User`s guide

$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