you know the exact name of the erased file, you can
go directly to Ward D to recover it. If you do not know the
name, Ward E provides a directory of all the recoverable
erased files (as well as all the active files), by extents,
on a disk.
▶04◀
WARD D
This Ward performs its functions by reading the
directory of a disk and then rewriting it, with minor
changes, back to the same disk. The program operates in two
modes: as an erased file recovery routine and as a fast,
but dangerous directory repair routine.
The primary use of Ward D is to recover erased files.
After you enter the program, you will be asked to place the
disk with the erased file in the previously designated
patient drive, give the unambigous filename of the erased
file, and hit return. The program will then read the
directory of the disk and search for the specified file. It
will indicate if it has found the file and, if so, will then
proceed to check whether any active file has cannibalized
the groups of the desired file. If any of the groups appear
in the directory entry of an active file, the program will
inform you that the file is not recoverable. If not it will
un-erase the file. If only a few groups have been lost to
active files, then it may be worthwhile to use Ward B to
recover the groups remaining. (NOTE: If many disk
operations have been carried out since the erasure, there is
a chance that groups have been used by a file which has
itself been subsequently erased. If so, Ward D may not warn
you that the reactivated file is essentially useless.)
▶04◀
Ward D can recover multiple extent files, but only if
the following conditions are satisfied:
1.) all groups in all extents are unallocated,
2.) all extents are in ascending order,
3.) no extents are missing.
If any one of these conditions is not met, Ward D will not
recover the file. Ward B is then the appropriate remedy.
(NOTE: You must rename any active file of the same name as
the erased file you wish to recover, prior to attempting
recovery. Erasing the namesake would only result in its
being un-erased.)
▶04◀
WARD E
This is a specialized directory program. It will
display all the files on a disk; with the erased, apparently
recoverable files enclosed in parentheses: e.g.,
(MUGWUMP.TXT). As in Ward D, the appearance of a file as
recoverable does not guarantee that it has not been damaged
by a file which has in turn been erased. This program is
very useful in searching disks for a recoverable version of
a lost file. Ward D would then be used to attempt the
recovery, as Ward E only reads the directory and does not
modify it in any way. It requires that you place the disk
to be searched in the designated patient drive and give your
consent for the program to proceed.
In order for a file to be recovered, it must show up in
all its extents during Ward E, and the extents must be in
ascending order. If, for example, extent 00 is missing,
Ward D will not attempt to recover extent 01 of the same
filename. If extent 01 preceeded extent 00 in the Ward E
display, WARD D would not attempt to recover either extent
because that version of extent 01 must have been from a
different generation of the file (CP/M always puts the
extents in ascending order). Ward B can be used to piece
together such fragmented files if necessary.
: Common Disk Problems and Recommended Recovery Procedures
_____________________________________________________________
I. File(s) accidentally ø Invoke DISK DOCTOR (p. 10),
erased, filename(s) ø go to Ward D (p. 19). If
known. ø portions of erased file(s)
ø have been reallocated, you
ø can go to Ward B (p. 16) to
ø salvage the remaining groups.
-----------------------------ø-------------------------------
II. File(s) accidentally ø Invoke DISK DOCTOR (p. 10),
erased, filename(s) ø go to Ward E (p. 19). Disk
unknown ø directory with filename(s) of
ø recoverable erased file(s)
ø in parentheses will be dis-
ø played. Continue as in I
ø above.
-----------------------------ø-------------------------------
▶04◀
III. All existing files are ø Invoke DISK DOCTOR (p. 10),
intact, but attemp- ø go to Ward A (p. 15). æThis
ting to create a new ø same procedure may also be
file results in CP/M ø used to screen new disks and
error message--BDOS ø eliminate malfunctioning
ERR ON d:BAD SECTOR. ø groups from further use.å
-----------------------------ø-------------------------------
IV. An attempt to access ø Invoke DISK DOCTOR (p. 10),
an existing file re- ø go to Ward C (p. 18). The de-
sults in CP/M error ø sired file should be acces-
message--BDOS ERR ON ø sible on the disk copy gene-
d:BAD SECTOR, but ø rated by Ward C, although
file directory is ø data in any bad sectors will
still accessible. ø be replaced by ASCII spaces.
-----------------------------ø-------------------------------
V. Any attempt to access ø Invoke DISK DOCTOR (p. 10),
directory (such as ø go to Ward C (p. 18). If the
entering DIR) re- ø desired file still cannot be
sults in CP/M error ø accessed on the disk copy
message--BDOS ERR ON ø generated by Ward C, go to
d:BAD SECTOR. ø Ward B (p. 16) to salvage
ø what data you can from the
ø original disk or the copy.
-----------------------------ø-------------------------------
TO THE USER: As the creator of a program cannot always con-
ceive of all uses to which it may be put; if you discover
other ways to use DISK DOCTOR to your advantage, please call
or write us at SuperSoft to tell us about it.
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