Assignment 2 (Replacement) Solution

1. Write the full UNIX command that could be used to find the ID numbers of all running processes.
   How could you find information about running processes in Windows? [4]
   • ps -e (or ps -ef)
   • task manager (right click taskbar or press Ctrl-Alt-Del) or System Information (Accessories, System Tools)
2. Why are Windows NT-based operating systems generally more stable than Windows 95-based operating systems? [2]
   • Windows NT uses true multitasking; the kernel is protected from applications; better memory management; not DOS-based
3. What memory management technique eliminates external fragmentation? Internal fragmentation? [2]
   • paging, segmentation
4. When is garbage collection of main memory most important? [1]
   • when memory is (nearly) full or when free space is needed
5. Memory accesses are much faster than disk accesses, but memory capacity is much smaller than disk capacity. 
   Based on these two principles, what techniques do operating systems use [4]
   • when memory space is too small for all scheduled processes? (two techniques)
     • paging and segmentation (swapping a process to/from disk)
   • to reduce the number of disk accesses? (two techniques)
     • cacheing and reading/writing a block at a time 
6. What is the advantage of using large pages? small pages? [2]
   • Large pages reduces page table size
   • Small pages reduces internal fragmentation
7. What causes fragmentation? [2]
   • deleting and adding files to the disk; holes filled by different files so pieces of files are scattered and not contiguous
8. A 32-bit address is divided into a 14-bit segment number, an 8-bit page number and a 10-bit displacement. [3]
   • How many pages can a segment have?
     • 2⁸ = 256
   • What is the page size?
     • 2¹⁰ bytes = 1024 bytes or 1 KB
   • How many segments can be addressed?
     • 2¹⁴ = about 16000
9. What methods would an operating system likely use to select a page table entry to remove from a cache when it becomes full? [2]
   • least recently used, least frequently used, or longest in the cache
10. What is one benefit and one cost of disk compression? [2]
    • benefit: each file uses less disk space; more information can be stored on the disk
    • cost: reduced performance because of the extra processing to compress and decompress during writes and reads
    • cost: entire disk is more easily corrupted and more difficult to recover
11. What is the purpose of the Windows scandisk program? [2]
    • detects and repairs disk errors and problems such as deleting lost cluster chains or converting them to files
12. A 4 GB hard disk has 0.5 KB per sector, 1,000,000 clusters, 2,000 cylinders and 8 double-sided platters. Calculate the following numbers. Approximations are acceptable. [4]
    1. sectors
       • 4 GB / 0.5 KB/sector = 8,000,000 sectors
    2. tracks
       • 2000 cylinders x 16 heads = 32,000 tracks
    3. sectors per cluster
       • 8,000,000 sectors / 1,000,000 clusters = 8 sectors/cluster
    4. sectors per track
       • 8,000,000 sectors / 32,000 tracks = 250 sectors/track
13. What are some important file types in Windows and UNIX, and how can you determine what type of files are in the current directory in each system? [6]
    • Windows file types are displayed by extensions
    • Windows file types include .exe=executable, .com=command, .sys=system, .dll=library, .txt=text, .tmp=temporary
    • UNIX file types are displayed using the ls -l command
    • UNIX file types include regular, special (block/character devices), directories and executable files.
14. Name the following directories in a UNIX system. [3]
    1. .
       • current
    2. ..
       • parent
    3. /
       • root
15. You have correctly installed a program on your computer, and you want to run it by typing the program name at the MS-DOS command prompt, but you receive an error message "bad command or filename". [3]
    1. What would you need to type to get the program to run from the DOS prompt?
       • the full path name of the program
    2. How could you remove the need to do this if you want to run the program more than once?
       • add the program's directory to the search path (the PATH environmental variable)
    3. How could you run the program every time the computer starts?
       • add the program to the autoexec.bat file or add the directory to the search path and put a shortcut in the startup folder
16. Compare the way Windows and UNIX allow you to add and access different disks. [4]
    • Windows adds a new drive letter sequentially e.g. D:, E:, F:, etc.
    • To access a disk, include the drive letter in the file path e.g. dir "d:\my documents"
    • UNIX mounts a disk onto an existing folder
    • The disk is accessed through the mount point or the /dev directory
17. Compare the way Windows and UNIX store and access pieces of files. [4]
    • Windows uses a File Allocation Table which contains chains of clusters which are allocated to files.
    • UNIX uses inodes which contain pointers to data and index blocks; the index blocks point to data or index blocks.