Operating System Fundamentals

Operating Systems have 2 fundamental purposes (to users and programmes) :
* controlling the hardware.
* to create services for the user to manipulate the hardware.

Operating Systems are built up of 3 parts:
* Kernel - core loaded after boot-proccess
* Shell - User Interface.
* File System - Standard for Disk formatting.

An operating system provides three basic kinds of services:
* runs commands given by user.
* executes programs.
* manages the hardware resources.

Some of the services provided by the operating system are:
* interfaces to run commands for user and applications.
* file and I/O support services.
* boot-up.
* tools and services for concurrent processing.
* allocation of hardware resources.
* program control services to protect users and programs from each other and
provide communication between them.

Operating System types:
* MS-DOS - single-user, single-tasking.
* MS Windows - single-user, multitasking.
* Unix - multi-user, multitasking.
* Distributed Systems

Operating System Categories:
* batch processing systems - user submits program as part of job, system runs it
without interaction.
* soft real-time systems - soft contraint on task completion time. Processing
Transactions.
* hard real-time systems - Very tight deadlines, if deadline not met then
application dead.
* online, interactive systems - user interacts directly with the program to
provide input data and guidance as it is executing.

Basic Single-job Operations(e.g MS-DOS):
* programs loaded and sun serially.
* all memory not used for OS open for application running.
* system calls pause software until system call is complete.
* when software runnign software guaranteed all resources.

Concurrent Operations:
* multitasking/multiprogramming is using multiple programs on one CPU.
* Operating System acts as master contorl unit.
* while one program is waiting for I/O, another program can run.
* time slicing is using a clock interrupt to switch to another program.

User Interface and Command Execution Services:
* The User Interface is not considered part of the Operating System -
only a link to it.
* Most OS's only use one shell, some such as unix use multiple
* critical parts(Kernel) of OS always in memory.
* other parts are loaded when needed.
* most Operating Systems have facilities ot store scripts (batches/shell scripts).
* CLI = commabnd line interface.
* GUI = Graphical User Interface.

File Management:
* mapping to save physical location of files.
* keeps track of file's requirements and free-space.
* maintains directory structures for them all to be stored and retreived (as name).
* allows movement and manipulation of files.
* information of every file.

I/O Services:
* driver allows OS to control hardware.
* drivers control transfers
* I/O Services can be fixed and in ROM or loaded as required in RAM.
* devices can be added and removed as the system configuration is changed.
* drivers are either automatically added and removed, or manually.

Process Control Managment:
* a process is a program in execution.
* even OS programs are all treated as seperate processes.
* when program initiated, new process is created and assigned resources.
* processes can send each other messages.
* process control management determines if each process is running, ready to run,
or blocked..
* processes are made-up of threads which share the processes' resources.
* sometimes on thread cna be run seperately fro mother threads

Memory Management:
* main purpose is to give each program the memory required for execution.
* insures each process is using its own resources only and allocating more
when required.
* maintains list of programs waiting to enter memory when space is available.
* allocates free space to processes needing it and free's a terminated programs
resources.
* some memory management techniques cause fragmentation.
* fragmentation reversed by compaction.

Scheduling:
* only needed in multitasking systems.
* allocates cpu time to maximise efficiency per system spec.
* high-level scheduling = admitting processes
* dispatching = selects between threads rather than processes.
* some processes are cpu intensive where others are I/O intensive.
* in a non-preemptive system a process is only replaced when it becomes
blocked(in waiting).
* in a preemptive system uses timeslices, each slice is signaled by fixed amount of
time signalised by clock interrupts.

Secondary Storage Management:
* manages I/O requests given from processes.
* also organises the requests in fastest order to gain best performance.

Security and Protection Services:
* provides protection from other processs to processes.
* all services must be requested by the process from the OS.
* username and password services permit acces to specific files.

Network and Communication Support Services:
* services aid OS's to performs naturally in distributed systems.
* primitive systems only permit file transfers.
* sophisticated systems allow remote logins and process execution, automatic file
location e.t.c over the network.

System Administration Support:
* add and remove users modifying priviledges.
* upgrades and installations e.t.c.
* system generation must configure deice drivers for the physical system and giving
parameters to the OS to modify system modules and/or setting concurrency limits of
users.