1. Special Topics on Operating System
R. C. Chang

2. Tentative Topics Linux Internal Microkernels Window CE Internal
Inferno
Other Embedded Operating Systems

3. Trends of Operating System
Server : UNIX/Linux, Windows/NT
Desktop : Windows 98/NT, UNIX/Linux
Embedded : WIN/CE, Inferno,VxWork, JINI,…

4. Operating System Structure
Monolithic Kernel
UNIX, Windows/98, Linux
First Generation Microkernel
Mach, Chorus,…
Second Generation Microkernel
L4, Exokernel,...

5. Linux Internal
Text Book: The Linux Kernel by David A. Rusling, (Please Download the book from Internet)
Reference Books: Linux Kernel Internals (Second Edition) by M. Beck et al., Addison-Wesley, 1998

6. Hardware Basics
A typical PC Motherboard

7. PC Hardware PC, SP, PS Memory Bus Controllers and Peripherals
D-Cache, I-Cache
Cache Coherence
Bus
ISA, PCI
Controllers and Peripherals
IDE, SCSI, NIC,…
Address Space

8. Software Basics What is an operating system? Memory management
Processes
$ ps
PID TTY STAT TIME COMMAND
158 pRe :00 -bash
174 pRe :00 sh /usr/X11R6/bin/startx
175 pRe :00 xinit /usr/X11R6/lib/X11/xinit/xinitrc --
178 pRe 1 N 0:00 bowman
182 pRe 1 N 0:01 rxvt -geometry 120x35 -fg white -bg black
184 pRe 1 < 0:00 xclock -bg grey -geometry padding 0
185 pRe 1 < 0:00 xload -bg grey -geometry label xload
187 pp :26 /bin/bash
202 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black
203 ppc :00 /bin/bash
1796 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black
1797 v :00 /bin/bash
3056 pp6 3 < 0:02 emacs intro/introduction.tex
3270 pp :00 ps $

9. Device Drivers The Filesystems Controller Chips EXT2,CDROM,...
NCR810 SCSI
The Filesystems
EXT2,CDROM,...

10. Kernel Data Structures
Link Lists
Hash Tables
Caches
Abstract Interfaces
/proc/filessystems

11. Memory Management Virtual Memory large address space protection
memory mapping
fair physical memory allocation
shared virtual memory

12. An Abstract Model of VM

13. Demand Paging
Load virtual pages that are currently bein used by the executing program
Page fault handling
Linux uses demand paging to load executable images into a processes virtual memory

14. Swapping
If a process needs to bring a virtual page into physical memory and there are no free physical pages available…
Clean or read-only pages --> Disacrd
Dirty page --> swapping
Linux uses a Least Recently Used page aging technique to choose pages

15. Physical and Virtual Addressing Modes
OS usually runs in physical address model
Alpha AXP processor does not have a special physical addressing model.
It divides up the memory space into several areas and designates two of them as physical address area
KSEG (0xfffffc )

16. Access Control Page_Dirty, Page_ACCESSED ASM: Address Space Match
KRE: kernel mode can read
URE: user mode can read
PFN: Page Frame Number
Page_Dirty, Page_ACCESSED
V : Valid
FOE: Fault on Execute
FOW: Write
FOR: Read

17. Caches Buffer Cache : block device drivers(hard disks)
Page Caches : Speed up access to images and data on disk.
Swap Cache: only modified(dirty) pages are saved in the swap file.
Hardware Caches : Data/Instruction, TLB

18. Linux Page Tables Each Platform that Linux runs on must provide
translation macro that allow the kernel to traverse
the page tables for a particular processor

19. Page Allocation and Deallocation
Physical page : mem_map structure
mem_map_t for each physical page(initialized at the boot time)
Important fields
count, age, man_nr (physical frame number)
free-area vector is used to find free pages

20. Free_area data structure
Allocated groups of pages

21. Page Allocation/Deallocation
Buddy algorithm
Pages are allocated in blocks which are powers of 2 in size
The page deallocation code recombine pages into large blocks of free pages whenever it can

22. Memory Mapping
Virtual memory is represented by an mm_structure data structure
vm_area_structure

23. Demand Paging Page fault Search for vm_area_struct (AVL Tree)
If no (access illegal virtual address)--> SIGSEGV
Check for type of access
Decide the page in swap file or somewhere in disk
if the page’ page table entry is invalid but not empty then it is in swap file
nopage operation(by using page cache)

24. The Linux Page Cache

25. The Linux Page Cache
Memory mapped files are read a page at a time and these pages are stored in the page cache
Search through page_hash_table
Linux --> single page read ahead

26. Swapping Out and Discarding Pages
Kernel Swap Daemon(kswapd)
kernel thread
keep enough free pages in the system
started by init process and wait for kernel swap timer to periodcally expire
if # of free pages > free_page_high do nothing
otherwise kswapd try to reduce the number of physical pages being used by the system:

27. Swap out pages Reduce the size of the buffer and page caches
clock algorithm
Swapping out System V shared memory pages
update multiple page table entries
Swapping out and discarding pages

28. The Swap Cache
When swapping pages out, Linux avoids writing pages if it does not have to
Swap cache --> a PTE for a swapped out page
If a swap cache is non-zero, a page in swap file has not been modified --> no need to write the page to the swap file

29. Swapping Pages In
Demand paging
swapin operation