Assignment 2 Solution

1. How many interrupt levels are available on Intel machines running DOS or Windows? [1]
   • DOS and Windows 9x System Information list 16 IRQs; Windows 2000 and XP list 24.
2. What is the advantage of preemptive multitasking over non-preemptive multitasking? [2]
   • The operating system can preempt (suspend) a non-cooperative process (which may be hung, is taking too long and not voluntarily giving up the processor).
3. How would you find and remove a hung process in Windows? In UNIX? [4]
   • In Windows, press Ctrl-Alt-Del and possibly click the Task Manager button. Select the hung process and click End Task. You may have to click End Task again a short time later.
   • In UNIX, type ps or ps -e to list the process IDs of processes. To end the process, type kill PID or kill -9 PID.
4. What are two reasons for defragmenting a disk? [2]
   1. A defragmented disk runs faster because disk seek times are shorter and disk accesses are fewer.
   2. A defragmented disk lasts longer because less head movement produces less wear on mechanical parts. 
5. Why are memory and disk blocks usually allocated in sizes of powers of two? [2]
   • Computers are binary digital devices which perform calculations using binary arithmetic. 
   • Memory and disks are therefore physically arranged to store information in sizes of powers of two. 
   • Addressing and calculation are made easier; e.g. a memory address or disk location can be broken into two parts (block address and location within a block).
6. What is the main difference between segments and pages? [1]
   • Pages have a fixed size, but segment size varies according to individual process requirements.
7. Give one advantage of paging over segmentation and one advantage of segmentation over paging. [2]
   • Paging reduces external fragmentation.
   • Segmentation reduces internal fragmentation and improves performance because of locality of reference.
8. A 32-bit address is divided into a page number and a displacement which produces a page size of 16 KB.
   1. How many pages could be referenced? [3]
      • If the page size is 16 KB, it must take 14 bits, since 2¹⁴=16K. The remaining 18 bits are used for the page number, so 2¹⁸=256K pages could be referenced.
   2. How much space would the page table use if each entry is four bytes? [1]
      • 256K pages x 4 bytes/page table entry = 1 MB
9. Why do modular object-oriented programs run faster than monolithic programs with GOTO statements? [2]
   • locality of reference: All code and procedures for the object are stored in the same general location on disk, so the number of disk accesses is less than for a process that jumps around in spaghetti code.
10. What is the advantage of having a separate register to point to the page table or segment table? [1]
    • Access to any register is much faster than accessing memory. The page table and segment table are both frequently used, so quick access is important for overall virtual memory performance.
11. What two characteristics of pages do page replacement policies use to remove pages? [2]
    1. Total time the page has lived in memory
    2. Time since the page was last used
12. In which hardware device would you normally find real memory? virtual memory? [2]
    • Real memory is found in memory banks such as RAM SIMMs or DIMMs which are inserted into slots on the motherboard.
    • Virtual memory is found in a page or swap file which is usually stored on a hard disk.
13. What is the advantage of direct memory access? [1]
    • Direct memory access allows a hardware device to access (read or write to) memory without requiring intervention by the processor. This improves performance.
14. Why do disks and tape transfer data in blocks of characters rather than a single character at a time? [2]
    • Each disk or tape access takes much longer than reading each character, so it is more efficient to read several characters at once, even if they are not all used.
15. What is the purpose of a buffer? [2]
    • A buffer is a temporary storage area in memory.
    • A buffer decouples communication between readers and writers in order to improve performance. 
    • The readers and writers can work independently, operate at different speeds and use different chunks of data. 
    • For example the processor is faster than disks and disks read and write in units of blocks.
16. Which two system files make up the core of MS-DOS? [2]
    • msdos.sys and io.sys
17. How do you refer to an I/O device in DOS? in UNIX? [2]
    1. DOS I/O devices often end with a colon, e.g. A:, C:, CON:, COM1:, LPT1:, etc.
    2. UNIX devices are treated as files and are listed in the /dev directory.
18. Which file loads DOS device drivers? [1]
    1. The config.sys file will load listed device drivers when the system starts.
19. What steps do you need to take to install a new hardware device on a PC running Windows? [5]
    1. Physically connect the device to the computer. Internal devices should be installed into the computer case. External devices should be plugged into one of the ports on the back of the computer.
    2. You may have to switch off and on or restart the computer, though not for some devices e.g. those connected to USB ports.
    3. Install device driver files and related software. This can occur automatically using Plug and Play hardware or through the Add New Hardware control panel.
    4. Configure the hardware and related software. This can often be done using a control panel. For example, you might need to set DMA, I/O and IRQ addresses if it is not done automatically or there is a conflict.
20. What is the capacity of a hard disk with 512 bytes per sector, 200 sectors per track, 1000 cylinders and 4 double-sided platters? [2]
    • 4 platters x 2 sides/platter x 1000 tracks/side x 200 sectors/track x 512 bytes/sector = 800 MB
21. What must be done to a new hard disk before installing two different operating systems like Linux and Windows? [2]
    • Partition the disk to divide it into two or more partitions
    • Format each of the partitions with a file system that is compatible with the operating system
22. When Windows is not shut down properly (e.g. if the power is cut), files and file tables can be damaged.
    What two programs can be used to repair the file system? [2]
    1. chkdsk is used by DOS, Windows 3.1 and Windows NT/2000/XP.
    2. scandisk is used by Windows 9x.
23. What is the purpose of a cache? [2]
    • A cache is a small, fast memory area to hold recently accessed data which speeds up later accesses to the same data.
24. What are two reasons that memory performance is much faster than disk performance? [2]
    • Memory is entirely electronic, whereas disks have physically moving parts.
    • Memory is uses random access whereas disks are accessed sequentially.