Download Design of MS-DOS

MS-DOS
• Worth looking at :
- Sets an historical perspective for modern PC-based
OS’s.
- Design Principles / Fundamentals
- Basis of many other Disk(based) Operating Systems.
• The first personal computer DOS, called Personal Computer Disk Operating System, was developed for IBM by Microsoft
Corporation.
• MS retained the rights to market a Microsoft version, called
MS-DOS. PC-DOS and MS-DOS are almost identical and
most users have referred to either of them as just "DOS."
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MS-DOS
• DOS was (and still is) a non-graphical line-oriented
command-driven operating system, with a relatively simple
but not overly "friendly" user interface.
Its prompt to enter a command looks like this (usually) :
C:\>
• Many versions - later ones fixing bugs and adding
functionality
• We will look briefly at ver 6.22
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MS-DOS V6
• Core of the OS is contained in three files :
io.sys
msdos.sys
command.com
(a hidden, protected system file)
(a hidden, protected system file)
(a protected system file) - the shell!
• io.sys
A program that provides basic facilities for handling the
computers input and output devices (e.g. keyboard and vdu)
Stored in the root directory of the boot drive.
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MS-DOS V6
• msdos.sys
The central DOS program that controls communication
between the internal microcomputer components.
Stored in the root directory of the boot drive.
• command.com
The shell or command interpreter.
It executes / interprets all commands entered at the keyboard
and directs them to the appropriate internal component (CP,
Disk, RAM etc..)
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MS-DOS V6
• The remainder of DOS is made up of its many utility programs.
These programs can be divided into two groups :
- Internal Commands
- External Commands
• Internals command utilities are part of (and are implemented in)
the command.com file.
EG : DIR, MD, CD etc….
• External DOS utilities are separate programs (files) with .EXE
or .COM extensions.
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MS-DOS Bootup
1. Power up machine - POST runs.
CPU checks circuitry (BIOS error checking) and looks for a
ROM.
2. Bootstrap instructions contained on ROM are executed.
3. Bootstrap program loads io.sys and then msdos.sys .
This step is the main operating system load (into memory)
If this step is successful, the next step: Dos Configuration is
invoked.
4. CPU now looks for a configuration file called config.sys .
If this file is present, the commands it contains are executed.
Typical commands include device driver settings and
memory manager program.
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MS-DOS Bootup
4. (cntd…)
Next the systems looks for command.com (the command
interpreter).
Once command.com is loaded, the machine is ready to
perform its duties.
It next looks for (and executes) a file called autoexec.bat, (a
file containing a batch of statements to be executed
automatically at system start-up).
• Note that both config.sys and autoexec.bat are machine
specific user-editable files.
They both reside as normal files on the root of the boot drive
(c:\.)
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MS-DOS Design
• In fact, once command.com has executed, the system is ready
to process any valid user request.
• The “readiness” is presented to the user as a “command
prompt” e.g. : c:\ or a:\
Exercise :
Observe the boot sequence of my machine.
Note, I am booting from a floppy disk with
MS-DOS V6.22 installed on it.
Demonstration of commands, program
execution, memory mgt etc…!
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