ZiLOG Z80-RIO Operating System User's Manual page 190

7.4 DFS ALLOCATION
7.4.1 SECTOR 0 FORMAT
As opposed to the floppy-disk system, which uses a RAM bit map,
allocation is maintained as a list of free sectors. The first
part of this list, plus the volume ID (Disk ID) and
statistics on disk usage, are maintained on sector 0
of each unit. The format of that sector is as follows:
VOLUME ID -- bytes 1-100
MAXSIZ -- bytes 101-103 -- the total number
of sectors on that unit, and thus
1 more than the highest accessible
sector address.
MAXID -- bytes 104-107 -- the current highest
file ID. This is incremented every time
a new file is created.
ROOTPT bytes 108-110 -- the pointer to the
root directory descriptor for the unit.
TFREE -- bytes 111-113 -- the total number of
sectors remaining on the free list.
NERR -- bytes 114-116 -- the number of sectors
unusable because of errors. This is
generated during the read pass of the
format routine.
NUSED -- bytes 117-119 total number of
sectors on the unit which have been
allocated.
NFREE -- bytes 120-121 -- an index into the
free array to follow.
FREE -- bytes 122-421 -- an array of 100
3-byte pointers, each to an
unallocated sector.
FMOD byte 422 -- internal use.
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