Lecture 4 Flashcards
Define an OS?
An operating system is the software that controls the overall operation of a computer system.
What are the functions of an OS?
User interface
Memory management
Interrupt handling
Processor scheduling
What is Application software?
Performs specific tasks for users
What is System software?
Provides infrastructure for application software
Consists of operating system and utility software
Wat is the Kernel?
Kernel: Performs basic required functions such as: File manager Device drivers Memory manager Scheduler and dispatcher
What is the memory manager?
Allocates space in main memory
May create the illusion that the machine has more memory than it actually does (virtual memory) by playing a “shell game” in which blocks of data (pages) are shifted back and forth between main memory and mass storage
What is the bootstrap?
Boot loader: Program in ROM (example of firmware)
Run by the CPU when power is turned on
Transfers operating system from mass storage to main memory
Executes jump to operating system
What is the Scheduler?
Adds new processes to the process table and removes completed processes from the process table
A scheduler is a program, a set of instructions, used by the OS to decide where each of the jobs which are in its control should be and in what order to manipulate them.
What is the Dispatcher?
Controls the allocation of time slices to the processes in the process table
The end of a time slice is signaled by an interrupt.
What are interrupts?
Some tasks are more important than others. Therefore, interrupts are used - to tell the processor to stop what it is doing because there is something more important which needs dealing with. Before the Fetch Decode Execute cycle, the CPU checks the contents of the interrupt register
3 goals of scheduling?
To provide an acceptable response time to all users
To maximise the time the CPU is usefully engaged
To ensure fairness on a multi-user system
A scheduling algorithm should also ensure all tasks are processed, by changing the priorities if necessary.
What is memory management?
All operating systems need memory management software:
To allocate memory to allow separate processes to reside in memory and then run at the same time.
To reallocate memory when necessary, for example reallocating the memory space of a closed program.
To protect processes and data from each other by blocking access to the memory space of another program.
What is segmentation?
. Segmentation is where each task has a whole “segment”. The whole segment has to be moved to and from virtual memory, however, segmentation avoids the need to split up a program.
What is paging?
Paging is where a program is split into pages, which all have the same size. Individual pages can be swapped in and out of virtual memory, which can lead to more efficient use of RAM. However, programs have to be split into individual pages.
What is virtual memory?
A computer will only have a limited amount of RAM. When the RAM is full, virtual memory has to be used, based on secondary storage (such as the hard disk). This allows programs to run that need more memory than is available. However, if virtual memory is overused, then the processor could end up spending more time moving segments or pages between RAM and virtual memory. This is known as disk thrashing, and can result in the computer performing sluggishly, or “hanging”.
