1. Operating System Support Focus on Architecture
Chapter 8
Operating System Support
Focus on Architecture

2. Functions of Operating System
Managing Resources
Access to System Utilities
Access to files
Access to I/O devices
Managing Interrupts & Bus Control (DMA)
Error detection and response
Accounting
Scheduling Processes (or tasks)
Program creation
Program execution
I/O Processing
Managing Memory Utilization
Partitioning,
Paging,
Virtual memory,
Segmentation

3. Types of Operating Systems
What do we want to Optimize in each?
Interactive
Batch
Uni-tasking
Multi-tasking
Real-Time

4. Batch Operating System Resident Monitor
Jobs are batched in queue
Monitor handles scheduling
Monitor controls sequence of
events to process batch
When one job is finished,
control returns to Monitor
which loads next job

5. Desirable Hardware Support for OS
Memory protection
To protect the Monitor & Utilities
Timer
To prevent a job monopolizing the system
Privileged instructions
Only executed by Monitor
e.g. I/O, files
Interrupts
Allows for relinquishing and regaining control
DMA
Allows for optimizing bus usage

6. The Case for Multi-programmed Batch Systems
I/O devices are very slow
 Waiting is inefficient use of computer
When one program is waiting for I/O, another can use the CPU

7. Multi-Programming with Three Programs

8. Utilization: Uni-programmed vs Multi-programmed

9. Multiprogramming Resource Utilization

10. Scheduling What are the challenges of scheduling ?
When do we start a new program ?
When a program is blocked, what program is executed next?
When several programs are blocked, which gets first dibs at the resource needed?
How long can one process monopolize a resource?

11. How can we model Processing? A Five State Process Model

12. Types of Scheduling Needed

13. Keeping Track of Process Information
If Processes can be interrupted and restarted, what is necessary to be able to stop and restart programs?

14. Process Control Block Layout

15. Scheduling Time Sequence Example:

16. Some Key Elements of O/S

17. Process Scheduling:
What happened to the Medium-term Queue ?

18. Memory Management What are the Memory Management Issues?
How can we keep the optimum “portions” of programs in memory to optimize the use of the resources ?
How can we protect one program from corrupting other programs ?
What do we do when all programs in memory and stalled and memory is full ?

19. Memory Management Uni-programming Multi-programming
Memory split into two
One for Operating System (monitor)
One for currently executing program
Multi-programming
“User” part is sub-divided and shared among active processes
Note: Memory size implications
- 16 bits  64K memory addresses
- 24 bits  16M memory addresses
- 32 bits  4G memory addresses
- 64 bits  ? memory addresses

20. Swapping
Problem: I/O is so slow compared with CPU that even in multi-programming system, CPU can be idle most of the time
Solutions:
Increase amount of main memory
Expensive
Swapping

21. How Does Swapping Work? Long term queue of “processes stored on disk”
Processes moved in as space becomes available
As a process completes it is moved out of main memory to make room for other process(es)
If none of the processes in memory are ready (i.e. all I/O blocked)
Swap out a blocked process (intermediate queue)
Swap in a ready process or a new process
But swapping is an I/O process…
Isn’t I/O slow?
So why does swapping make sense ?

22. Implementation of Swapping

23. Other Structures to help with Memory Management
Partitioning
Paging

24. Partitioning Partitioning: Fixed Size Partitions:
Splitting memory into sections to allocate to processes
(including Operating System!)
Fixed Size Partitions:
Equal-sized partitions
Potentially a lot of wasted memory
Variable-sized partitions
Process is stored into smallest reasonable “hole”
Dynamic partitions
memory leak
need periodic compaction

25. Fixed Partitioning

26. Effect of Dynamic Partitioning – memory leaks

27. What about Relocation Challenges?
Can’t expect that process will load into the same place in memory as last time, but
Instructions contain addresses
For Locations of data
For Addresses for instructions (branching)
How about having logical address and physical addresses ?
Logical address - relative to beginning of program
Physical address - actual location in memory (this time)
Mechanisms:
Use Base Address
Automatic (hardware) Conversion

28. Paging Then: Split memory into equal sized, “small” chunks
- Page frames
Operating System maintains list of free frames
Then:
Allocate the required number page frames to a process
A process does not require contiguous page frames
Each process has its own page table

29. Allocation of Free Frames

30. Paging Addresses- Logical and Physical

31. Paging Implementation Issues
Demand paging
Do not require all pages of a process in memory
Bring in pages as required
Page fault
Required page is not in memory
Operating System must swap in required page
May need to swap out a page to make space
Perhaps select page to throw out based on recent history

32. Paging has a Potential for “Thrashing”
Too many processes in too little memory
Operating System spends all its time swapping
Little or no real work is done
Solutions
Good page replacement algorithms
Reduce number of processes running
Add more memory

33. Virtual Memory Differentiation “memory models”: Implications:
We do not need all pages of a process in memory for it to run - We can swap in pages only as required
So - we can now run processes that are bigger than total memory available!
Differentiation “memory models”:
Main memory is called real memory
physical memory
User/programmer can see much bigger memory space
virtual memory
Implications:
Page Tables can become huge and can’t fit into memory
need multiple level tables, or
inverted tables (Why inverted tables ?)

34. Alternate Inverted Page Table Structure

35. Problem with Inverted Page Table
Every virtual memory reference causes two physical memory accesses
Fetch page table entry
Fetch data
Use Translation Lookaside Buffer
special cache for page table(s)

36. TLB and Cache Operation (special Cache for tables)

37. Segmentation
Segmentation is “partitioning” memory that is visible to the programmer
- Note: Paging is not visible to the programmer
Usually different segments are allocated to program and data
There may be a number of program and data segments per process (program)
e.g. to support protection levels, priority levels, organization, flexibility, etc.

38. Advantages of Segmentation
Simplifies handling of growing data structures
Allows programs to be altered and recompiled independently, without re-linking and re-loading
Lends itself to sharing among processes
Lends itself to protection
Can paging and segmentation be combined?

39. Example: Pentium II Address Translation Mechanism
“Segment” uses 2 bits to provide 4 levels of protection, typically:
0: OS kernel, 1: OS, 2: apps needing special security, 3: general apps

40. Pentium II (Uses hardware for segmentation & paging)
Unsegmented, unpaged
virtual address = physical address
Used in Low complexity, High performance systems
Unsegmented, paged
Memory viewed as paged linear address space
Protection and management via paging (Ex: Berkeley UNIX)
Segmented, unpaged
Collection of local address spaces
Protection to single byte level, Translation table needed is on chip when segment is in memory, provide predictable access times
Segmented, paged
Segmentation used to define logical memory partitions subject to access control
Paging manages allocation of memory within partitions (Ex: Unix System V)

41. Scheduling: OS Review uni-programming multi-programming time-sharing
long-term scheduler (queue of all jobs potentially schedulable)
short-term scheduler (queue of processes that are ready to execute)
medium-term scheduling (queue of jobs that can reside in memory)
blocked monitoring (queue of processes blocked for resources)
new – ready – running – blocked – exit state machine
Memory management:
partitioning
paging – frames, pages, page fault, page table, logical/physical addr
virtual memory – inverted page table, Translation Lookaside Buffer
segmentation