An Overview of Microsoft Disk Operating System

1
An Overview of Microsoft Disk Operating System
2
Introduction to MS-DOS

• An operating system is a set of interrelated
  programs that manage and control computer
  processing.The Microsoft Disk Operating System,
  MS-DOS, is a traditional microcomputer operating
  system that consists of four major components.
•         The Operating-system loader
• - It brings the operating system
  from the startup into RAM.
• - Bootstrapping
•         The MS-DOS BIOS
• - BIOS stands for Basic
  Input/Output systems
• - Loaded from the file IO.SYS
  during initialization.
• - Layer that sits between
  operating system kernel and hardware.
•         The User Interface (shell)
• - Conventional program that
  allows the user to interact with the operating
  system.
• - Default MS DOS user
  interface is a shell program called Command.com.
  
• The MS-DOS Kernel
• - Heart of the operating system
• - it is a proprietary program supplied
  by Microsoft corporation.

3
MS-DOS Kernel

• The kernel provides a collection of
  hardware-independent services called system
  functions.
• - File Management
• - Memory Management
• - Device Input and Output
• - Process control

4
File Management

• Block Devices are accessed on a sector basis. The
  MS- DOS kernel through the device driver sees a
  block as a logical fixed size array of sectors.
  the device driver in turn translates the logical
  sector requests from the MS DOS into physical
  locations on the block device.
• MS DOS file system is divided into four parts
• - Boot Sector
• - Always at the beginning of a
  partition.
• - Contains OEM identification, a
  loader routine , and a BIOS parameter block.
• - File Allocation Tables (FAT)
• - Provides a map to the storage
  locations of files on the disk by indicating
  which
• clusters are allocated to each
  file.
• - Second copy of FAT as back up.
• - Free cluster is found by
  scanning the FAT for a zero value.
• - Root directory
• - Root directory entries are 32
  bytes long.
• - Each entry includes a filename
  and extension, size, starting FAT entry, the time
  and date the file was created and the files
  attributes.
• - Files Area
• - Contains subdirectories, file
  data and unallocated clusters.
• - The area is divided into fixed
  size clusters and the use for a particular
  cluster is specified by the corresponding FAT
  entry

5
Memory Management

• Based on a pool of variable sized memory blocks.
• Two basic functions are to allocate a block from
  the pool and to return the allocated block to the
  pool.
• MS-DOS compatible personal computers can be
  fitted with three kinds of RAMs
• Conventional Memory
• Expanded Memory
• Extended Memory
• Conventional Memory
• - Up to 1MB of Memory is directly
  addressable.
• - Physical addresses for references are
  generated by a 16-bit segment register combined
  with 16-bit offset.
• - Out of 1MB MS-DOS occupies 640KB of
  the conventional memory.
• - This 640KB is divided into three zones
• - The interrupt vector table
• - Occupies lowest 1024 bytes
  of memory.
• - Its address and length are
  hardwired into the processor and cannot be
  changed.
• - The operating system area
• - Begins immediately above
  the interrupt vector table.
• - Holds the operating system,
  its tables and buffers, and device drivers.
• - Amount of memory occupied
  varies from version to version of MS-DOS.
• - The transient program area

6
How MS-DOS supports Conventional Memory
Management

• The MS-DOS kernel supports three memory
  management functions, invoked with interrupt 21H,
  which operate on the TPA
• - Function 48H (Allocate Memory
  block)
• - Function 49H (Free Memory block)
• - Function 4AH (Resize Memory
  block)
• The memory manager can use any of three
  allocation strategies
• - First fit the arena entry at
  the lowest address that is large enough to
  satisfy the request.
• - Best fit the smallest available
  arena entry that satisfies the request,
  regardless of its position.
• - Last fit the arena entry at the
  highest address that is large enough to satisfy
  the request
• Default approach used by MS-DOS is First-fit
  approach.
• Expanded Memory
• - To circumvent the 1MB limit of
  conventional memory, expanded memory was
  designed.
• - As much as 8MB of expanded memory
  can be installed in a single machine.
• - Made available to the application
  software in 16KB pages.
• Expanded Memory Manager
• - Provides an interface between
  application programs and expanded memory.
• - Divided into Driver and Manager.
• - Manager controls the status,
  allocation, mapping and deallocation of expanded
  memory.

7
Device Input and Output

• MS-DOS recognizes two types of devices block
  devices e.g. floppy disk or fixed disk drives
  and character devices, e.g keyboard, display,
  printer, and communication ports.
• MS-DOS identifies each block device by a drive
  letter assigned when the devices controlling
  software, the device driver, is loaded. A
  character device on the other hand, is identified
  by a logical name built into the device driver.
• Distinction between character and block devices
  is that the MS-DOS always adds new block device
  drivers to the tail of the driver chain but adds
  new character device drivers to the head of the
  chain.
• Techniques to access character devices
• - Handle type function calls
• A handle is a 16 bit number
  returned by the operating system whenever a
  device is opened or created.
• - Character input and output
  functions
• Interrupt 21H functions 01H to
  0CH. These functions are designed to communicate
  directly with the keyboard, display, printer and
  serial port.
• Process Control
• Process, or task, control includes program
  loading, task execution, task termination, task
  scheduling and intertask communication.
• MS-DOS is not a multitasking operating system ,it
  is a single tasking operating system.