RTOS has a cost savings advantage for real-time applications, such as those with random inputs requiring a flexible data routing facility, display systems simplified by a device independent interface language, and complex applications needing added storage protection and data queuing

Hoffman, R. L.

Kopp, R. S.

Mueller, H. H.

Pollan, W. D.

Van Sant, B. W.

Weiler, P. W.

English

NASA Technical Reports Server

September 1969	 Brief 69-10386 
S NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Real-Time Operating System/360 
The Real-Time Operating System (RTOS) is a real- the number of requests to be processed. 	 Thus, by 
time control program developed to support the Apollo thorough queue management, the system's work flow 
Spaceflight missions. The program has a cost savings can be regulated and core lockouts can be prevented. 
advantage for other real-time applications, such as The heart of RTOS is the routing capability. 	 In 
those having random inputs which require a flexible, a real-time environment data must be processed im-
fast data routing facility; display systems which can mediately when they appear. 	 Data routing, acting 
be	 simplified	 by	 a	 device	 independent	 interface as the interface between the hardware interrupt fa-
language; complex applications (having many asyn- cility and the independent task management function, 
chronous paths of logic) which need added storage identifies the criteria which link 	 a particular data 
protection and data queuing; and high response prob- type to an independent task.	 If an input message 
lems that require rapid access to a large number of matches the current data definitions, it is routed to 
programs and/or data. RTOS is an example of high the task that will process it.	 If no match is found, 
reliability software that has demonstrated its value the message is discarded. 	 Messages for a given task 
during many hours of' Manned Spaceflight support. can be allowed to accumulate until a specified num-•	
Elements of this control program are now considered ber is reached before a request for the task is gen-
standard for real-time supervisors. erated.	 Certain real-time functions require work re-
Independent task management is the basic control quests to be generated after a given amount of time 
facility in RTOS/360.	 Processing under OS/360 is has elapsed.	 Time routing, acting as the interface 
conducted by means of tasks or "units of work". An between the time management and task management 
OS task depends on its creator in order to exist. 	 It modules, accomplishes the job. 
has the storage protect key of its creator, must have In order to meet the demands of a real-time en-
at least one load module executing under it, and its vironment, special input/output facilities are needed 
dispatching priority is a function of its creator's pri- to handle input/output requests . rapidly	 and effi-
ority. An independent task, however, does not require ciently, and to support special real-time I/O devices 
the existence of its creator in order to exist—in fact, in use at the Manned Spacecraft Center. These fun 
independent tasks can be created by the system. 	 An tions are achieved through the use of the special Real-
independent task has a unique protect key; it can be Time Access Method. With this access method, it is 
dormant with no load modules executing under it, and necessary to OPEN each data set or device only once 
it assumes its priority from the request it is processing. during the course of a real-time job step, and it allows 
These characteristics make it possible for the real-time all tasks access to the device. The Real-Time Access 
system to receive and process varying data loads Method supports standard and special purpose graphic 
rapidly and efficiently. and communication devices.	 By creating device in-
In order to handle multiple requests for a given dependence, RTOS allows substitution of sequential 
independent task, RTOS must build and maintain a devices (printers and tape units) for the special real-
queue of work requests for the task. 	 Information time I/O devices during testing. 
about each request is held in elements of a real-time A special facility was developed to handle the large 
work queue.	 By varying the characteristics of the amounts of data processed by the Real Time Corn-
queue, it is possible to control the order of or limit puting Complex (RTCC). A special set of utility pro-
(cOilttflLlCd overkal) 
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grams was written to support data tables—arrays 
of data maintained on direct access disk storage in 
OS/360 partitioned format. The standard OPEN/ 
CLOSE logic was eliminated, increasing the speed 
at which data could be read or updated. Many differ-
ent tasks can access a given table at the same time. 
Data table integrity and consistency are achieved by 
"locking" the table, which prevents other tasks from 
accessing the table until it is "unlocked". Thus, 
various segments of a table can be read through 
different requests and the user is assured that no up-
date has taken place between requests. 
One of the major features of RTOS is the ability 
to support many different types of display devices 
each with different internal format requirements. The 
Display Formatting Language (DFL), which consists 
of a set of reentrant subroutines loaded at system 
initialization, insulates the user from the unique char-
acteristics of a given display and the internal format 
changes resulting from modifications to these devices. 
In the standard OS/360 environment system, 
library routines are included within high-level-lan-
guage-load modules produced by the linkage editor, 
frequently causing multiple copies of some routines 
to be present in main core at the same time. Real-
time linkage resolves the problem by allowing load 
modules to reference common resident reentrant 
library routines. In addition, certain numerical con-
stants are frequently needed by many different pro-
grams. By using real-time linkage to maintain these 
constants in common tables, the system coordinators 
can insure that all programs are using identical con-
stants. Real-time linkages also allow parameters such 
as task priorities to be held in a common table. Thus, 
it is possible to "tune" the system by simply modify-
ing the desired priorities rather than reassembling a 
large number of programs. 
In the RTOS environment, Large Core Storage 
(ICS) is used as secondary storage. Associated with 
the LCS units is the storage channel feature which 
allows a selector channel to access memory inde-
pendent of the central processor. Programs and data 
are brought into LCS from direct access storage in 
anticipation of their use by a set of support programs 
called LCS allocation. When a load module is re-
quested by a task, it is transferred from LCS into 
main memory for execution by a set of support 
routines called the Large Core Storage Access 
Method. LCS provides a significant reduction in the 
overhead related to the repeated loading of modules 
into main memory. As LCS becomes filled, the allo-
cation routines examine the modules and purge those 
which have the least use. 
RTOS has a complete spectrum reliability and 
testing features to qualify its use in complex real 
time environments. Both an input simulator and 
statistics gathering facility are available for extensive 
debug and analysis of the application. Catastrophic 
Brief 69-10386
hardware recovery facilities for CPU, channel, and 
devices are provided through extensive switching and 
restart features. Intermittent hardware and software 
failures are recovered efficiently through retry and 
refresh logic to ensure continuous support of the 
application. 
Notes: 
1. It is possible to run RTOS on an IBM System/360 
Model 50; 512K bytes of high speed memory are 
required. 
2. Two updates to the program have recently been 
achieved. The first update added the following 
capabilities: 
1. Dynamic Large Core Storage (LCS) alloca-
tion 
2. System Task 
3. Alternate Device Support 
4. High Speed Restart 
5. Performance and Reliability improvements 
which represent an improvement in the total 
system. 
The second update added the following capabili-
ties:
1. OS/360 Release 14 
2. OS/360 Release 15 Input/Output Super-
visor 
3. OS/360 Release 16 FORTRAN H Compiler 
4. Multi-jobbing Capabilities 
5. Job Step Timing 
6. Public/Private Device Allocation 
7. Job Scheduler Allocation Recovery 
8. Management Information System (MIS) 
9. Houston Automatic Spooling Priority System 
(HASP) Version II 
10. Capability to introduce jobs to HASP from 
another job 
11. Single 2314 System Residence Volume 
12 System Recovery Facility (SRF) 
13. General Performance improvements to RTOS 
facilities 
14. Main Core Storage 2K Block History Utility 
15. Stand Alone Restore program for 2314's. 
Inquiries should be made to: 
COSMIC 
Computer Center 
University of Georgia 
Athens, Georgia 30601 
Reference: B69-10386 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. L. Hoffman, H. H. Mueller, R. S. Kopp,

W. D. Pollan, B. W. Van Sant, and P. W. Weiler of

IBM Corporation
under contract to

Manned Spacecraft Center

(MSC-12148) 
Category 01
..


Real-Time Operating System/360

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