Transcript History of Operating Systems

Operating systems history & principles
This lesson includes the following sections:
• History of OS Development
• Process Allocation
• Memory Management
• Scheduling
• Resource Allocation
• The File System
A history of operating systems development
An operating system is the most important and most
complex piece of software accompanying a computer.
The effort in design and development of an operating
system is comparable to that of the computer
hardware.
The modern operating system is a composite of systems
designed since the late 1940s and to date one can
delineate four generations (each with significant
milestones) of operating systems.
First generation OS (circa 1945-55)
There was no OS, and very little support software!
• machine operation was hands on, time was blocked out
for operating the machine.
• programmers were physically in the room, with paper
tapes, punch cards etc.
• loading programs was done with buttons at the console.
• difficult, arduous process and “programmers” were
highly trained professionals.
Very inefficient use of expensive (~$1M) equipment. The
majority of time was spent analysing and thinking.
human thinking time = idle machine
Second generation OS (circa 1955-65)
To keep machines busy, batch operating systems were
developed.
• programs handed (on cards) to an operator.
• several programs were grouped into a batch, then
translated onto tape.
• tape would then be loaded onto the computer.
• programs run singularly, with results written to tape.
• output tape was then translated to printed form.
Second generation OS - pluses & minuses
“turn around” time was hours to days - even to find a
syntax error! Programmers saw this as cumbersome.
However, system utilization increased dramatically.
• No programmer set up or “thinking” time.
• As one job terminated, the computer would load the
next job in the batch and begin execution.
Since programmers were separated from the machine,
job control languages were developed to specify to the
operating system the operations to perform on their
program.
Third generation OS (circa 1965-85)
The use of integrated circuits dramatically improved
computer speeds. Batch systems ran “continuously”,
but only one job at a time.
If a job was paused (to execute an IO process) the CPU
was idle, if only for a few milliseconds.
The same philosophy that led to 2nd generation systems
held - how to make computers work continuously?
Many milestone concepts were developed for third
generation (multiprogramming) operating systems.
multiprogramming OS
supported many user programs simultaneously in memory.
when a running job paused for IO, one of several ready
jobs would be executed.
system utilization further improved
efficient allocation of resources became very important.
memory protection became very important.
time-sharing, interactive communication with the
operating system by many users, was developed.
Fourth generation OS (circa 1985- )
Small, cheap PCs resulted in a move away from a
centralized environment to a distributed environment.
Shared peripherals, email and networks lead to new
operating systems, ones that supported local computation
and remote access (for users and resources)
Network operating systems
Network OS
Manages all the resources of the single computer and
those of the telecommunications network (LAN).
Users could use the computers as usual with the OS
providing all of the services previously described.
Users could also access many shared network services, as
though they were local.
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file servers
print servers
mail servers
compute servers
Operating system modules
There are several modules to a modern operating system,
each responsible for a separate task, and together make
for the safe and efficient execution of the computer
system and the application software.
A modern multi-programming, time-sharing operating
system needs to perform many functions, and some of the
larger and more important modules are explained briefly
here.
Process allocation - the dispatcher
The OS is responsible for initiating and coordinating
the processes (program execution) that must be
performed.
The existence of a number of processes implies the CPU
must be shared amongst them. It keeps a queue of
programs to be executed, and as the CPU is freed, a
new process is allocated to it.
There are three possible states of a process, running
(being executed), ready (executable and awaiting CPU
time), blocked (unable to proceed, until the completion
of some task).
Process allocation - CPU time slices
Equitable time sharing of the CPU is achieved by
allocating a quantum of time to a process for its
continuous execution. Time slices is ~0.1 sec.
After a quantum is used up, the process moves from a
running to the ready state, yielding the CPU to some
other ready process.
When is dispatcher invoked?
• a running process becomes blocked
• the quantum of a running process expires
• a running process terminates
Dispatcher sometimes considers process priority.
Memory management - the security guard
Has three major aspects
1 the allocation of sufficient memory for the program
and its associated data.
2 protection of allocated memory
3 utilisation of memory
A program will possess program addresses starting at 0.
Program addresses differ from the memory addresses in
which the object program eventually resides.
Mapping between addresses is done during execution
using information maintained by the operating system.
Scheduling - the efficiency expert
Scheduler module is responsible for initiating processes
and takes account of:
• amount of required resources
• amount of resources currently available
• priority of the job
• the length of the waiting time.
The scheduler maintains the information about resources
currently available, and updates this as resources are
allocated and released.
Resource allocation
(1) static allocation: All resources required by a process are
allocated at initiation, and when the process is complete,
the resources are reclaimed (a process can be initiated only
when all the required resources are available).
(2) dynamic allocation: Resources are allocated to a
process as required, and are released when the process
doesn't need it any more (a process may be initiated at any
time, but it may become blocked if the resource is
unavailable).
Dynamic allocation leads to better resource utilisation but
action must be taken to avoid deadlock (ie. two processes
each hold a resource which is required by the other.)
Deadlock resolution - the traffic officer
Deadlock is analogous to a gridlock traffic jam, where
opposing streams of traffic are trying to cross each
other’s path - each occupies the space required by the
other.
Program A, needs disk drive, laser printer, prints file
Program B, needs laser printer, disk drive, prints file
If the OS satisfies the first request of each program, then
both are blocked awaiting the release of resources.
Effective deadlock resolution algorithms can be complex.
A simple prevention algorithm is, if a resource is not
available, give up ALL your resources and issue new
requests.
The file system
Computer systems need to store information for long
periods, eg. programs, its own OS etc. Secondary storage
is typically a magnetic disk.
Information is usually stored in the form of a file.
OS responsibility for files are typically:
• creation, deletion of files
• provision of access to files
• secondary storage management
• protection from unauthorised access
• protection against loss and corruption.
A user is not concerned with the physical location of the
file, so it is normally accessed through a name.
The file system
The OS maintains a directory of the names of files and
their corresponding locations on the storage media. These
are further broken into a system master directory, and
several individual user directories.
The Master directory contains a list of (allowed) user
names, and the locations of user directories.
The User directory contains, for each “file”, the file
name, its physical location, its size, its access control
information, other administrative information such as
creation and last modification dates.
What else does it do?
The OS has many other responsibilities, such as IO
processing, maintaining the system clock, recovery from
power failure, access security and literally dozens of other
tasks - some large, some trivial but all essential to a
functioning computer system.