1. In a nut shell 1

2.  Goals  A little history  System components  Threads & CPU scheduling  Virtual memory  Environmental subsystems  File System: NTFS 2

3.  Preemptive multitasking  Goals: ◦ Security, reliability, ease-of-use, Windows & POSIX application compatibility, high- performance, extensibility, portability, international language support ◦ Commercial  “the layered architecture of the system … makes it so easy to use”

4.  Windows NT ◦ Adopted Windows 95 user interface and incorporated web- server & web-browser ◦ User-interface routines and all graphics code were moved into the kernel to improve performance  Side effect: Decrease in system reliability 4

5.  Windows XP (Oct 2001) ◦ Successor to Windows NT/2000, replacement for Windows 95/98 ◦ Reliability requirement for Windows XP more stringent than Windows 2000 (which was the most reliable, stable system released by Microsoft) ◦ “extensive manual and automatic code review to identify over 63,000 lines in the source [code] that might contain issues not detected by testing” and then set about a review & correction process 5

6. 6 System Components

7.  Microkernel: Executes in protected mode ◦ HAL: Hardware abstraction layer  Some hardware independence  HAL provides memory mapping, configuring I/O buses, setting up DMA, motherboard specific facilities  Device drivers (I/O manager) can still work directly with hardware ◦ Kernel  thread scheduling, interrupt & exception handling, CPU synchronization, power failure recovery  Never paged out, never preempted  User mode processes ◦ Environmental subsystems  User mode operating systems (e.g., MS-DOS, OS/2, Win32, POSIX)  Logon systems ◦ User applications 7

8.  Processes ◦ Virtual memory address space ◦ Base priority ◦ “affinity” (assignment) to one or more processors ◦ One or more threads  Threads ◦ Units of execution; dispatched by kernel ◦ States: ready, standby, running, waiting, transition, terminated 8

9.  Priority-based, preemptive scheduling  32 priority levels; each has a queue of threads ◦ Variable class: 0-15 ◦ Real-time: 16-31 ◦ Higher numbers indicate higher priority  Scheduler traverses from highest to lowest priority queues ◦ Round robin, combined with priority scheme 9

10.  Variation on standard round-robin CPU scheduling  When thread’s time quantum runs out ◦ Variable class: priority lowered  unless already base priority ◦ Returned to relevant priority level in queue, in ready state ◦ Why lower the thread priority? Are there conditions under which a variable class thread will not have its priority lowered? 10

11.  When variable priority thread released from wait state ◦ Dispatcher boosts priority, depending on type of wait ◦ High boost:  Waiting for keyboard I/O  Thread associated with user’s active GUI window ◦ Low boost: waiting for disk I/O 11

12.  32-bit processors ◦ 4K page size  12 bit page offset ◦ 4GB virtual address space ◦ Upper 1-2 GB of all processes, used by operating system in kernel mode  64 bit processors ◦ 8K page size  13 bit page offset ◦ 8 TB virtual address space 12

13.  Demand paging with clustering ◦ Bring in neighboring pages on a page fault: “Prefetching”  Max/min working set size per process ◦ Processes have initial working set size of 50 frames  If a process is at its working set maximum and a page fault occurs ◦ Local page replacement  When number of free frames drops below a certain value ◦ Automatic working-set trimming ◦ memory manager removes pages from processes until processes at working set minimum ◦ FIFO or variation of clock algorithm depending on hardware (p. 363)

14.  2-level page tables  Each process has a page directory (outer page table) ◦ 1024 page-directory entries (PDE’s) ◦ Size 4 bytes for each PDE ◦ Each PDE points to a page table  Page table (inner page table) ◦ 1024 page-table entries (PTE’s) ◦ Size 4 bytes for each PTE ◦ Each PTE points to a 4 KB frame of physical memory ◦ Page tables are swapped out to disk when necessary  Total size of all page tables, per process: 4 MB 14 Logical Address structure

15. 15

16.  20 bits for frame number  12 bits remain to describe state of page ◦ Accessed or written ◦ Caching attributes ◦ Access mode ◦ Global ◦ PTE valid 16

17.  Some implement user-level operating systems  Some implement services crucial to all user- level operating systems ◦ E.g., GUI, security management  Win32 ◦ Executes in unprotected mode ◦ Provides all keyboard, mouse, graphical display  Other environmental subsystems use this ◦ Separate processes with own input queues ◦ Window manager dispatches input to input queue of appropriate process

18.  A user-level operating system ◦ Executes in unprotected mode  Instruction execution unit: Emulates Intel 486 instructions  Routines to emulate DOS ROM BIOS & “Int 21” software interrupt services  Virtual device drivers: Screen, keyboard, comm. Ports ◦ Partial support only: No direct h/w access 18

19.  User-level operating systems considered to be “clients”  Kernel considered to be “server”  Communication between client and server provided by message passing  “Local procedure call” (LPC) facility ◦ like Remote Procedure Calls  Message passing within a single computer  Used to implement system calls 19

20.  Server (e.g., kernel) ◦ Publishes a globally visible connection port object  Client (e.g., Win32), to obtain services provided by server ◦ Opens a handle to server connection port  Sends connection request ◦ Server creates channel & returns handle to client ◦ Channel: pair of private communication ports  Client-to-server messages  Server-to-client messages  Some specifics dependent on message sizes ◦ Small messages (e.g., up to a few hundred bytes) are copied from sender to receiver ◦ Shared memory is used for larger messages 20

21.  Goals ◦ data recovery, security, fault tolerance, large file & file system sizes, multiple data streams, UNICODE names, compression  NTFS is a journaling file system ◦ It provides recovery of structure of file system (metadata) should there be a system failure  NTFS volumes can occupy a portion of a disk, an entire disk, or can span disks 21

22.  In NTFS a file is not a simple stream of bytes as it is in some operating systems (e.g., Unix)  Rather, files are structured objects comprised of typed attributes  Some types of attributes ◦ Conventional data of a file ◦ Standard attributes: name, creation time, security descriptor  Examples of other uses of these typed attributes ◦ Mac file resource fork on a Windows XP file server ◦ Thumbnail of an image 22

23.  Cluster: Unit of disk allocation  Allocates sectors on disk in contiguous groups ◦ A number of 512 byte disk sectors (power of two size)  Example ◦ Cluster size is 4096 bytes with disks > 4GB ◦ If the (block) sector size on disk is 512 bytes, this means each cluster is going to be 8 (blocks) sectors 23

24. 24 “How NTFS Works”, www.microsoft.com

25.  MFT (Master file table) ◦ Describes all files; file control block information (analogous to Unix inode information)  Partial copy of MFT  Log file ◦ Temporary record of all metadata updates  Volume file ◦ Name of volume, NTFS version, consistency bit  Attribute-definition table ◦ Types of attributes used in volume & operations on types  Root directory: top-level in hierarchy  Bitmap file: free/used clusters on disk  Boot file: startup code for Windows XP  Bad-cluster file: Bad areas on volume 25

26.  MFT: Master file table ◦ One or more records per file ◦ Each record is 1-4 KB in size ◦ Describes attributes of file ◦ Resident attributes: small sized  data stored in MFT record ◦ Nonresident attributes: larger sized  data stored in contiguous extents on disk  For small files, even the data of the file may be stored in the MFT record  Each file in NTFS volume has an ID-- its file reference ◦ 48 bit file number, 16 bit sequence number  Directories: particular kinds of files; B+ tree rep’n

27.  If a system failure (e.g., power failure) occurs when a file system is being written, can loose meta data (organization info) and/or file data ◦ Metadata loss tends to be more difficult  In NTFS, all file-system data-structure (metadata) updates are performed in log file transactions ◦ Before a data-structure is altered, transaction first writes a log record that contains redo and undo information ◦ After the data structure has been changed, transaction writes a commit record to log to signify that transaction succeeded  After a crash, log file transactions that had not been committed can be undone 27