Operating systems and control systems practice questions.

A user sends three long documents to the same printer within a few seconds. Which operating system function stops the three documents from being interleaved on the paper?

• Memory management through paging
• Providing a graphical user interface
• Peripheral management, by queuing the jobs for the printer
• File management, by storing each document in its own directory

Answer: Peripheral management, by queuing the jobs for the printer

The operating system holds each print job in a queue on secondary storage and passes them to the printer one at a time through its driver. Paging and directory structures manage memory and stored data, not access to an attached device.

A computer has 512 MiB of primary memory installed and the user starts a program that needs 800 MiB. The program runs successfully. Which explanation is correct?

• Only the pages currently in use are held in primary memory
• The operating system compresses the program until all of it fits into memory
• The program is moved into cache memory, which has no fixed limit
• The processor executes the program directly from secondary storage

Answer: Only the pages currently in use are held in primary memory

Virtual memory divides the program into pages and keeps only the pages being used in primary memory, fetching the others from secondary storage on demand. Cache is smaller than primary memory rather than larger, and the processor can only execute instructions that have been loaded into primary memory.

A computer has almost no free primary memory left and becomes nearly unusable: its secondary storage is accessed continuously although the processor is mostly idle. Which explanation fits best?

• The cache has been filled with instructions from the user interface
• The window manager is redrawing the whole screen after every keystroke
• A device driver is polling the keyboard far too often
• Pages are being swapped in and out faster than useful work is done

Answer: Pages are being swapped in and out faster than useful work is done

When the memory demanded by the running programs exceeds the primary memory available, the operating system spends nearly all of its time transferring pages instead of executing instructions. The symptom is a busy disk together with an idle processor; redrawing or polling would keep the processor busy instead.

A key is pressed while a program is running and the keyboard raises an interrupt. What does the processor do next?

• It checks every attached device in turn until it finds the keyboard
• It saves its current state and runs the handler routine
• It abandons the running program and returns to the user interface
• It stores the keypress in secondary storage until the program ends

Answer: It saves its current state and runs the handler routine

An interrupt makes the processor finish the current instruction, save its registers so that the interrupted program can resume, and execute the service routine for that device. Checking each device in turn is polling, which wastes processor time on devices that have nothing to report.

Why does an operating system prevent an application from writing directly to any address in primary memory?

• Direct writes are slower than requests made through the operating system
• Applications can only address secondary storage, not primary memory
• One faulty program could overwrite the code or data of another
• Primary memory can only be written to while no program is running

Answer: One faulty program could overwrite the code or data of another

Memory protection gives every process its own address space so that a fault in one program cannot corrupt another program or the operating system itself. Speed is not the reason: the boundary check is made by hardware as the address is issued.

A programmer saves data to a solid-state drive and to a magnetic drive using exactly the same command. Which idea does this illustrate?

• The operating system hides device detail behind a uniform interface
• Both drives are attached to the same bus inside the machine
• The two drives use the same file format for every document
• The application contains a driver for each type of drive it supports

Answer: The operating system hides device detail behind a uniform interface

Abstraction means the operating system offers one set of file operations while the driver deals with how a particular device actually stores the data. Without it every application would have to be rewritten each time a new kind of drive appeared.