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G53OPS
Operating Systems
Graham Kendall
Operating Systems
G53OPS Operating Systems
Introduction to Operating Systems
• This section of the course based on (Tanenbaum,
1992) Chapter 1
• Module Content
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What is an Operating System?
History of Operating Systems
Operating System Concepts
Operating System Structure
• References
– Levy, S. 1994. Hackers.
– Tanenbaum, A., S. 1992. Modern Operating Systems. Prentice
Hall.
G53OPS Operating Systems
Introduction to Operating Systems
• Give three functions of an operating
system
G53OPS Operating Systems
What is an Operating System? - 1
Word Processor
Spreadsheet
Accounting
Compilers
Editors
Command Interpreter
Application
Programs
Systems Programs
Operating System
Machine Language
Microprogramming
Hardware
Physical Devices
G53OPS Operating Systems
What is an Operating System? - 2
• Physical Devices
– Integrated circuits, cathode ray tubes, wire etc.
Word Processor
Spreadsheet
Accounting
Compilers
Editors
Command Interpreter
Application
Programs
Systems Programs
Operating System
Machine Language
Microprogramming
Hardware
Physical Devices
G53OPS Operating Systems
What is an Operating System? - 2
• Microprogramming
– Firmware
– Provides basic operations that allow communication
with the physical devices
– Normally in Read Only Memory (and that is why it is
called firmware)
Word Processor
Spreadsheet
Accounting
Compilers
Editors
Command
Interpreter
Operating System
Application
Programs
Systems
Programs
Machine Language
Microprogramming
Physical Devices
Hardware
G53OPS Operating Systems
What is an Operating System? - 3
• Machine Language
– Defines the instruction set of the computer
– Software but is considered as hardware
Word Processor
Spreadsheet
Accounting
Compilers
Editors
Command
Interpreter
Operating System
Application
Programs
Systems
Programs
Machine Language
Microprogramming
Physical Devices
Hardware
G53OPS Operating Systems
What is an Operating System? - 3
• Operating System
– Layer between the hardware and the software
– Allows us to use the hardware in a user friendly way
– Layer of abstraction
Word Processor
Spreadsheet
Accounting
Compilers
Editors
Command
Interpreter
Operating System
Application
Programs
Systems
Programs
Machine Language
Microprogramming
Physical Devices
Hardware
G53OPS Operating Systems
Other Views of an Operating System - 1
• A Resource Manager
– Operating System is seen as a way of providing the
users of the computer with the resources they need at
any given time.
– Some resource requests may not be able to be met
(memory, CPU usage etc.) but the operating system is
able to deal with scheduling problems such as these.
– Other resources have a layer of abstraction placed
between them and the physical resource (e.g. a printer).
G53OPS Operating Systems
Other Views of an Operating System - 2
• Extended or Virtual Machine
– The operating system is seen as a way of not having to
deal with the complexity of the hardware (e.g. floppy
disc controller).
G53OPS Operating Systems
History of Operating Systems - 1
• Charles Babbage - Analytical Engine.
– Attributed with designing the first digital computer
– Never built the computer
– No operating system.
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Charles Babbage - 26th December
1791 to 18th October 1871
G53OPS Operating Systems
History of Operating Systems - 1
• First Generation (1945-1955)
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Developed during the war
Vacuum Tubes
Many people were developing automatic calculating machines
Filled entire rooms
No operating system
No programming languages
Had to physically wire the computer to carry out the intended
instructions
G53OPS Operating Systems
History of Operating Systems - 2
• Second Generation (1955-1965)
– Transistors
– More reliable
– Became feasible to manufacture computers that could
be sold to customers
– Job of computer operator invented
– Batch jobs introduced in order to improve throughput
(via tapes)
– Can be seen as the first operating system
G53OPS Operating Systems
History of Operating Systems - 3
• Third Generation (1965-1980)
– Integrated Circuits as a replacement for transistors
– IBM introduced its System/360 range and ICL
introduced its 1900 range
– The start of multiprogramming which led to a need for
a more complex operating system – but no virtual
memory
– Spooling – both input and output
– Concept of time sharing introduced which allows
interactive working
– Operating systems had to become a lot more complex
in order to deal with all these issues
G53OPS Operating Systems
History of Operating Systems - 4
• Fourth Generation (1980-present)
– Large Scale Integration (LSI)
– Led directly to the development of the personal
computer (PC)
– One of the requirements for the original IBM PC was
an operating system - Bill Gates supplied MS-DOS
– On non-Intel processors, the UNIX operating system
was also being used.
G53OPS Operating Systems
History of Operating Systems - 7
• Another View
– First, Second and Third generation as before and for
same reasons
– Fourth generation started in 1971 with the introduction
of LSI, then VLSI (Very Large Scale Integration) and
then ULSI (Ultra Large Scale Integration)
– Really, just arguing about when the PC revolution
started. Was it in the early 70’s when LSI first became
available? Or was it in 1980, when the IBM PC was
launched?
G53OPS Operating Systems
History of Operating Systems - 5
• Fifth Generation (Sometime in the future)
• What will constitute a fifth
generation computer?
G53OPS Operating Systems
History of Operating Systems - 5
• Fifth Generation (Sometime in the future)
– Computer generations were influenced by new
hardware (vacuum tubes, transistors, integrated circuits
and LSI).
– Fifth generation may break with that tradition
– One view is that a fifth generation computer will
interact with humans in a way that is natural to us
Computers will be able to reason in a way that imitates
humans
– Being able to accept (and understand!) the spoken word
and carry out reasoning is complex, requiring advances
in software and maybe hardware
G53OPS Operating Systems
History of Operating Systems - 6
• Fifth Generation (Sometime in the future)
• What advances will have to
be made to allow a fifth
generation computer to be
realised?
G53OPS Operating Systems
History of Operating Systems - 6
• Fifth Generation (Sometime in the future)
– Advances will be needed in AI including NLP
– Maybe need parallel processing
– Maybe a non-silicon computer
– The first time a generation has not been motivated by
advances in hardware
G53OPS Operating Systems
Case Study – 1 (ICL Mainframe Operating Systems)
• Manual Executive
– Introduced with its 1900 range
– Manual operating system
– Operators had to load and run each program
G53OPS Operating Systems
Case Study – 2 (ICL Mainframe Operating Systems)
• George 1/2/2+
– GEneral ORGanisational Environment
– Ran “on top” of manual exec - so it was not an operating system as
such
– Allowed you to submit jobs to the machine and G2+ would
schedule them
– Batch programs together into a single job
– Parameterised macros (or JCL – Job Control Language) allowed
automation of tasks
Submit many jobs at the same time so that G2+ would run them
one after another
– Adjustable scheduling algorithm (via the operators console)
– Other scheduling factors (e.g. tape decks or memory)
– Under G2+ the operators still look after individual jobs
G53OPS Operating Systems
Case Study – 3 (ICL Mainframe Operating Systems)
• George 3/4
– Operators no longer looked after individual jobs.
Instead they looked after the system
– Jobs could be submitted via interactive terminals by a
scheduling team
– Development staff could issue batch jobs and also runs
jobs in an interactive environment.
– Any problems were referred to development or
technical staff
G53OPS Operating Systems
Case Study – 4 (ICL Mainframe Operating Systems)
• George 3/4
– Operators, in some peoples opinion were little more
than “tape monkeys”
– G3 was an operating system in its own right
– To use the machine you had to run the job in a user
– The Job Control Language (JCL) was much more
extensive than that of G2+.
– It allowed interactive sessions
– It had a concept of filestore
G53OPS Operating Systems
Case Study – 5 (ICL Mainframe Operating Systems)
• VME (Virtual Machine Environment)
– Still the operating system used on ICL mainframes
today – and the foreseeable future
– VME creates virtual machines that jobs run in. If you
log onto VME, you will create a virtual machine.
– VME is written to cater for the many different
workloads that mainframes have to perform
– Job control language which (SCL) is a lot more
sophisticated
G53OPS Operating Systems
Case Study – 6 (ICL Mainframe Operating Systems)
• VME (Virtual Machine Environment)
– Still concept of filestore – but all disc based
– Amount of filestore available to users or group of users
is under the control of the operating system (and thus
the technical support teams).
– Like G3, the operators control the entire system
– Move towards lights out working.
G53OPS Operating Systems
Operating System Concepts - 1
• System Calls
• Each system call has a procedure associated
with it so that system calls can be done in a
familiar way.
• The procedures places the parameters into
registers and informs the operating system
• OS is informed via a TRAP instruction
(sometimes known as a kernel call or a
supervisor call).
G53OPS Operating Systems
Operating System Concepts - 2
• CPU switches from user mode to kernel (or
supervisor) mode
• After carrying out the work the operating
system will return a result in the same way
as a user written function written in a high
level language.
• Example of an operating system call (via a
procedure) is
– count = read(file, buffer, nbytes);
• Use system calls to manipulate objects
G53OPS Operating Systems
Operating System Concepts - 2
• Give some examples of
processes that should only be
allowed in kernel
(supervisor) mode
• Disable all interrupts
• Set the time of day
• Change the memory map
G53OPS Operating Systems
Operating System Concepts - 3
• Processes
– A running program with all the other information that is
needed to control its execution
– Needs to be able to start a process from exactly the
same point as it was left.
– Process information held in a process table.
– A process may create a child process.
– One of the main tasks of an operating system is to
schedule processes
– Processes may also communicate with other processes.
The OS must deal with this.
G53OPS Operating Systems
Operating System Concepts - 4
• Files
• System calls to (for example) create files,
delete files, move files, rename files, copy
files, open files, close file, read files, write
files.
• Files and Directories
G53OPS Operating Systems
Operating System Concepts - 5
• Example of abstraction
– Text and binary files
– Standard input and standard output
– Pipes.
• DIR | SORT
– Redirection
• DIR > dir.txt
G53OPS Operating Systems
Operating System Concepts - 6
• The Shell
• Allows the user to interact with the OS
• For example, by means of a CLI or a
graphical user interface
• Is the shell part of the OS. If
not why not?
G53OPS Operating Systems
Operating System Concepts - 6
• Is the shell part of the OS. If
not why not?
• No
• Less flexibility
• Making it a separate program allows
different users to have different shells
• In general, the more code outside of the OS,
the better
G53OPS Operating Systems
Operating System Concepts - 6
• The Shell
• Shell is not part of the operating system but
worth mentioning as it makes heavy use of
operating system features
• Example (in UNIX this time)
– cat file1 file2 file3 | sort > /dev/lp &
G53OPS Operating Systems
Operating System Structure - 1
• How is an OS structured internally (up to
now – only an external view)?
• Monolithic Systems
• No structure at all
• OS is simply a collection of procedures
• OS is constructed by compiling all the
procedures into one huge monolithic system
• But they can naturally fall into a structure
G53OPS Operating Systems
Operating System Structure - 2
Main
Procedure
Service
Procedures
Utility
Procedures
G53OPS Operating Systems
Operating System Structure - 3
• Layered Systems
– Layer 0 was responsible for the multiprogramming
aspects of the operating system. It decided which
process was allocated to the CPU. It dealt with
interrupts and performed the context switches when a
process change was required.
– Layer 1 was concerned with allocating memory to
processes.
– Layer 2 deals with inter-process communication and
communication between the operating system and the
console.
G53OPS Operating Systems
Operating System Structure - 4
• Layered Systems
– Layer 3 managed all I/O between the devices attached
to the computer. This included buffering information
from the various devices.
– Layer 4 was where the user programs were stored.
– Layer 5 was the overall control of the system (called
the system operator)
G53OPS Operating Systems
Operating System Structure - 5
• Moving through the hierarchy (from 0 to 5)
you do not worry about the layers you have
“left behind”
• For example, user programs (level 4) do not
have to worry about where they are stored in
memory or if they are currently allocated to the
processor or not, as these are handled in level 0
and level 1.
G53OPS Operating Systems
Operating System Structure - 6
• Virtual Machines
• Virtual machine?
– MS-DOS prompt from with Windows 95/98/NT
– ICL’s mainframe operating system is called VME (Virtual
Machine Environment)
• Both of these (Windows 95/98/NT and VME)
are recent developments
• VM/370 was one of the first OS’s to provide a
virtual machine to each user
• Each user was even able to run different
operating systems
G53OPS Operating Systems
Operating System Structure - 7
Virtual 370’s
Hardware
Instructions
TRAP
VM/370
370 Bare Hardware
G53OPS Operating Systems
Operating System Structure - 8
• Client-Server Model
• Server provides services to any client that
requests it
• Model is heavily used in distributed systems
– E.g. print server or database
• Operating systems can be designed as a
client/server
G53OPS Operating Systems
Operating System Structure - 9
• Benefits include
– Can result in a minimal kernel
– As each server is managing one part of the operating
system, the procedures can be better structured and more
easily maintained.
– If a server crashes it is less likely to bring the entire
machine down as it won’t be running in kernel mode. Only
the service that has crashed will be affected.
Client
Process
Client
Process
Process
Server
File Server
Kernel
Message sent from
client to server
Memory
Server
………
Server
User Mode
Kernel Mode
G53OPS
Operating Systems
Graham Kendall
End of Operating Systems