ZiLOG Z80-RIO Operating System User's Manual page 17

The file system maintains a special file on each disk which is
named 'DIRECTORY'. In this file are the names of all files
(including itself) on the disk and the location of the
first record of each file. The first record of each file
is one sector long (regardless of the record length of the file)
and is called the 'descriptor record'. All the file attributes
including entry point (where execution may begin), date of
creation, date of last modification, first data record
address, last data record address, record length, and
record count are contained in this record. Each record of
the file contains pointers (disk addresses) to the previous
record and the subsequent record in the file. Note that
records which are logically in order according to file
contents may, in fact, reside in an arbitrary order on the
di~k.
This 'linked' structure allows maximum utilization
of the disk. The disk allocation algorithm in ZDOS
~ttempts
to localize the disk sectors used for a single
file. Note that the sectors which comprise a single file
record are physically contiguous on the disk and are
therefore always read or written as a single disk access.
ZDOS maintains a bit map to keep track of allocated vs.
unallocated disk sectors. This map resides on three
sectors of the diskette which are preallocated by the diskette
formatting utility and is read into memory by the Initialize
command or automatically by ZDOS when the diskettes are
exchanged. The map is written from memory to the diskette
when a file is closed following an allocation change.
If the diskette is formatted as a 'system' disk, additional
space is preallocated for the system bootstrap routine and
the GET/SAVE command package (see the Debug command,
section 5.13).
Under DFS, the unallocated hard disk sectors are linked
in a free chain. The allocation and deallocation of
sectors is a matter of removing sectors from or adding
sectors to the free chain. System disks contain the
'BOO~STRAP' file contains the file system that is loaded
at eystem initialization.
While files created by the RIO Operating System or PROM
debugger on the Development System or Micro Computer System
are compatible, the bootstrap is not. Thus files may be
interchcnged between systems (procedure files are generally
not transferable) but a system disk will bootstrap
correctly only on the system for which it was designed.
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