06.019 What information is included in the DOS 3.3 VTOC and catalog sectors?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
06.019 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.
$3C-3F Bit map for Track $01
....
$80-81 Bit map for Track $12 (18)
Example: The entry shown is 3F 7F 00 00. Only the first two
bytes (3F 7F) are used:
Sector- F E D C B A 9 8 7 6 5 4 3 2 1 0
Bit- 0 0 1 1 1 1 1 1 0 1 1 1 1 1 1 1
Hex- 3 F 7 F
This shows that on Track $12 only sectors $F, $E, and $7 (15, 14,
and 7) are used. The other sectors on the track are free for use.
....
$C0-C3 Bit map for Track $22 (34)-- usually the last track
$C4-FF Unused on normal disks (may contain extra bit maps on disks with
more than 35 tracks)
Catalog
Starting at Track $11/Sector $0F (17/15 in decimal) and working downward
in the track (e.g. Sector $0E, $0D, ...), each catalog sector contains a
pointer to the next catalog sector, and seven file entries:
Byte Meaning
---- -------
$00 Unused
$01 Track number of next catalog sector ($00 if no more)
$02 Sector number of next catalog sector
$03-0A Unused
$0B-2D First file entry
$2E-50 Second file entry
$51-73 Third file entry
$74-96 Fourth file entry
$97-B9 Fifth file entry
$BA-DC Sixth file entry
$DD-FF Seventh file entry
Each file entry looks like this:
Byte Meaning
---- -------
$00 Track number of this file's first track/sector list
$01 Sector number of this file's first track/sector list
$02 File type:
Bit Meaning
--- -------
7 0=unlocked, 1=locked
6-0 File type ($00=Text, $01=Integer, $02=Applesoft,
$04=Binary, $08=S, $10=Relocatable, $20=A, $40=B)
$03-20 File name (high bits set; padded with blanks on right)
$21-22 Number of sectors allocated to this file
----------------------------
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