OPERATING SYSTEMS 1.2.1

OPERATING SYSTEMS 1.2.1

1. FUNCTION & PURPOSE OF OS

   • OS are software that control the hardware of the system
   • allows other software to be installed & run
   • provides a user interface & helps to control system security
   • eg. windows, iOS

2. MEMORY MANAGEMENT

   • each different program will be using its own data - w/out MM one program could change the data of another
     • thus the OS will restrict each program to access/amend its own area of data only
   • OS manages memory in a computer
   • ensures programs can't access each others memory unless legitimately required to
   • ensures no space is wasted by breaking programs into chunks
   • SEGMENTATION
     • is the dividing of memory into logical units
   • PAGING
     • is dividing of memory into physical units
   • VIRTUAL MEMORY
     • is the use of secondary storage as an extension of a computer's physical memory
     • when a system is running low on physical memory (ie. RAM), it is able to use an area of the hard disk as VM
     • when OS believes a page is not likely to be needed in near future, it is moved from RAM to VM
     • if done often, it will significantly slow system down
       • referred to as DISK THRASHING

3. INTERRUPTS

   • when a device sends a signal to the processor to get attention
   • an interrupt will have a priority indicating how urgently it requires attention
   • when an interrupt is raised, the OS runs the relevant interrupt service
   • INTERRUPT SERVICE ROUTINES (ISR)
     • each interrupt has a priority level // if its priority is higher than the process currently being executed needs to be serviced first
     • OS has ISR that determine what happens when a particular interrupt is carried out
     • at the end of each f-d-e cycle, processor checks to see if there are any interrupts
     • if there are & they are of a higher priority than current task, following steps are carried out:
       • 1. the contents of PC & other registers are copied to area of memory called stack
       • 2. the relevant ISR can then be loaded by changing PC to value where ISR starts in memory
       • 3. when ISR = complete, previous values of PC & other registers can be restored from memory to CPU

4. SCHEDULING

   • multi-tasking OS need to make sure that multiple processes can run alongside each other, simultaneously - thus OS needs to carry out scheduling
   • SCHEDULER
     • a program that manages amount of time different processes have in CPU
   • ROUND ROBIN
     • each process is given fixed amount of time - if it hasn't finished by end of that time period, it goes to back of queue so next process in line can have its turn
     • most efficient way because it gets round to all processes
   • FIRST COME FIRST SERVED
     • first process to arrive is dealt w/ by CPU until it is finished; meanwhile, any other processes that come along are queued up for their turn
   • MULTI-LEVEL FEEDBACK QUEUES
     • uses a number of queues. each of these queues have a different priority // algorithm can move jobs between these queues depending on jobs' behaviour
   • SHORTEST JOB FIRST
     • this picks the job that will take the shortest time & run it until it finishes
       • algorithm will need to know time each job takes in advance
   • SHORTEST REMAINING TIME
     • scheduler estimates how long each process will take - it then picks one that will take least amount of time & runs that
     • if a job is added w/ shorter remaining time, scheduler is switched to that one

5. TYPES OF OS

   • MULTI-TASKING
     • run multiple programs simultaneously
     • possible because of the speed processors work at
   • DISTRIBUTED
     • allow multiple computers to work together as a single system
   • EMBEDDED
     • built into a device, often w/ limited resources e.g. comps built into tv set-top boxes, cars, wash9ing machines etc.
     • likely only have one job & is not likely to have a need for multi-tasking
     • some but not all) embedded devices run on an embedded OS which are specifically designed for the device & will run w/ efficiency in mind i.e. operate on low-powered CPUs w/ little RAM
   • REAL-TIME
     • guaranted to carry out actions w/in given period of time, usually w/in small fraction of a 2nd
     • safety critical systems will often run on real-time OS
   • MULTI-USER
     • allow more than 1 user to access a computer's resources at one time - common on mainframe comps where there may be many users accessing them simultaneously

6. BIOS

   • stands for 'basic input/output sysytem'
   • when a comp is 1st switched on, it looks to BIOS to get it up & running, & so processor's program counter-points to BIOS's memory
   • will usually 1st check that comp is functional, memory = installed & accessible & processor = working
     • this is called the POWER-ON SELF-TEST (POST)
     • once it has done this, it can use boot loader program to load the OS kernel into memory
   • usually stored on flash memory so that it can be updated; this also allows settings such as boot order of disks to be changed & saved by user

7. DEVICE DRIVERS

   • piece of software, usually supplied w/ a device, that tells OS how it can communicate w/ the device
   • OS are expected to communicate w/ wide variety of devices, each w/ different models & manufacturers

8. VIRTUAL MACHINES

   • it is possible to write a program that has same functionality as a physical comp
   • is a program that acts as though it were a comp
   • USES
     • to run an OS inside another e.g. when testing programs' compatibility on multiple platforms
     • to run intermediate code
       • this intermediate code can then be run off any comp w/ a VM
         • compiler will produce byte code from source code which can then be read from VM
           • any device w/ this VM can read this intermediate code