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37 Cards in this Set
- Front
- Back
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Define operating systems |
The software that managers the computer and controls the computers basic functions such as process scheduling and controlling peripherals |
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What is the OS responsible for? |
The OS is responsible for managing the computers resources. It does this by splitting the OS into layers: -memory manager -responsible for splitting RAM into chunks and assigning them into different processes based on need -process manager/scheduler - responsible for managing processes, dealing with multitasking and deciding the order in which processes will run -file manager - responsible for managing the file systems of secondary storage |
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Define the kernel |
The core of the OS that provides basic services for other parts of the OS, such as handling communication between hardware and software |
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Define paging |
Fixed blocks of memory are allocated to memory. These blocks of memory are known as pages and are a physical method of memory allocation |
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Define segmentation |
Memory is logically split into variable sizes so processes are allocated the exact amount of memory required |
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What is an advantage of segmentation over paging |
RAM is managed more efficiently so there will be more RAM available for new processes as less is already being taken up |
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Define virtual memory |
Pages thst have not been used recently will be sent to a file in the HDD, therefore making more RAM available. If needed again then it can be sent back into RAM |
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Why is virtual memory needed in some systems? And why is this disadvantageous |
Some systems don't have enough RAM available to store all current processes. This is a drawback as using virtual memory is a slower process than keeping everything in RAM |
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Define disk thrashing |
When RAM is full the OS may move pages back and forth between virtual memory and RAM. This is known as thrashing and will slow down the speed of the system |
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Define interrupts |
Signals sent from devices to the CPU to inform that the device needs attention |
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Describe interrupts and give examples of common interrupts |
When an interrupt occurs the CPU runs some OS code on the next instruction cycle to inform the OS of the interrupt. The current toy running process will be interrupted so this event can be handled Examples include: -key has been pressed -saving data to HDD -Sending data to printer |
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Define buffers |
Small blocks of memory inside hardware devices such as printers and keyboard where work currently handled by that device is stored. Example: A keyboard key is pressed, an interrupt is sent to the CPU and the keystroke is stored in the CPU's buffer. The buffer stores the keystrokes until the OS is ready |
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Define interrupt service routines (ISR) |
Interrupts are dealt with by calling an ISR. ISR's check the priority level for interrupts and will deal with the interrupts I'd it is a higher priority than the current process. If not then it will be dealt with in the next instruction cycle |
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How are interrupts recorded? |
A signal is sent to the CPU via the control bus. The interrupt register is updated |
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How do interrupts affect the FDE cycle? |
At each iteration of the FDE cycle the CPU checks to see if any interrupts have occurred. If the interrupts are of a higher priority then the current process then these steps are carried out: -the contents of the program counter and other registers are stored in a stack in memory -the relevant ISR is loaded - when the ISR is complete, the values from the registers are sent from the stack back to the CPU |
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Define the process scheduler |
A part of the OS which decides the order in which processes will be processed |
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What are the goals of the scheduler? |
-Minimise starvation - when a process doesn't get a chance to run due to higher priority processes being processed first -Eliminate deadlock - when a process has to wait for a system resource that is currently held by another process -ensure fairness of all processes - ensure reasonable response time |
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Describe the round Robin scheduling algorithm |
Each process is given a fixed amount of time. If it hasn't finished in the time given, it goes to the back of the queue so the next process can be processed |
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Advantage and disadvantage of round Robin |
+processes are guaranteed processor time, limiting starvation - does not take priorities into consideration |
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Define first come first served scheduling algorithm |
The first process to arrive will be processed by the CPU. All other processes will queue until the CPU has finished the previous process |
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Advantages and disadvantages of first come first served scheduling algorithm |
+all processes are guaranteed to be processed, regardless of the time it takes to process -priorities aren't considered -processes may have to queue for a long time |
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Define shortest job first scheduling algorithm |
Picks the job thst will take the shortest time and will run it until it finishes |
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Advantages and disadvantages of shortest job first scheduling algorithm |
+many jobs processed quickly +prioritises the processes -larger processes may not get a chance to be run, increasing starvation -OS has to figure out how long each process will take. Difficult to prioritise |
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Define shortest remaining time scheduling algorithm |
Picks the job thst will take the shortest time and runs it. However, if a job is added to the queue with a shorter time remaining then the scheduler will swap the processes |
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Advantage and disadvantage of shortest remaining time scheduling algorithm |
+prevents larger processes from starvation -swapping makes the algorithm more complex |
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Define multi level feedback queue scheduling algorithm |
There are three queues. The scheduler decides which queue the process goes in based on priority. Processes in the high priority queue will be processed first on a FCFS basis. Each process is given a fixed amount of time and if it doesn't finish processing the it will be demoted to a lower priority queue |
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Advantage and disadvantage of multi level feedback queue scheduling algorithm |
+prioritises jobs -may not process low priority jobs as continuously processing higher priority jobs |
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Define embedded operating systems |
Designed for a specific device with efficiency in mind to operate on low powered CPU's and little RAM. Likely to have one job and is not likely to have a need for multi-tasking Used in devices such as cars, ATM's and printers |
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Define BIOS |
Basic input output system The systems software that is the first thing that is run when the computer is switched on |
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Describe the tasks the BIOS carries out when the pc is turned on |
BIOS is loaded from RAM and performs the following tasks: -initialises and tests hardware by running the POST (power on self test) -selects the primary boot device -the boot loader program is run from that device and if successful, runs the OS -if unsuccessful it moves onto the next devices and repeats until all devices have been tried -if no boot devices have suitable boot loaders, the system will stop |
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Define device drivers |
A piece of software usually supplied with a device, that tells the OS how it can communicate with that device |
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Define virtual machines |
A general purpose computer thst will emulate the hardware of your system, including its own RAM, CPU and HDD's. Virtusl machines are sandboxed environments where the virtual machine has no idea it's inside a host system |
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Advantages of virtual machines |
+can be backed up and duplicated +more than one vm can be run at one time off a physical machine. It is for these reasons that businesses are virtualising there networks. Allows them to run their servers off a group of vm's running from few physical machines |
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Define a system virtual machine (system virtualization) |
Where a full OS is virtualized on the host system |
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Define a process Virtusl machine (process virtualization) |
Where intermediate code can be executed through a vm |
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Define intermediate code |
Source code is compiled to intermediate code to prevent the incompatibilities that exist when trying to run compiled code on different CPU architectures or different OS's. Process ritual machines run intermediate code |
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What are the benefits of looking intermediate code? |
Code can be run off many devices, if they run a process VM. This means that developers can program one solution and have it run off many devices instead of making a desktop, mobile, smart TV version etc. |
