SLR 4
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Describe two ways that an operating system could manage physical memory
• Paging • block of memory of fixed size • Segmentation • blocks of memory split logically / variable size
Explain one benefit of memory management to the user
• security • does not let programs access memory reserved for other programs • multitasking • allows multiple programs to run at once
Describe how virtual memory allows a user to run programs when physical memory is full
• pages not currently in use are moved from memory to secondary storage • to create room in memory • pages moved back to memory when required
Explain, giving an example, why a firm’s computers use operating systems capable of multi-tasking
• multitasking allows the user to run more than one program at the same time • e.g. running CAD software while checking emails
Explain why a computer’s operating system uses a FCFS algorithm when sending documents to a printer
• for printer queue • all documents/users have equal priority • whichever document is received first is printed first • FIFO/LILO
Explain why a computer’s operating system uses a round-robin algorithm for allocating processor time
• to enable multitasking to take place • to switch between active processes and those running in the background • to limit each process to a certain amount of time/allow processes an equal share of processor time • to ensure the OS cycles through all processes • so that users can receive an immediate response
Describe what BIOS will do to start up the computer
• contains the computer start up instructions • loads settings/configurations • carries out POST • and reports errors • loads bootstrap
Apart from storing BIOS, ROM can also be used in other ways. Describe how an embedded system can make a different use of ROM and why it is an advantage
• storing OS • that can’t be overwritten • so that access is faster
Define what is meant by the term ‘device driver’, giving one example of a device driver that a home user would need
• software/program • that allows the OS to communicate with hardware • printer driver, webcam driver etc.
Explain why the programmers of anti-virus software may make use of virtual machines when developing updates
• allow them to run the update on a number of different OSs • without needing multiple physical machines • they can put viruses on the VM to test if the update can catch them • but protect the physical machine from the virus
Describe how a multi-level feedback queue works
• Puts processes into different categories/ queues • Each queue has a different priority /quantum /scheduling approach - Processes can move between queues… • …based on their behaviour /priority • Designed to minimise I/O bottle necks • …balances longer and shorter processes
Describe what the effect would be of a computer having its BIOS overwritten
• The computer would not be able to boot / load the OS – Or set up its initial configuration / hardware checks – Making the computer unusable
Points why memory management is so important (9)
• Ensures RAM is used efficiently • removes unnecessary data • paging / segmentation • virtual memory • paging causes internal fragmentation while segmentation causes external fragmentation • RAM is much more expensive than secondary storage so VM is useful • over use of VM slows computer down • segmentation errors can cause memory leakage • security - applications can only access memory allocated to them
Two roles of an operating system are to handle interrupts and manage scheduling. Describe two other roles of an operating system.
• Resource/memory management e.g. • Moving data between RAM and secondary storage/ virtual memory / paging and/or segmentation • Allocating/deallocating memory • Manage hardware/peripherals e.g. • Tracking all devices connected to the system • Device drivers • File management e.g. • Storing files in secondary storage • Searching for /copying / moving / renaming files/folders • Security/user management e.g. • Controlling who can access the system /Managing user profiles • Controlling who can access certain resources on the system / Managing access rights • Provide a user interface e.g. • Allowing the user to interact with the software/hardware/computer • Providing utilities e.g. • Used to monitor / manage / maintain the computer • To manage the security • Providing a platform on which to run software e.g. • Allows additional software to be installed on the computer • To allow the user to complete additional tasks
Explain how an operating system deals with an interrupt
• Interrupt checked for at start/end of each fetch-execute cycle • If the interrupt is of a lower/equal priority to the current process then the current process continues • (If interrupt raised) contents of registers copied to stack • Flags are set to determine if interrupts are enabled / disabled • Program counter changed to point to Interrupt Service Routine (ISR) / ISR runs • After interrupt complete, previous register values restored back from stack • Flag is reset • If higher priority interrupt received during servicing of interrupt… • …this is added to stack and new interrupt dealt with
Describe how a stack is used when handling interrupts
• Stacks use LIFO/FILO storage • As processes are halted by an ISR they are pushed on the stack • When they are returned, they are popped from the top of the stack • So they are returned to in correct order
Application software is run on thin client computers. A thin client computer is a very low-powered computer connected to a powerful central server. The operating system and all the applications run within a virtual machine on the server. The thin client computer will only display the output of the virtual machine and capture and send input to the virtual machine. State one advantage of running the application software within the virtual machine.
• Reduced hardware is needed on computers / reduced hardware costs • Improved security by keeping all software running from one physical device • Easier/cheaper to manage as only one physical device runs the programs. • Can add/remove resources/memory/process to the VM • If it is infected by malware it can be deleted and recreated/ the rest of the system isn’t affected/is protected against malware • No direct access to hardware • Resources can be used more flexibly between the machines
Two roles of an operating system are to handle interrupts and manage scheduling. Describe two other roles of an operating system.
• Resource/memory management e.g. • Moving data between RAM and secondary storage/virtual memory /paging and/or segmentation • Allocating/deallocating memory • Manage hardware/peripherals e.g. • Tracking all devices connected to the system • Device drivers • File management e.g. • Storing files in secondary storage • Searching for /copying / moving / renaming files/folders
A secondary school is upgrading their computers. They decide to install “thin client” computers. A thin client computer is when users access their computer in the usual way with a keyboard, mouse and monitor. However, all processing takes place on the virtual machine on a server rather than the computer at their desk. Describe one advantage of using virtual machines in this way. Describe one disadvantage of using virtual machines in this way.
• Allows for more efficient use of computing resources • … Less likely to have workstations run idle • … can allocate more resources to power users dynamically • Thin clients are often physically smaller than desktops… • …allowing more desk space/workspace for staff/students. • Can cause network overload… • …if the network does not have sufficiently good bandwidth • Could create process lag… • …If lots of large processes run at the same time • If a virtual machine hangs/crashes the user is unable to reset it… • …whereas they can physically reboot on a local machine. • Thin-clients may not be compatible with some peripherals… • …meaning some of the school’s peripherals become unusable.
Each virtual machine will run an operating system. One type of operating system is multi-user. Describe two other types of operating system and give an example of where each may be used.
• Multi Tasking … • … is optimised to deal with multiple processes simultaneously/concurrently • …Can run multiple programs/tasks at the same time • Home/personal computer • Distributed … • … Designed to run the same task over multiple machines • SETI or other cloud-based processes/ Clusters of desktops used for rendering/calculations. • Embedded … • … Will have a restricted set of operations (to conserve resources) • … is read only • Domestic appliance such as a washing machine • Real-Time … • …optimised to handle and respond to inputs with no wait/within a guaranteed response time/immediate. • Used in “mission critical” systems such as aircraft control.
The business uses virtual storage to hold regular backups of all of its data. Explain why virtual storage is well-suited for storing backups.
• Stored away from the computer(s)/remote… • … so in case of disaster, data is not also damaged • All of the data (from multiple machines) can bebacked up at the same time • Can be accessed from elsewhere / other machines • Storage can be expanded as necessary/no limit on size • Speed of access is not a priority for a backup • Can make recovery from another site easier • No physical space needed for backup hardware • No on site maintenance required • Allows more local storage capacity for data
One computer owned by the business monitors critical-safety features of manufacturing. All input data must be processed within a predictable timescale of a fraction of a second. State the type of operating system that should be used by this computer.
Real time
Give the name of three types of operating system other than Real Time, and for each state its purpose.
• Multi-tasking… • …runs multiple programs at the same time • Multi-user… • … allows multiple users at the same time (must be clear that candidate is not discussing an OS that simply has multiple accounts) • Distributed… • …allows multiple computers to work together on a single task • Embedded… • …has a dedicated/limited function • …is read-only / cannot be changed
One role of an operating system is the Interrupt Service Handler. This allows processes being executed by the CPU to be interrupted. One example of an interrupt would be removing an external hard disk drive from a computer. State why this would need to interrupt the current fetch-decode-execute cycle of the CPU.
• GUI will need to remove open file manager windows • OS will need to remove open files/release locks • Inform the CPU to cease transferring files