1. The VMS Operating System
 CS 351: Operating Systems
Spring 2001
Section 1   
Chad Cruys, Joshua Krug, William Menke, Matt Stehle, Kenneth Ward

2. Overview of VMS Also known as OpenVMS Developed by Digital/Compaq
First released in 1978 for the VAX system
Later released for the Alpha system also (1992)

3. Overview of VMS The 32-bit operating system quickly gained popularity
Ran on minicomputers and workstations
Compaq has added many server applications to the OS
Runs JAVA, Apache Server, and various servers

4. Overview of VMS
A new design and marketing strategy of OpenVMS is to provide integration of VMS and Windows NT
By partnering with Microsoft to develop NT and VMS integrated solutions, OpenVMS seems to continue to play a major role in enterprise information systems.

5. Success of VMS
Compaq reports that 66% of fund transfers between banks take place on systems running VMS
90% of CPU manufacturers use VMS to run their assembly lines
A large portion of stock transfers and many cellular phone billing services run on VMS systems
VMS is the top rated healthcare operating system for real-time, mission critical computing

6. Technological Success
OpenVMS set the bar for reliability and security
Password security is unmatched by UNIX or Microsoft
Provides security from denial-of-service attacks, hot-swappable hardware, and the ability to run multiple instances of the operating system on the same machine to increase reliability

7. Process State Structure
The process state structure for VMS involves a process priority queue where processes are ranked in priority from 0 to 31
16-31 is a high priority real-time process
0-15 is a normal process
Normal processes are moved up in the queue if they are critical to the operating environment

8. Process State Structure
States: suspended, executable, running, or waiting
executable process would be similar to Stalling's ready state

9. Process State Structure
A register called the processor status long word (PSL) contains information about the process
information contained includes the current access mode, the current interrupt priority level (IPL), the stack alignment, and several reserved fields

10. Process State Structure
The interrupt priority word contains a ranking at which one process may interrupt another
There are four access modes: kernel (mode 0), executive (mode 1), supervisor (mode 2), and user (mode 3)

11. Symmetric Multiprocessing (SMP)
Multiple processors used
All processors perform simultaneously
Prioritized processes

12. OpenVMS VAX processes Consist of: Maximum of 8,192 per system
individual address space
registers
code (executable image)
Maximum of 8,192 per system
Processes prioritized (0-31)
0-15 for time-sharing or not time critical
16-32 for real-time

13. OpenVMS Alpha Kernel Threads
Consist of:
individual address space
registers
code (executable image)
Maximum of 16,384 per system
Kernel threads prioritized (0-63)
0-15 for time-sharing or not time critical
16-63 used for real-time

14. Interprocess Communication
Shared memory sections
Event flags
Lock manager for queuing

15. Synchronization Facilities
Uses locks
Access regulation
shared address spaces
shared I/O resources
Mutual exclusion
Lock Manager
Multileveled locks
Asynchronous System Traps (ASTs)
Cluster systems
RMS
XQP
Application programs

16. Synchronization Facilities (Cont.)
System coordination
Shared resources
Updating data
Guarantees data integrity
Multithreaded applications
DECthreads
multiple threads executed at once
concurrent processing
mutexes
condition variables
coordinated access

17. File System Vital part of operating system
Link between hardware and software teams
Timesharing system
Separates from I/O operations
Initial opening of I/O
Update virtual-to-logical map

18. File System (Cont.) Files-11 On-Disk Structure (ODS)
Latest version ODS-5
On-disk structures
CD-ROM
volume
file structures
Random file access
Disk
read access
write access
read only access

19. File System (Cont.) Levels of On-Disk Structures ODS-1 ODS-2
ODS-5 (latest)
ODS-5
Supports Extended File Specifications

20. Memory Management Two means for memory management in VMS Swapper Pager
Controls number of processes in memory
Pager
Moves pages in and out of memory

21. Pager
Upon creation of a process, only necessary pages are moved into memory
When a page fault occurs:
A process-local page replacement policy is used
FIFO replacement algorithm
Recently replaced pages are placed in cache

22. Memory Data Structure 32-bit virtual address
First two bits are the segment
Next 21 bits are the page
Last nine are the offset
512 bits per page

23. File Data Structures File ID File Number File Sequence Number
Relative Volume Number
File Number Extension

24. File Data Structures File Header Header Area Ident Area Map Area
Access Control List Area
Reserved Area
End Checksum

25. Process Data Structures
Process Quotas
Privileges
User Identification Code
Username
Process ID
Process’ Scheduling Priority

26. Process Data Structures
Virtual Address Space Description
Program
Control
System

27. Deadlock Permanent blocking of multiple processes Two causes
Competing for system resources
Attempting to communicate with each other
Lock Manager
TRANSACTION TIMEOUT
Transaction services aborts if time limit exceeded

28. Scheduling
Three parts
Process Time
Process Priority
Process State

29. Scheduling Time Round-robin with preemption QUANTUM
Quantum remaining field
10-milliseconds timer interrupt called and remaining value is decremented
0 means next process in queue is scheduled to run

30. Scheduling Priority 0-15 (Normal processes)
If a resource is needed, it can get a boost of up to 6 priority points
i.e. I/O-bound can get boost, CPU-bound can’t
16-31 (Real-time processes)
No boosting allowed

31. Scheduling State Similar to Stallings’ states
Readily computable vs. Waiting for resource
Memory resident vs. Must be swapped into memory