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As was described in CHAPTER 3, a 16 sector diskette consists of 560 data areas
of 256 bytes each, called sectors. These sectors are arranged on the diskette in
35 concentric rings or tracks of 16 sectors each. The way DOS allocates these
tracks of sectors is the subject of this chapter.
A file (be it APPLESOFT, INTEGER, BINARY, or TEXT type) consists of one or more
sectors containing data. Since the sector is the smallest unit of allocatable
space on a diskette, a file will use up at least one sector even if it is less
than 256 bytes long; the remainder of the sector is wasted. Thus, a file
containing 400 characters (or bytes) of data will occupy one entire sector and
144 bytes of another with 112 bytes wasted. Knowing these facts, one would
expect to be able to use up to 16 times 35 times 256 or 143,360 bytes of space
on a diskette for files. Actually, the largest file that can be stored is about
126,000 bytes long. The reason for this is that some of the sectors on the
diskette must be used for what is called "overhead".
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*** INSERT FIGURE 4.1 ***
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Overhead sectors contain the image of DOS which is 1oaded when booting the
diskette, a list of the names and locations of the files on the diskette, and an
accounting of the sectors which are free for use with new files or expansions of
existing files. An example of the way DOS uses sectors is given in Figure 4.1.
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The map in Figure 4.1 shows that the first three tracks of each diskette are
always reserved for the bootstrap image of DOS. In the exact center track (track
17) is the VTOC and catalog. The reason for placing the catalog here is simple.
Since the greatest delay when using the disk is waiting for the arm to move from
track to track, it is advantageous to minimize this arm movement whenever
possible. By placing the catalog in the exact center track of the disk, the arm
need never travel more than 17 tracks to get to the catalog track. As files are
allocated on a diskette, they occupy the tracks just above the catalog track
first. When the last track, track 34, has been used, track 16, the track
adjacent and below the catalog, is used next, then 15, 14, 13, and so on, moving
away from the catalog again, toward the DOS image tracks. If there are very few
files on the diskette, they will all be clustered, hopefully, near the catalog
and arm movement will be minimized. Additional space for a file, if it is
needed, is first allocated in the same track occupied by the file. When that
track is full, another track is allocated elsewhere on the disk in the manner
described above.
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The Volume Table Of Contents is the "anchor" of the entire diskette. On any
diskette accessible by any version of DOS, the VTOC sector is always in the same
place; track 17, sector 0. (Some protected disks have the VTOC at another
location and provide a special DOS which can find it.) Since files can end up
anywhere on the diskette, it is through the VTOC anchor that DOS is able to find
them. The VTOC of a diskette has the following format (all byte offsets are
given in base 16, hexadecimal):
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00 Not used
01 Track number of first catalog sector
02 Sector number of first catalog sector
03 Release number of DOS used to INIT this diskette
04-05 Not used
06 Diskette volume number (1-254)
07-26 Not used
27 Maximum number of track/sector pairs which will fit
in one file track/sector list sector (122 for 256
byte sectors)
28-2F Not used
30 Last track where sectors were allocated
31 Direction of track allocation (+1 or -1)
32-33 Not used
34 Number of tracks per diskette (normally 35)
35 Number of sectors per track (13 or 16)
36-37 Number of bytes per sector (LO/HI format)
38-3B Bit map of free sectors in track 0
3C-3F Bit map of free sectors in track 1
40-43 Bit map of free sectors in track 2
BC-BF Bit map of free sectors in track 33
C0-C3 Bit map of free sectors in track 34
C4-FF Bit maps for additional tracks if there are more
than 35 tracks per diskette
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A four byte binary string of ones and zeros,
representing free and allocated sectors respectively.
Hexadecimal sector numbers are assigned to bit
positions as follows:
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+0 FEDC BA98
+1 7654 3210
+2 .... .... (not used)
+3 .... .... (not used)
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Thus, if only sectors E and 8 are free and all
others are allocated, the bit map will be:
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If all sectors are free:
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An example of a VTOC sector is given in Figure 4.2. This VTOC corresponds to the
map of the diskette given in Figure 4.1.
*** INSERT FIGURE 4.2 ***
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In order for DOS to find a given file, it must first read the VTOC to find out
where the first catalog sector is located. Typically, the catalog sectors for a
diskette are the remaining sectors on track 17, following the VTOC sector. Of
course, as long as a track/sector pointer exists in the VTOC and the VTOC is
located at track 17, sector 0, DOS does not really care where the catalog
resides. Figure 4.3 diagrams the catalog track. The figure shows the
track/sector pointer in the VTOC at bytes 01 and 02 as an arrow pointing to
track 17 (11 in hexadecimal) sector F. The last sector in the track is the first
catalog sector and describes the first seven files on the diskette. Each catalog
sector has a track/sector pointer in the same position (bytes 01 and 02) which
points to the next catalog sector. The last catalog sector (sector 1) has a zero
pointer to indicate that there are no more catalog sectors in the chain.
*** INSERT FIGURE 4.3 ***
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In each catalog sector up to seven files may be listed and described. Thus, on a
typical DOS 3.3 diskette, the catalog can hold up to 15 times 7, or 105 files. A
catalog sector is formatted as described on the following page.
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00 Not used
01 Track number of next catalog sector (usually 11 hex)
02 Sector number of next catalog sector
03-0A Not used
0B-2D First file descriptive entry
2E-50 Second file descriptive entry
51-73 Third file descriptive entry
74-96 Fourth file descriptive entry
97-B9 Fifth file descriptive entry
BA-DC Sixth file descriptive entry
DD-FF Seventh file descriptive entry
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00 Track of first track/sector list sector.
If this is a deleted file, this byte contains a hex
FF and the original track number is copied to the
last byte of the file name field (BYTE 20).
If this byte contains a hex 00, the entry is assumed
to never have been used and is available for use.
(This means track 0 can never be used for data even
if the DOS image is "wiped" from the diskette.)
01 Sector of first track/sector list sector
02 File type and flags:
Hex 80+file type - file is locked
00+file type - file is not locked
00 - TEXT file
04 - BINARY file
08 - S type file
10 - RELOCATABLE object module file
20 - A type file
40 - B type file
(thus, 84 is a locked BINARY file, and 90 is a
locked R type file)
03-20 File name (30 characters)
21-22 Length of file in sectors (LO/HI format).
The CATALOG command will only format the LO byte of
this length giving 1-255 but a full 65,535 may be
stored here.
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Figure 4.4 is an example of a typical catalog sector. In this example there are
only four files on the entire diskette, so only one catalog sector was needed to
describe them. There are four entries in use and three entries which have never
been used and contain zeros.
*** INSERT FIGURE 4.4 ***
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Each file has associated with it a "Track/Sector List" sector. This sector
contains a list of track/sector pointer pairs which sequentially list the data
sectors which make up the file. The file descriptive entry in the catalog sector
points to this T/S List sector which, in turn, points to each sector in the
file. This concept is diagramed in Figure 4.5.
*** INSERT FIGURE 4.5 ***
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The format of a Track/Sector List sector is given below. Note that since even a
minimal file requires one T/S List sector and one data sector, the least number
of sectors a non-empty file can have is 2. Also, note that a very large file,
having more than 122 data sectors, will need more than one Track/Sector List to
hold all the Track/Sector pointer pairs.
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00 Not used
01 Track number of next T/S List sector if one was
needed or zero if no more T/S List sectors.
02 Sector number of next T/S List sector (if present).
03-04 Not used
05-06 Sector offset in file of the first sector described
by this list.
07-0B Not used
0C-0D Track and sector of first data sector or zeros
0E-0F Track and sector of second data sector or zeros
10-FF Up to 120 more Track/Sector pairs
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A sequential file will end when the first zero T/S List entry is encountered. A
random file, however, can have spaces within it which were never allocated and
therefore have no data sectors allocated in the T/S List. This distinction is
not always handled correctly by DOS. The VERIFY command, for instance, stops
when it gets to the first zero T/S List entry and can not be used to verify some
random organization text files.
An example T/S List sector is given in Figure 4.6. The example file (HELLO,
from our previous examples) has only one data sector, since it is less than 256
bytes in length. Counting this data sector and the T/S List sector, HELLO is 2
sectors long, and this will be the value shown when a CATALOG command is done.
*** INSERT FIGURE 4.6 ***
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Following the Track/Sector pointer in the T/S List sector, we come to the first
data sector of the file. As we examine the data sectors, the differences between
the file types become apparent. All files (except, perhaps, a random TEXT file)
are considered to be continuous streams of data, even though they must be broken
up into 256 byte chunks to fit in sectors on the diskette. Although these
sectors are not necessarily contiguous (or next to each other on the diskette),
by using the Track/Sector List, DOS can read each sector of the file in the
correct order so that the programmer need never know that the data was broken up
into sectors at all.
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The TEXT data type is the least complicated file data structure. It consists of
one or more records, separated from each other by carriage return characters
(hex 8D's). This structure is diagrammed and an example file is given in Figure
4.7. Usually, the end of a TEXT file is signaled by the presence of a hex 00 or
the lack of any more data sectors in the T/S List for the file. As mentioned
earlier, if the file has random organization, there may be hex 00's imbedded in
the data and even missing data sectors in areas where nothing was ever written.
In this case, the only way to find the end of the file is to scan the
Track/Sector List for the last non-zero Track/Sector pair. Since carriage
return characters and hex 00's have special meaning in a TEXT type file, they
can not be part of the data itself. For this reason, and to make the data
accessible to BASIC, the data can only contain printable or ASCII characters
(alphabetics, numerics or special characters, see p. 8 in the APPLE II REFERENCE
MANUAL) This restriction makes processing of a TEXT file slower and less
efficient in the use of disk space than with a BINARY type file, since each
digit must occupy a full byte in the file.
*** INSERT FIGURE 4.7 ***
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The structure of a BINARY type file is shown in Figure 4.8. An exact copy of the
memory involved is written to the disk sector(s), preceded by the memory address
where it was found and the length (a total of four bytes). The address and
length (in low order, high order format) are those given in the A and L keywords
from the BSAVE command which created the file. Notice that DOS writes one extra
byte to the file. This does not matter too much since BLOAD and BRUN will only
read the number of bytes given in the length field. (Of course, if you BSAVE a
multiple of 256 bytes, a sector will be wasted because of this error) DOS could
be made to BLOAD or BRUN the binary image at a different address either by
providing the A (address) keyword when the command is entered, or by changing
the address in the first two bytes of the file on the diskette.
*** INSERT FIGURE 4.8 ***
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A BASIC program, be it APPLESOFT or INTEGER, is saved to the diskette in a way
that is similar to BSAVE. The format of an APPLESOFT file type is given in
Figure 4.9 and that of INTEGER BASIC in 4.10. When the SAVE command is typed,
DOS determines the location of the BASIC program image in memory and its length.
Since a BASIC program is always loaded at a location known to the BASIC
interpreter, it is not necessary to store the address in the file as with a
BINARY file. The length is stored, however, as the first two bytes, and is
followed by the image from memory. Notice that, again, DOS incorrectly writes
an additional byte, even though it will be ignored by LOAD. The memory image of
the program consists of program lines in an internal format which is made up of
what are called "tokens". A treatment of the structure of a BASIC program as it
appears in memory is outside the scope of this manual, but a breakdown of the
example INTEGER BASIC program is given in Figure 4.10.
*** INSERT FIGURES 4.9 AND 4.10 ***
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Additional file types have been defined within DOS as can be seen in the file
descriptive entry format, shown earlier. No DOS commands at present use these
additional types so their eventual meaning is anybody's guess. The R file type,
however, has been used with the DOS TOOLKIT assembler for its output file, a
relocatable object module. This file type is used with a special form of BINARY
file which can contain the memory image of a machine language program which may
be relocated anywhere in the machine based on additional information stored with
the image itself. The format for this type of file is given in the documentation
accompanying the DOS TOOLKIT. It is recommended that if the reader requires
more information about R files he should refer to that documentation.
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From time to time the information on a diskette can become damaged or lost. This
can create various symptoms, ranging from mild side effects, such as the disk
not booting, to major problems, such as an input/output (I/O) error in the
catalog. A good understanding of the format of a diskette, as described
previously, and a few program tools can allow any reasonably sharp APPLE II user
to patch up most errors on his diskettes.
A first question would be, "how do errors occur". The most common cause of an
error is a worn or physically damaged diskette. Usually, a diskette will warn
you that it is wearing out by producing "soft errors". Soft errors are I/O
errors which occur only randomly. You may get an I/O error message when you
catalog a disk one time and have it catalog correctly if you try again. When
this happens, the smart programmer immediately copies the files on the aged
diskette to a brand new one and discards the old one or keeps it as a backup.
Another cause of damaged diskettes is the practice of hitting the RESET key to
abort the execution of a program which is accessing the diskette. Damage will
usually occur when the RESET signal comes just as data is being written onto the
disk. Powering the machine off just as data is being written to the disk is also
a sure way to clobber a diskette. Of course, real hardware problems in the disk
drive or controller card and ribbon cable can cause damage as well.
If the damaged diskette can be cataloged, recovery is much easier. A damaged
DOS image in the first three tracks can usually be corrected by running the
MASTER CREATE program against the diskette or by copying all the files to
another diskette. If only one file produces an I/O error when it is VERIFYed, it
may be possible to copy most of the sectors of the file to another diskette by
skipping over the bad sector with an assembler program which calls RWTS in DOS
or with a BASIC program (if the file is a TEXT file). Indeed, if the problem is
a bad checksum (see CHAPTER 3) it may be possible to read the bad sector and
ignore the error and get most of the data.
An I/O error usually means that one of two conditions has occured. Either a bad
checksum was detected on the data in a sector, meaning that one or more bytes is
bad; or the sectoring is clobbered such that the sector no longer even exists on
the diskette. If the latter is the case, the diskette (or at the very least, the
track) must be reformatted, resulting in a massive loss of data. Although DOS
can be patched to format a single track, it is usually easier to copy all
readable sectors from the damaged diskette to another formatted diskette and
then reconstruct the lost data there.
Many commercially available utilities exist which allow the user to read and
display the contents of sectors. Some of these utilities also allow you to
modify the sector data and rewrite it to the same or another diskette. A simple
version of such a utility is provided in APPENDIX A. The ZAP program given
there will read any track/sector into memory, allowing the user to examine it or
modify the data and then, optionally, rewrite it to a diskette. Using such a
program is very important when learning about diskette formats and when fixing
clobbered data.
Using ZAP, a bad sector within a file can be localized by reading each
track/sector listed in the T/S List sector for the file. If the bad sector is a
catalog sector, the pointers of up to seven files may be lost. When this occurs,
a search of the diskette can be made to find T/S List sectors which do not
correspond to any files listed in the remaining "good" catalog sectors. As
these sectors are found, new file descriptive entries can be made in the damaged
sector which point to these T/S Lists. When the entire catalog is lost, this
process can take hours, even with a good understanding of the format of DOS
diskettes. Such an endeavor should only be undertaken if there is no other way
to recover the data. Of course the best policy is to create backup copies of
important files periodically to simplify recovery. More information on the
above procedures is given in APPENDIX A.
A less significant form of diskette clobber, but very annoying, is the loss of
free sectors. Since DOS allocates an entire track of sectors at a time while a
file is open, hitting RESET can cause these sectors to be marked in use in the
VTOC even though they have not yet been added to any T/S List. These lost
sectors can never be recovered by normal means, even when the file is deleted,
since they are not in its T/S List. The result is a DISK FULL message before
the diskette is actually full. To reclaim the lost sectors it is necessary to
compare every sector listed in every T/S List against the VTOC bit map to see if
there are any discrepancies. There are utility programs which will do this
automatically but the best way to solve this problem is to copy all the files on
the diskette to another diskette (note that FID must be used, not COPY, since
COPY copies an image of the diskette, bad VTOC and all).
If a file is deleted it can usually be recovered, providing that additional
sector allocations have not occured since it was deleted. If another file was
created after the DELETE command, DOS might have reused some or all of the
sectors of the old file. The catalog can be quickly ZAPped to move the track
number of the T/S List from byte 20 of the file descriptive entry to byte 0. The
file should then be copied to another disk and then the original deleted so that
the VTOC freespace bit map will be updated.
.nx ch5