1. Computer Organization and Architecture
Operating System Support

2. Objectives and Functions
Convenience
Making the computer easier to use
Efficiency
Allowing better use of computer resources

3. Layers and Views of a Computer System

4. Operating System Services
Program creation
Program execution
Access to I/O devices
Controlled access to files
System access
Error detection and response
Accounting

5. O/S as a Resource Manager

6. Types of Operating System
Interactive
Batch
Single program (Uni-programming)
Multi-programming (Multi-tasking)

7. Early Systems Late 1940s to mid 1950s No Operating System
Programs interact directly with hardware
Two main problems:
Scheduling
Setup time

8. Simple Batch Systems Resident Monitor program
Users submit jobs to operator
Operator batches jobs
Monitor controls sequence of events to process batch
When one job is finished, control returns to Monitor which reads next job
Monitor handles scheduling

9. Desirable Hardware Features
Memory protection
To protect the Monitor
Timer
To prevent a job monopolizing the system
Privileged instructions
Only executed by Monitor
e.g. I/O
Interrupts
Allows for relinquishing and regaining control

10. Multi-programmed Batch Systems
I/O devices very slow
When one program is waiting for I/O, another can use the CPU

11. Single Program

12. Multi-Programming with Two Programs

13. Multi-Programming with Three Programs

14. Time Sharing Systems
Allow users to interact directly with the computer
i.e. Interactive
Multi-programming allows a number of users to interact with the computer

15. Scheduling Key to multi-programming Long term Medium term Short term
I/O

16. Long Term Scheduling
Determines which programs are submitted for processing
i.e. controls the degree of multi-programming
Once submitted, a job becomes a process for the short term scheduler
(or it becomes a swapped out job for the medium term scheduler)

17. Medium Term Scheduling
Part of the swapping function (later…)
Usually based on the need to manage multi-programming
If no virtual memory, memory management is also an issue

18. Short Term Scheduler Dispatcher
Fine grained decisions of which job to execute next
i.e. which job actually gets to use the processor in the next time slot

19. Process States

20. Process Control Block Identifier State Priority Program counter
Memory pointers
Context data
I/O status
Accounting information

21. Key Elements of O/S

22. Process Scheduling Process Request Long-Term Queue Short-Term Queue
CPU
End
I/O
I/O Queue
I/O
I/O Queue
I/O
I/O Queue

23. Memory Management Uni-program Multi-program Memory split into two
One for Operating System (monitor)
One for currently executing program
Multi-program
“User” part is sub-divided and shared among active processes

24. 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 main memory
Expensive
Leads to larger programs
Swapping

25. What is Swapping? Long term queue of processes stored on disk
Processes “swapped” in as space becomes available
As a process completes it is moved out of main memory
If none of the processes in memory are ready (i.e. all I/O blocked)
Swap out a blocked process to intermediate queue
Swap in a ready process or a new process
But swapping is an I/O process...

26. Partitioning
Splitting memory into sections to allocate to processes (including Operating System)
Fixed-sized partitions
May not be equal size
Process is fitted into smallest hole that will take it (best fit)
Some wasted memory
Leads to variable sized partitions

27. Fixed Partitioning

28. Variable Sized Partitions (1)
Allocate exactly the required memory to a process
This leads to a hole at the end of memory, too small to use
Only one small hole - less waste
When all processes are blocked, swap out a process and bring in another
New process may be smaller than swapped out process
Another hole

29. Variable Sized Partitions (2)
Eventually have lots of holes (fragmentation)
Solutions:
Coalesce - Join adjacent holes into one large hole
Compaction - From time to time go through memory and move all hole into one free block (c.f. disk de-fragmentation)

30. Effect of Dynamic Partitioning

31. Relocation
No guarantee that process will load into the same place in memory
Instructions contain addresses
Locations of data
Addresses for instructions (branching)
Logical address - relative to beginning of program
Physical address - actual location in memory (this time)
Automatic conversion using base address

32. Paging Split memory into equal sized, small chunks - frames
Split programs (processes) into equal sized small chunks - pages
Allocate the required number page frames to a process
Operating System maintains list of free frames
A process does not require contiguous page frames
Use page table to keep track

33. Logical and Physical Addresses - Paging

34. Virtual Memory Demand paging Page fault
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
Select page to throw out based on recent history

35. Thrashing Too many processes in too little memory
Operating System spends all its time swapping
Little or no real work is done
Disk light is on all the time
Solutions
Good page replacement algorithms
Reduce number of processes running
Fit more memory

36. Bonus We do not need all of a process in memory for it to run
We can swap in pages as required
So - we can now run processes that are bigger than total memory available!
Main memory is called real memory
User/programmer sees much bigger memory - virtual memory

37. Virtual Memory

38. Virtual Memory
In order to be executed or data to be accessed, a certain segment of the program has to be first loaded into main memory; in this case it has to replace another segment already in memory
Movement of programs and data, between main memory and secondary storage, is performed automatically by the operating system. These techniques are called virtual-memory techniques

39. Virtual Memory

40. Virtual Memory Organization
The virtual programme space (instructions + data) is divided into equal, fixed-size chunks called pages.
Physical main memory is organized as a sequence of frames; a page can be assigned to an available frame in order to be stored (page size = frame size).
The page is the basic unit of information which is moved between main memory and disk by the virtual memory system.

41. Demand Paging
The program consists of a large amount of pages which are stored on disk; at any one time, only a few pages have to be stored in main memory.
The operating system is responsible for loading/ replacing pages so that the number of page faults is minimized.

42. Demand Paging
We have a page fault when the CPU refers to a location in a page which is not in main memory; this page has then to be loaded and, if there is no available frame, it has to replace a page which previously was in memory.

43. Address Translation
Accessing a word in memory involves the translation of a virtual address into a physical one:
- virtual address: page number + offset
- physical address: frame number + offset
Address translation is performed by the MMU using a page table.

44. Example

45. Address Translation

46. The Page Table
The page table has one entry for each page of the virtual memory space.
Each entry of the page table holds the address of the memory frame which stores the respective page, if that page is in main memory.

47. The Page Table
Each entry of the page table also includes some control bits which describe the status of the page:
whether the page is actually loaded into main memory or not;
if since the last loading the page has been modified;
information concerning the frequency of access, etc.

48. Memory Reference with Virtual Memory

49. Memory Reference with Virtual Memory
Memory access is solved by hardware except the page fault sequence which is executed by the OS software.
The hardware unit which is responsible for translation of a virtual address into a physical one is the Memory Management Unit (MMU).

50. Translation Lookaside Buffer
Every virtual memory reference causes two physical memory access
Fetch page table entry
Fetch data
Use special cache for page table
TLB

51. TLB Operation

52. TLB and Cache Operation

53. Pentium II Address Translation Mechanism

54. Page Replacement
When a new page is loaded into main memory and there is no free memory frame, an existing page has to be replaced
The decision on which page to replace is based on the same speculations like those for replacement of blocks in cache memory
LRU strategy is often used to decide on which page to replace.

55. Page Replacement
When the content of a page, which is loaded into main memory, has been modified as result of a write, it has to be written back on the disk after its replacement.
One of the control bits in the page table is used in order to signal that the page has been modified.

56. Summary Objective and functions OS Scheduling Memory management
virtual memory
page replacement