Operating Systems

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Minimizing Rotational Latency

Use Scan based on sector numbers not cylinder number. For rotational latency Scan is the same as C-Scan. Why?
Ans: Because the disk only rotates in one direction.

Homework: 24, 25

5.4.4: Error Handling

Disks error rates have dropped in recent years. Moreover, bad block forwarding is normally done by the controller (or disk electronics) so this topic is no longer as important for OS.

5.5: Clocks

Also called timers.

5.5.1: Clock Hardware

5.5.2: Clock Software

  1. Time of day (TOD): Bump a counter each tick (clock interupt). If counter is only 32 bits must worry about overflow so keep two counters: low order and high order.

  2. Time quantum for RR: Decrement a counter at each tick. The quantum expires when counter is zero. Load this counter when the scheduler runs a process (i.e., changes the state of the process from ready to running). This is presumably what you did for the (processor) scheduling lab.

  3. Accounting: At each tick, bump a counter in the process table entry for the currently running process.

  4. Alarm system call and system alarms:
  5. Profiling

Homework: 27

5.6: Character-Oriented Terminals

5.6.1: RS-232 Terminal Hardware

Quite dated. It is true that modern systems can communicate to a hardwired ascii terminal, but most don't. Serial ports are used, but they are normally connected to modems and then some protocol (SLIP, PPP) is used not just a stream of ascii characters. So skip this section.

Memory-Mapped Terminals

Not as dated as the previous section but it still discusses the character not graphics interface.


Tanenbaum description of keyboards is correct.

5.6.2: Input Software

5.6.3: Output Software

Again too dated and the truth is too complicated to deal with in a few minutes.

5.7: Graphical User Interfaces (GUIs)


5.8: Network Terminals


5.9: Power Management


5.10: Research on Input/Output


5.11: Summary


Chapter 6: File Systems


  1. Size: Store very large amounts of data.
  2. Persistence: Data survives the creating process.
  3. Access: Multiple processes can access the data concurrently.

Solution: Store data in files that together form a file system.

6.1: Files

6.1.1: File Naming

Very important. A major function of the file system.

6.1.2: File structure

A file is a

  1. Byte stream
  2. (fixed size) Record stream: Out of date
  3. Varied and complicated beast.

6.1.3: File types


  1. (Regular) files.

  2. Directories: studied below.

  3. Special files (for devices). Uses the naming power of files to unify many actions.
        dir             # prints on screen
        dir > file      # result put in a file
        dir > /dev/tape # results written to tape
  4. “Symbolic” Links (similar to “shortcuts”): Also studied below.

“Magic number”: Identifies an executable file.

Strongly typed files:

6.1.4: File access

There are basically two possibilities, sequential access and random access (a.k.a. direct access). Previously, files were declared to be sequential or random. Modern systems do not do this. Instead all files are random and optimizations are applied when the system dynamically determines that a file is (probably) being accessed sequentially.

  1. With Sequential access the bytes (or records) are accessed in order (i.e., n-1, n, n+1, ...). Sequential access is the most common and gives the highest performance. For some devices (e.g. tapes) access “must” be sequential.
  2. With random access, the bytes are accessed in any order. Thus each access must specify which bytes are desired.

6.1.5: File attributes

A laundry list of properties that can be specified for a file For example:

6.1.6: File operations

Homework: 6, 7.

6.1.7: An Example Program Using File System Calls

Homework: Read and understand “copyfile”.

Notes on copyfile

6.1.8: Memory mapped files (Unofficial)

Conceptually simple and elegant. Associate a segment with each file and then normal memory operations take the place of I/O.

Thus copyfile does not have fgetc/fputc (or read/write). Instead it is just like memcopy

while ( *(dest++) = *(src++) );

The implementation is via segmentation with demand paging but the backing store for the pages is the file itself. This all sounds great but ...

  1. How do you tell the length of a newly created file? You know which pages were written but not what words in those pages. So a file with one byte or 10, looks like a page.
  2. What if same file is accessed by both I/O and memory mapping.
  3. What if the file is bigger than the size of virtual memory (will not be a problem for systems built 3 years from now as all will have enormous virtual memory sizes).

6.2: Directories

Unit of organization.

6.2.1-6.2.3: Single-level, Two-level, and Hierarchical directory systems


These are not as wildly different as they sound.

6.2.4: Path Names

You can specify the location of a file in the file hierarchy by using either an absolute or a Relative path to the file

Homework: 1, 9.

6.2.5: Directory operations

  1. Create: Produces an “empty” directory. Normally the directory created actually contains . and .., so is not really empty

  2. Delete: Requires the directory to be empty (i.e., to just contain . and ..). Commands are normally written that will first empty the directory (except for . and ..) and then delete it. These commands make use of file and directory delete system calls.

  3. Opendir: Same as for files (creates a “handle”)

  4. Closedir: Same as for files

  5. Readdir: In the old days (of unix) one could read directories as files so there was no special readdir (or opendir/closedir). It was believed that the uniform treatment would make programming (or at least system understanding) easier as there was less to learn.

    However, experience has taught that this was not a good idea since the structure of directories then becomes exposed. Early unix had a simple structure (and there was only one type of structure for all implementations). Modern systems have more sophisticated structures and more importantly they are not fixed across implementations.

  6. Rename: As with files

  7. Link: Add a second name for a file; discussed below.

  8. Unlink: Remove a directory entry. This is how a file is deleted. But if there are many links and just one is unlinked, the file remains. Discussed in more detail below.

6.3: File System Implementation

6.3.1: File System Layout

6.3.2: Implementing Files

Contiguous allocation

Homework: 12.

Linked allocation

FAT (file allocation table)