Transcript Ch. 12 MS-DOS Operating System

Ch. 12 MS-DOS Operating System
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History
Design Goals
Memory Management
Processor Management
Device Management
File Management
User Interface
Additional Commands
Understanding
Operating Systems
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MS-DOS, PC-DOS or DOS
• Developed to run single-user, stand-alone desktop
computers.
– Exemplifies early OS because it manages jobs
sequentially from single user.
• Advantages: fundamental operation & straightforward
user commands.
• Disadvantages:
1. Lack of flexibility & limited ability to meet needs of
programmers & experienced users.
2. Written for a single family of microprocessors (Intel
family of chips: 8086, 8088, 80186, and 80286).
Understanding
Operating Systems
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History of DOS
Version
1.0
1.1
2.0
3.0
Release
date
1981
1982
1983
1984
3.1
3.2
3.3
4.0
5.0
6.0
6.22
1984
1986
1987
1988
1991
1993
1994
Understanding
Operating Systems
Features
CP/M compatible; supported only 1 directory
Allowed double-sided 5¼ inch disks
Eliminated some defects in version 1
Increased memory requirement to 36K,
supported PC/AT
First release to support networking
Supported token ring and 3½ inch disks
IBM PS/2 computer
Supported hard disks larger than 32 megabytes
Better use of extended memory
Better use of conventional memory
Provided users with capabilities previously
available only as third party applications
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Design Goals
User
COMMAND.COM
DOS Kernel
BIOS
Hardw are
Understanding
Operating Systems
MS-DOS
• Accommodate single novice
user in single-process
environment.
• Standard I/O support includes
keyboard, monitor, printer, &
secondary storage unit.
• User commands are based on
English words/phrases
indicative of action to be
performed.
• Commands are interpreted by
command processor.
• Layering approach “protects”
user from hardware.
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First 2 DOS Layers : BIOS & Kernel
1. BIOS (Basic Input/Output System) -- interfaces directly with
various I/O devices.
– Device drivers (control flow of data to/from each device).
– Receives status info about success/failure of each I/O operation &
passes it on to the processor.
2. DOS kernel -- routines needed to interface with the disk drives.
• Read into memory at initialization time from MSDOS.SYS file on
boot disk.
• Accessed by application programs.
• Provides collection of hardware-independent services, such as
memory management, & file & record management (system
functions).
Understanding
Operating Systems
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Third DOS Layer : Command
Processor
3. Command processor (shell) -- sends prompts to user,
accepts commands, executes commands, & issues
appropriate responses.
– Resides in a file called COMMAND.COM, which consists of 2
parts stored in 2 different sections of main memory.
– Not interpretive.
– MS-DOS Version 4.0 -- menu-driven DOS shell.
– OS/2 -- designed to replace MS-DOS.
• In its heyday, MS-DOS ran enormous collection of
software packages making it difficult to discontinue.
Understanding
Operating Systems
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DOS Memory Manager
• Relatively simple job because it’s managing single job for
single user.
– Uses a first-fit memory allocation scheme since it is
most efficient strategy in a single-user environment.
• Main memory:
1. Read Only Memory (ROM) -- very small in size &
contains program, section of BIOS, with sole task of
starting up system (bootstrapping).
– Retrieves remaining resident OS & loads it into RAM.
2. Random Access Memory (RAM) -- where programs are
loaded and executed.
Understanding
Operating Systems
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Reserved for BIOS
Unused
Transient part of
Command.COM
Transient program
Area (user memory)
1M
640K
RAM Layout for
1MB of Memory
TSR programs
Resident part of
Command.COM
Installable Drivers
Buffer Cache
MS-DOS Kernel
BIOS Interface
Interrupt Vectors
Understanding
Operating Systems
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DOS Main Memory Allocation
• First versions had simple contiguous memory allocation
scheme that gave all available memory to resident
application program.
– Applications couldn’t dynamically allocate memory blocks.
– Version 2.0, MS-DOS support dynamic allocation, modification, &
release of main memory blocks by applications.
• Amount of memory each application owns depends on
type of file from which program is loaded & size of TPA.
– Programs.COM -- given all of TPA, whether or not they need it.
– Programs.EXE -- given amount of memory they need.
• Except for COM files, there can be any number of files in
TPA at one time.
Understanding
Operating Systems
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Memory Block Allocation
• Allocates memory by using first-fit algorithm & linked list
of memory blocks.
• With Version 3.3, use best-fit or last-fit strategy.
– With last-fit, allocates highest addressable memory
block big enough to satisfy program’s request.
• Size of a block can vary from 16 bytes (paragraph) to
maximum available memory.
Understanding
Operating Systems
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First 5 Bytes of Memory Block Define
Block’s Structural Characteristics
Byte
Contents
byte 0
ASCII 90h if it’s the last block, or ASCII 77h if it’s not.
bytes 1–2
Includes the number zero to indicate a busy block and the pointer to the
Program Segment Prefix (PSP) that is created by the EXEC function when
the program is loaded.
bytes 3–4
Gives the number of paragraphs contained in the block.
Understanding
Operating Systems
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Free/Busy Block List
• Whenever request for memory comes in, DOS looks
through free/busy block list to find free block that fits.
– If list becomes disconnected, system stops & must be
rebooted.
• Well-designed application program releases memory block
it no longer needs.
Busy
Free
Busy
Free
Busy
Block 1
Block 2
Block 3
Block 4
Block 5
Understanding
Operating Systems
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Process Management
• MS-DOS wasn’t written in reentrant code because it was designed for
a single-user, single-task environment.
– Reentrant code is basis for multitasking.
– Programs can’t break out of middle of DOS internal routine &
restart routine from somewhere else.
– There's no interleaving & no need for sophisticated algorithms or
policies to determine which job will run next or for how long.
• Some software programs give illusion that two jobs can run together.
– E.g., Microsoft Windows versions 1.0 to 3.0.
– Possible because MS-DOS interrupt handlers give programmers
capability to save all info about parent program that allows its
proper restart after child program finishes.
Understanding
Operating Systems
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Interrupt Handlers Are Responsible
for Synchronizing Processes
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PC has 256 interrupts & interrupt handlers, & they are accessed via
interrupt vector table residing in lowest bytes of memory.
1. Internal hardware interrupts -- generated by certain events occurring
during program’s execution (e.g., division by zero).
2. External hardware interrupts -- caused by peripheral device
controllers or by coprocessors & assigned by manufacturers.
3. Software interrupts -- generated by system & application programs to
access DOS & BIOS functions, which, in turn, access system
resources.
– Terminate and Stay Resident (TSR) -- a process without
releasing its memory, thus providing memory-resident
programming facilities.
Understanding
Operating Systems
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Device Management
• Ability to reorder requests to optimize seek & search time is not
feature because it’s designed for a single-user environment.
– All requests are handled on a first-come first-served basis.
– Version 3.0 BIOS can support spooling so users can schedule
several files to be printed one after the other.
• MS-DOS written for simple systems using keyboard, monitor, printer,
mouse, 1-2 serial ports, & maybe second printer.
– Use one of each type of I/O device for each port, so no device
channels.
– Devices do not require special management from OS.
– Device drivers are only items needed by Device Manager to make
system work.
Understanding
Operating Systems
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File Name Conventions
• File name consists of :
– Drive name (usually A, B, C, or D) is followed by a colon (:).
– Directories/subdirectories are 1-8 characters long & preceded by
back slash (\).
– Primary file name is 1-8 characters long.
– Period.
– Extension is 1-3 characters long with possible special meaning.
• Relative name --primary name & extension, if used.
• Absolute name -- drive designation & directory location (path)
followed by its relative name.
Understanding
Operating Systems
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Managing Files
• Earliest versions of MS-DOS kept every file in a single
directory.
• Version 2.0 implemented hierarchical directory structure.
– Inverted tree directory structure.
• When disk is formatted its tracks divided into sectors of
512 bytes.
• 2-8 sectors are grouped into clusters & that's how File
Manager allocates space to files.
• Supports hidden files—files that are executable but not
displayed in response to DIR commands.
Understanding
Operating Systems
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FORMAT Creates Three
Special Areas on Disk
1. Boot record -- first sector of every logical disk & contains
disk boot program & table of disk's characteristics.
2. Root directory -- where system begins its interaction with
user. Lists system’s primary subdirectories & files (name,
extension, size, date & time of modification, starting
cluster #, file attribute codes).
3. FAT (File Allocation Table) -- contains status info about
disk’s sectors: which are allocated, which are free, and
which can’t be allocated because of formatting errors.
Understanding
Operating Systems
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Managing Files in DOS
• Looks at data in a disk file as a continuous string of bytes.
– I/O operations request data by relative byte (relative to beginning
of file) rather than by relative sector.
– Transformation from physical sector to relative byte address is
done by File Manager.
• Supports noncontiguous file storage & dynamically allocates disk
space to a file, provided there’s enough disk room.
• Compaction (MS-DOS Version 6.0) available via utility used to
defragment disk (DEFRAG.EXE).
• CHKDSK command used to determine need for compaction.
• Restricting user access to computer system & resources isn’t built into
MS-DOS.
Understanding
Operating Systems
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User Interface
• Command-driven operating system.
• When user presses Enter key, shell (COMMAND.COM)
interprets command & calls on next lower level routine to
satisfy request.
• User commands include some or all of these elements in
this order:
command source-file destination-file switches
Understanding
Operating Systems
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Command
DIR
CD or CHDIR
COPY
DEL or ERASE
RENAME
TYPE
PRINT
DATE
TIME
MD or MKDIR
FIND
FORMAT
CHKDSK
Stands for
Directory
Change Directory
Copy
Delete
Rename
Type
Print
Date
Time
Make Directory
Find
Format Disk
Check Disk
PROMPT
DEFRAG
(filename)
System Prompt
Defragment Disk
Understanding
Operating Systems
Action to be performed
List what’s in this directory.
Change working directory.
Copy a file. Append one to another.
Delete the following file or files.
Rename a file.
Display text file on screen.
Print one or more files on printer.
Display and/or change system date.
Display and/or change system time.
Create a new directory or subdirectory.
Find a string. Search files for a string.
Logically prepare disk for file storage.
Check disk for disk/file/directory
status.
Change system prompt symbol.
Compact fragmented files.
Run, execute, file.
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Batch Files & Redirection
• By creating customized batch files, users can quickly
execute combinations of DOS commands to configure
their system, perform routine tasks, or make it easier for
non-technical users to run software.
• MS-DOS can redirect output from one standard input or
output device to another.
command > destination
Understanding
Operating Systems
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Filter Commands (SORT, MORE)
• Filter commands accept input from default device,
manipulate data in some fashion, & send results to default
output device.
• SORT accepts input from keyboard, sorts that data, &
displays it on screen.
– Sort the file by column.
• MORE causes output to be displayed on screen in groups
of 24 lines, one screen at a time, & waits until user presses
Enter key before displaying next 24 lines.
Understanding
Operating Systems
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Pipe
• Cause standard output from one command to be used as
standard input to another command.
– Symbol is a vertical bar, |.
– Alphabetically sort directory & display sorted list on screen:
DIR | SORT
• Combine pipes and other filters.
TYPE INVENTRY.DAT | MORE
MORE < INVENTRY.DAT
DIR | SORT | MORE
DIR | SORT > SORTFILE
MORE < SORTFILE
Understanding
Operating Systems
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Other Commands
• FIND -- filter command that searches for specific string in
given file or files & displays all lines that contain string
from those files.
• PRINT -- allows user to set up series of files for printing
while freeing up COMMAND.COM to accept other
commands.
Understanding
Operating Systems
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