1. Lesson Objectives Aims Key Words
Be able to answer exam questions on the topic of operating systems
Key Words

2.  In order to function, all computing devices require an operating system.
(a) Describe the purpose of memory management in the operating system.
[3]
(b) Describe the purpose behind process scheduling in the operating system.

3. ➊ (a)Allows memory to be partitioned, through either segmentation or paging, and allocated to processes. It ensures that processes cannot access each other’s segments/pages for security and stability purposes. Memory management also allows for the use of virtual memory when RAM is almost full. (b)Scheduling will ensure that all processes get processed, taking into consideration any priorities that may exist. It will process the maximum number of processes in the least amount of time ensuring high throughput of processes. It ensures that all users of a system will get fast response times by making the most efficient use of the processor.

4. Describe how page tables are used by processes to manage their memory address space. [6]

5. ➋Memory is allocated to a process and recorded using a page table
➋Memory is allocated to a process and recorded using a page table. Each process will have its own page table which will say where in RAM the page resides. A process will not be aware of what pages it uses, but rather will use a virtual address space. On memory access, a virtual address is converted into a real address using the page table. A process sees its memory pages as a continuous block, but each page may reside in different parts of memory.

6. Describe, using an example, how and why a process may become suspended

7. ➌ When a process is suspended it will have its process control block saved into virtual memory. This will include the stack and the contents of registers (PC, ACC etc.). A process may need to be suspended if there is a limited amount of RAM remaining and a new process has been started. The memory manager will try and choose a process to suspend that is not currently being used. Once a process has been suspended it must be loaded back into RAM in order to continue processing.

8. Describe how buffers and interrupts are used when saving a large file to the hard drive. [6]

9. ➍ Data will be sent to the hard drive and stored into the hard drive’s buffer. As the buffer works much faster than the hard drive itself, the CPU will be free to run other processes while the data is saved and the process which is saving the file becomes blocked. Once the buffer is full, the hard drive will slowly save that data. When the buffer is empty and the data saved, an interrupt is sent back to the CPU. On the next FDE cycle, the interrupt register will be checked and depending on priorities, the blocked process will be brought back into the ready to run queue and more data sent to the buffer. This algorithm repeats until the file is fully saved.

10. Describe how a distributed operating system works and how micro kernels are used. [6]

11. ➎ A distributed OS will have a number of computers, known as nodes, connected over a network. Each node will have a micro kernel, which has a small set of key features such as device management, memory management and network communication. Scheduling is done on top of the micro kernel as a process could be run on any node. Scheduling itself would be done by a single node, while other distributed functions like file management may be done by another node.

12. Intensive care systems usually run on an embedded, real time, operating system
A) State what is meant by the term real time [1]
B) Explain why a real time operating system would be suitable for this purpose [2]

13. The system will give a response within a guaranteed amount of time.b)
If something happens to a patient, response must be immediate
Other types of system may have delays in their response
This could result in a patient not receiving treatment in time

14. Explain why software designed for windows does not work on Linux? [2]

15. Software is usually dependent on libraries
These may exist in windows but not linux and vice versa
Software will/may make system calls, these are different on windows/linux

16. Review/Success Criteria