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Program Files.
All tasks, procedures, and
overlays are installed in structures referred to as
program files. These files are based on the
expandable relative-record file type and contain
program images in blocks corresponding to file
records. An internal directory is maintained within
the program file. This internal directory contains
pointers to each image on the file as well as relevant
information about the images. Typically, the DXIO
operating system requires two disk accesses to load a
disk-resident task, procedure, or overlay: one for the
directory entry and one for the image.
Roll in/Roll out
Program 1
Program 2
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One program file is designated as the system
program file. The system program file initially
contains only programs that constitute parts of the
DXIO operating system. Other program files can be
created to hold application programs.
Program Identification.
Program parts stored in
program files can be retrieved by task, procedure, or
overlay numbers specified at installation time. A
program can be installed with or without the
replicative attribute.
Overlays
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Root
Root
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Overlav2
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Figure 2. Memory Conservation through Disk Management
~eliDory ~anageliDent
The DXIO operating system uses the 990 mapping
option to dynamically allocate memory to the disk-
resident task segment, procedure segment, and file-
blocking buffers. The allocated blocks can be
released from memory and rolled to disk as needed.
The roll-in/ roll-out mechanism, shown in figure 2,
ensures efficient use of main memory and CPU time.
The DX I 0 operating system incorporates an
algorithm that permits programs of high priority to
preempt memory space from those of lesser or equal
priority. Any program can preempt space from a
suspended program. Whenever insufficient memory
space is available to permit the operating system to
execute a program, the DXIO system seeks lower-
priority or suspended-task segments and dispatches
these programs to disk. This process is called roll
out. Similarly, when the task and priority mix
indicates, the rolled-out program is rolled in from
the disk and execution resumes. The memory-
mapping feature permits a program segment to be
restored to a different physical memory space than it
occupied at the time it was rolled out. The priority-
oriented roll-in/ roll-out mechanism guarantees high-
10
priority tasks immediate access to memory to
respond to users or other external stimuli.
File blocks are allocated in the dynamic memory
area. Any blocks of data retained by the DXIO
operating system from recent disk transfers can be
preempted by the operating system if the memory
space is needed. These blocks are written to their
appropriate file location on the disk (if they have
been updated) to acquire memory space.
SysteliD COliDliDand Interpreter
The DXIO system command interpreter (SCI)
provides an interactive, conversational user interface
at a terminal and also provides a background batch-
processing mode. The SCI is a collection of more
than 170 procedures that provide system functions
ranging from setting the time of day or initiating
compiles to backing up disks. Commands are at the
operator's fingertips via the SCI. This can save 10 to
30 percent of development effort on major programs.
The completeness and flexibility of the functions
performed by the SCI make it without parallel in
the minicomputer market. Many of the functions
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