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Question 11

What is compaction?

Compaction refers to the mechanism of shuffling the memory portions such that all the free portions of the memory can be aligned (or merged) together in a single large block. OS to overcome the problem of fragmentation, either internal or external, performs this mechanism, frequently. Compaction is possible only if relocation is dynamic and done at run-time, and if relocation is static and done at assembly or load-time compaction is not possible.

Question 12

What is memory-mapped I/O? How is it different frim I/O mapped I/O?

Memory-mapped I/O, meaning that the communication between the I/O devices and the processor is done through physical memory locations in the address space. Each I/O device will occupy some locations in the I/O address space. I.e., it will respond when those addresses are placed on the bus. The processor can write those locations to send commands and information to the I/O device and read those locations to get information and status from the I/O device. Memory-mapped I/O makes it easy to write device drivers in a high-level language as long as the high-level language can load and store from arbitrary addresses.

Question 13

List out some reasons for process termination.

  • Normal completion
  • Time limit exceeded
  • Memory unavailable
  • Bounds violation
  • Protection error
  • Arithmetic error
  • Time overrun
  • I/O failure
  • Invalid instruction
  • Privileged instruction
  • Data misuse
  • Operator or OS intervention
  • Parent termination.

Question 14

What is Belady's anomaly?

Usually, on increasing the number of frames allocated to a process' virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is known as the Belady's Anomaly. This is true for certain page reference patterns. This is also called the FIFO anomaly.

Question 15

What are short-, long- and medium-term scheduling?

Long term scheduler determines which programs are admitted to the system for processing. It controls the degree of multiprogramming. Once admitted, a job becomes a process.

Medium term scheduling is part of the swapping function. This relates to processes that are in a blocked or suspended state. They are swapped out of main-memory until they are ready to execute. The swapping-in decision is based on memory-management criteria.

Short term scheduler, also know as a dispatcher executes most frequently, and makes the finest-grained decision of which process should execute next. This scheduler is invoked whenever an event occurs. It may lead to interruption of one process by preemption.

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