1. IBM OS/360 E. Casey Lunny 1

2. IBM OS/360 ● Launched in December 1965 ● 21 releases ● Retired August 1972 ● Ran on IBM System 360 ● 360° of coverage ● Predecessor of 370 and 390 2

3. Second Generation OS ● First Generation – Batch System – Real-Time System ● Commercial and scientific applications ran on separate lines of hardware ● 360 as “second generation” – “First general purpose mainframe computer” – Accommodate diversity of applications and operating modes – Review of mainframe interface 3

4. Elements Covered ● Definition of Job/Task ● Degree of Multiprogramming ● Memory Management ● Job Scheduling ● Deadlock ● File Security 4

5. Processor Support ● Number of Processors – Originally uniprocessor – In 1968 support added for multiple processors – SMP ● Modes – Kernel – Problem – No “superuser” 5

6. Job ● Definitions – Job (Process) – Job-Step – Task (Thread) – Sub-task 6

7. Degrees of Multiprogramming ● Evolved over time, with optional components ● Primary Control Program (PCP) ● Multiprogramming with a Fixed Number of Tasks (MFT) – Four then upgraded to 15 tasks ● Multiprogramming with a Variable Number of Tasks (MVT) 7

8. Memory Management ● PCP – One task at a time – Single partition ● MFT – 4-15 tasks – Multiple fixed partitions 8

9. Memory Management ● MVT – Original implementation ● Multiple variable partitions created on the fly ● Memory allocated as requested ● External fragmentation is an issue – Improved implementation ● Task tagged with one of several fixed sizes ● Fit into predefined slots ● “Buddy system” 9

10. Job Scheduling ● FIFO ● Priority scheduling – 0 to 14, increasing priority – Uses input work queue – Priorities set by user – Initiator can look ahead to determine resources that will be needed 10

11. Deadlock ● Rudimentary method of resolution ● No ability to know whether a request filled ● Especially in PCP, entire system could be locked up waiting for I/O ● Implemented arbitrary limit of 30 minutes before canceling the job 11

12. File Security ● Password-based ● Master password ● Data sets could be flagged as “protected” ● Password entered on operator console ● Checked against passwords stored in control table 12

13. Conclusion ● Merged separate lines of hardware ● First general purpose mainframe ● Moving away from traditional batch systems ● Financially successful 13