Table of Contents
Advertisement
Physical Disk Characteristics
Figure 5 shows the disk-volume layout. Under the
DXIO operating system, each disk volume contains
overhead space reserved for user files. The DXIO
system uses one disk drive from which the operating
system is loaded. That disk is designated as the
system disk and is used by the DXlO system to
perform its internal disk-based functions. The DXlO
system disk or secondary disks are described in the
following paragraphs.
System Overhead
Area~
Tracks 0 and 1 are
always allocated to system overhead on the system-
disk and secondary-disk volumes. Each disk under
the DXIO operating system is identified by a user-
assigned name. User-assigned names are a part of
specific volumes and are recorded on its associated
disk. The assigned name is called the volume ID.
The DXIO operating system maintains a map of
used/unused disk space for each volume on the disk.
A system loader is stored on disk to initialize
memory with a DXIO image at initial-program-load
(IPL) time. The DXIO system maintains a map of
unusable surface on the disk. The bad-disk-surface
map is initialized to reflect disk condition at disk
initialization.
VCATALOG
Directory
I
I
User Files
User
System
Directories
Files
Lev el 1
Lev
User Files
User
el 2
Directories
I
I
User Files
User
el3
Directories
Lev
I
I
I
r---
L --..,
r
1
I
I
Level N
Figure 6. Files and Directory Structure
15
Dynamic File Area.
Area on a disk volume
other than tracks 0 and 1 is dynamically allocated to
files. Each disk volume has a specific file directory
named VCATALOG where the DXIO operating
system maintains a volume table of contents. As
shown in figure 6, the files described in
VCAT ALOG can be data files or directory files.
At IPL time, any disk drive can
be
selected to
contain the system volume. The DXIO operating
system maintains certain files on system-disk volumes
to support DXIO internal disk-based functions.
System files also can be maintained on volumes
installed in secondary drives. Such disks can be used
to back up system volumes or can be selected as an
alternate system at IPL time.
The DXIO operating system maintains user files
on disk volumes. User files can be data files or
directory files. User-directory files contain file names
that are unique to the user directory and can be
duplicated in the system directory or other user
directories without conflict.
Any given file on a disk volume is referenced by
its pathname. The path name for a file is a
concatenation of the volume name, the directory
levels (excluding VCATALOG) leading to the file,
and the file name itself. Components of the
pathname are separated by periods. The following is
an example pathname:
VOLONE.AGENCY.RECORDS
If
temporary files are used by a task, they are
automatically deleted from the disk volume when the
task terminates. Otherwise, operator commands and
supervisor calls are available to delete a file (or a
directory).
Logical Input/Output
The DXIO operating system supports the assignment
of four-character names to each peripheral device at
system-generation time. Peripherals are identified by
these names when operating within the DXIO
system. Files are identified by pathnames. However,
a volume in which a file resides may be alternately
referenced by the physical device name of the disk
drive rather than volume ID. File identifiers follow
the device name, volume name, or system-disk
designation. Generically, the names of devices and
files are called access names. Access names can
be
either device names or file pathnames.
Logical Unit Numbers.
The DXIO operating
system performs input/ output to logical units instead
of physical units making programs more flexible in
the disposition of input and output. For example, a
Advertisement
Table of Contents
