User`s guide

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From: Neil Parker and Rubywand
019- What information is included in the DOS 3.3 VTOC and
catalog sectors?
VTOC
The Volume Table of Contents (VTOC) is Sector $00 (0) on Track $11 (17).
This is the key sector from which all searches start out.
Example: A typical DOS 3.3 Disk's VTOC sector
Byte 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E
0000- 04 11 0F 03 00 00 FE 00 00 00 00 00 00 00 00
0010- 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0020- 00 00 00 00 00 00 00 7A 00 00 00 00 00 00 00
0030- 15 01 00 00 23 10 00 01 00 00 00 00 00 00 00
....
0080- 3F 7F 00 00 00 00 00 00 00 00 00 00 00 00 00
....
00F0- 00 00 00 00 00 00 00 7A 00 00 00 00 00 00 00
Byte Meaning
---- -------
$00 Unused (always $04 on my disks)
$01-02 Track/Sector location of first catalog sector-- the standard
location is Track $11/Sector $0F (17/15)
Note: The location of first catalog sector may be different on some
disks. DOS 3.3 can adjust; but, some utilities (e.g. Copy II Plus)
assume T/S $11/$0F and will not be able to find the catalog.
$03 DOS version number ($03 for DOS 3.3)
$04-05 Unused
$06 Volume Number-- $01-$FE (1-254) is the standard range;
$FE (254) is the standard default VN
Note: This entry merely records the VN for handy reference.
VN is set when a disk is INITed and it is embedded in the
Address header of each sector.
$07-26 Unused
$27 Max number of Track/Sector pairs in each sector of
a file's Track/Sector list-- normally $7A (122)
$28-2F Unused
$30 Last track where sectors were allocated-- in the example
it is Track $15 (21)
$31 Direction of allocation-- $01 (+1)=inward; $FF (-1)=outward
$32-33 Unused
$34 Number of tracks per disk-- normally $23 (35)
$35 Number of sectors per track-- normally $10 (16)
$36-37 Number of bytes per sector-- normally $0100 (256)
$38-3B Bit map for Track $00-- four bytes per entry (only two bytes are
used); each bit in the two-byte entry indicates whether a sector
is in use (0) or free for use (1). See example just below.