MSDOS and Windows

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MS-DOS and Windows

• CIS 250 Operating Systems
• Lecture 11

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Today

• Talk about several operating systems
• Final Exams coming up
• Study Session??

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Operating Systems MS-DOS

• Origins
• Seattle Computer Products 86-DOS
• Microsoft and IBM roles
• Is actually a family of several versions
• Each version changed important qualities
• Upgrades necessary to handle more than 640k
  memory, more than 32Meg hard disks, etc

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Target

• Customers were small business owners and a few
  personal users
• IBM wanted some design cross compatibility with
  its mainframe softwareOS/2
• IBM eventually takes over OS/2 production
• Single CPU, small memory/storage, single process
  environment

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Basic Structure

• Bottom Layer BIOS (Basic I/O System)
• Provided support for non-storage devices
• DOS Kernel Layer
• Supported BIOS communication to Shell Layer, and
  handled disk drives
• Shell Layer
• Processes user commands, executes them
• Not interpreted like the UNIX shells
• COMMAND.COM file is an important part

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Memory Management

• Uses a First-Fit scheme
• Initially strictly contiguous, but later became
  dynamic
• Special areas reserved in top and bottom of
  memory
• Eg CONFIG.SYS file, with its device drivers

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Processor Management

• No multitasking (ie no multiprogramming)
• Can have a job start a child process, and then
  restart the parent later
• Interrupt handlers provide this capability
• In 5 State Modelcan only have one job in the
  run-ready-wait states
• Consequently, no processor scheduling in the
  sense that we studied
• Also, no reentrant code

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Device Management

• Requests for items on disk handled FCFS
• No special seek algorithms, which helps keep the
  OS small
• After version 3.0, spooling is supported
• No channelsone I/O device per port
• Can add device drivers, unless greater extension
  needed (BIOS flash)

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File Management

• MS-DOS supports all three kinds of file access
  Sequential, Direct, and Indexed Sequential
• File naming restricted to 8 and 3
• Remaining 21 bytes of the 32 byte directory entry
  has reserved, time, date, size and other
  information about the file
• 26 block devices allowed, from A to Z
• Character device drivers kept in a linked list!
  (eg LPT1, AUX1 )
• Hierarchical directory structure introduced
• Separator is the backslash character
• Limit of 512 files in root directory!

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MS-DOS Analysis

• It was adequate for the 16-bit machines it was
  designed for
• Not as user friendly as MAC-OS (though Windows
  3.1 later tried in vain)
• Gradually became apparent that the introduction
  of 32-bit machines would completely obsoletize
  the operating system
• Microsoft decided to introduce new operating
  systems called Windows 95 and Windows NT to
  address these problems

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A Few Words on OS/2

• OS/2 originally developed by Microsoft for IBM
• IBM took over development, turned it into a solid
  32-bit OS with many features and better
  compatibility than MS-DOS (or Win95?)
• Not popular outside IBM clients

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Windows

• Successor to MS-DOS
• Two major families
• Windows 95, 98, 98 2nd Edition, ME
• Windows NT 1.0-4.0, 2000, XP (Pro and Home)
• Basic history of each family
• Note Windows 3.1 (and earlier) are GUIs, not
  operating systems!

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Target Markets

• Windows 95 Line Home users, games, email, small
  business, Internet, MS-DOS
• Windows NT Line Business hardware, client-server
  environments, government security and procurement
  requirements, distributed computing support
• Both needed multiprocessing, portability

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Windows NT

• Design goals included extensibility, portability,
  reliability, compatibility, performance
• NT kernel and user modes
• Written in C/C
• APIs

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Windows 95

• Big goal was to provide backward compatibility
• NOT backward compatible with OS/2!
• Does support MS-DOS, Windows 3.x, Win32, and
  Win32s APIs

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MemoryWindows NT

• NT implements virtual memoryvariation on Demand
  Paging
• 4 Kbyte pages and 32-bit virtual addresses
• VM Manager with reserve and commit
• Localized FIFO page replacement policy
• Pages clustered for program start-up swap
• OS resides in top of memory
• NT Executive parts may be swapped out, but not NT
  Kernel (now an override with Customizer XP)
• No direct process memory control

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MemoryWindows 95

• Demand paged system with 4 Gigabyte linear
  address model
• No support for non-Intel platforms!!
• Simpler VM Manager
• LRU Page Replacement policy

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Processor ManagementWindows NT

• Object oriented processes
• Threaded codethreads generated in WIN32 API
• Relationships among processes not maintained
• Preemptive symmetric multitasking environment
• Windows NT recognizes 32 priority levels
• Real-time threads 16-31, time-sharing 1-15
• Priorities increase for I/O (disk or input
  device), decreased for using up quantum
• Round robin scheduler, complex synchronization
  mechanisms

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Processor ManagementWindows 95

• Preemptive multitasking environment with
  threading
• Round robin scheduler20 millisecond time slices

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DevicesWindows NT

• Had to support multiple external file systems,
  including FAT, FAT32, POSIX, OS/2, and more
• Layered interaction with devices for security
  reasons
• Support for UPS systems

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DevicesWindows 95

• Allows more direct access to the hardware through
  DirectX
• Introduced plug-and-play

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File Management

• Windows NT has its own NTFS file system, but
  supports other file systems
• FAT, HPFS, CDFS
• Supports virtual I/O
• Windows 95 bears some similarities to NT, but
  runs FAT32

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Networking

• Windows NT was designed (in part) as a OSI-based
  Network Operating System
• Windows 95 can provide support similar to NT for
  client-server environments, but with less
  security
• Support for NetBEUI, TCP/IP, IPX/SPX, DECnet,
  AppleTalk, and others

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Summary of Windows

• Many similarities between Windows families
  despite different target markets
• Windows XP folds together the two families of
  Windows operating systems
• Based on NT technologies
• Compatibility functions allow older Win95 and
  Win98 software to run (albeit there are problems!)