NOTE: These notes are by Allan Gottlieb, and are
reproduced here, with superficial modifications, with his permission.
"I" in this text generally refers to Prof. Gottlieb, except
in regards to administrative matters.
================ Start Lecture #16
Also called timers.
5.5.1: Clock Hardware
- Generates an interrupt when timer goes to zero
- Counter reload can be automatic or under software (OS) control.
- If done automatically, the interrupt occurs periodically and thus
is perfect for generating a clock interrupt at a fixed period.
5.5.2: Clock Software
- 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.
- 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.
- Accounting: At each tick, bump a counter in the process table
entry for the currently running process.
- Alarm system call and system alarms:
- Users can request an alarm at some future time.
- The system also on occasion needs to schedule some of its own
activities to occur at specific times in the future (e.g. turn off
the floppy motor).
- The conceptually simplest solution is to have one timer for
- Instead, we simulate many timers with just one.
- The data structure on the right works well.
- The time in each list entry is the time after the
preceding entry that this entry's alarm is to ring.
- For example, if the time is zero, this event occurs at the
same time as the previous event.
- The other entry is a pointer to the action to perform.
- At each tick, decrement next-signal.
- When next-signal goes to zero,
process the first entry on the list and any others following
immediately after with a time of zero (which means they are to be
simultaneous with this alarm). Then set next-signal to the value
in the next alarm.
- Want a histogram giving how much time was spent in each 1KB
(say) block of code.
- At each tick check the PC and bump the appropriate counter.
- A user-mode program can determine the software module
associated with each 1K block.
- If we use finer granularity (say 10B instead of 1KB), we get
increased accuracy but more memory overhead.
Chapter 6: File Systems
- Size: Store very large amounts of data.
- Persistence: Data survives the creating process.
- Access: Multiple processes can access the data concurrently.
Solution: Store data in files that together form a file system.
6.1.1: File Naming
Very important. A major function of the file system.
- Does each file have a unique name?
Answer: Often no. We will discuss this below when we study
- Extensions, e.g. the ``html'' in ``class-notes.html''.
- Conventions just for humans: letter.teq (my convention).
- Conventions giving default behavior for some programs.
- The emacs editor thinks .html files should be edited in
html mode but
can edit them in any mode and can edit any file
in html mode.
- Netscape thinks .html means an html file, but
<html> ... </html> works as well
- Gzip thinks .gz means a compressed file but accepts a
- Default behavior for Operating system or window manager or
- Click on .xls file in windows and excel is started.
- Click on .xls file in nautilus under linux and gnumeric is
- Required extensions for programs
- The gnu C compiler (and probably others) requires C
programs be named *.c and assembler programs be named *.s
- Required extensions by operating systems
- MS-DOS treats .com files specially
- Windows 95 requires (as far as I can tell) shortcuts to
end in .lnk.
- Case sensitive?
Unix: yes. Windows: no.
6.1.2: File structure
A file is a
- Byte stream
- Unix, dos, windows (I think).
- Maximum flexibility.
- Minimum structure.
- (fixed size) Record stream: Out of date
- 80-character records for card images.
- 133-character records for line printer files. Column 1 was
for control (e.g., new page) Remaining 132 characters were printed.
- Varied and complicated beast.
- Indexed sequential.
- Supports rapidly finding a record with a specific
- Supports retrieving (varying size) records in key order.
- Treated in depth in database courses.
6.1.3: File types
- (Regular) files.
- Directories: studied below.
- 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
- ``Symbolic'' Links (similar to ``shortcuts''): Also studied
``Magic number'': Identifies an executable file.
- There can be
several different magic numbers for different types of
- unix: #!/usr/bin/perl
Strongly typed files:
- The type of the file determines what you can do with the
- This make the easy and (hopefully) common case easier and, more
- It tends to make the unusual case harder. For example, you have a
program that turns out data (.dat) files. But you want to use it to
turn out a java file but the type of the output is data and cannot be
easily converted to type java.
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
- 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.
- 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
- do not dump
- key length (for keyed files)
6.1.6: File operations
Essential if a system is to add files. Need not be a separate system
call (can be merged with open).
Essential if a system is to delete files.
Not essential. An optimization in which the translation from file name to
disk locations is perform only once per file rather than once per access.
Not essential. Free resources.
Essential. Must specify filename, file location, number of bytes,
and a buffer into which the data is to be placed.
Several of these parameters can be set by other
system calls and in many OS's they are.
Essential if updates are to be supported. See read for parameters.
Not essential (could be in read/write). Specify the
offset of the next (read or write) access to this file.
- Get attributes:
Essential if attributes are to be used.
- Set attributes:
Essential if attributes are to be user settable.
Tanenbaum has strange words. Copy and delete is not acceptable for
big files. Moreover copy-delete is not atomic. Indeed link-delete is
not atomic so even if link (discussed below)
is provided, renaming a file adds functionality.
6.1.7: An Example Program Using File System Calls
Notes on copyfile
- Normally in unix one wouldn't call read and write directly.
- Indeed, for copyfile, getchar() and putchar() would be nice since
they take care of the buffering (standard I/O, stdio).
- If you compare copyfile from the 1st to 2nd edition, you can see
the addition of error checks.
6.1.7: 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 ...
- 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.
- What if same file is accessed by both I/O and memory mapping.
- 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).
Unit of organization.
6.2.1-3: Single-level, Two-level, and Hierarchical directory systems
- One directory in the system (Single-level)
- One per user and a root above these (Two-level)
- One tree
- One tree per user
- One forest
- One forest per user
These are not as wildly different as they sound.
- If the system only has one directory, but allows the character / in
a file name. Then one could fake a tree by having a file named
rather than a directory allan, a subdirectory gottlieb, ..., a file
- Dos (windows) is a forest, unix a tree. In dos there is no common
parent of a:\ and c:\.
- But windows explorer makes the dos forest look quite a bit like a
tree. Indeed, the original gnome file manager for linux, looks A LOT
like windows explorer.
- You can get an effect similar to (but not the same as) one X per
user by having just one X in the system and having permissions that
permits each user to visit only a subset. Of course if the system
doesn't have permissions, this is not possible.
- Today's systems have a tree per system or a forest per system.
6.2.4: Path Names
You can specify the location of a file in the file hierarchy by
using either an absolute versus or a
Relative path to the file
- An absolute path starts at the (or a if we have a forest) root.
- A relative path starts at the
current (a.k.a working) directory.
- The special directories . and .. represent the current directory
and the parent of the current directory respectively.
Homework: 1, 9.
6.2.5: Directory operations
- Create: Produces an ``empty'' directory.
Normally the directory created actually contains . and .., so is not
- 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.
- Opendir: Same as for files (creates a ``handle'')
- Closedir: Same as for files
- 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). Modern systems have more
sophisticated structures and more importantly they are not fixed
- Rename: As with files
- Link: Add a second name for a file; discussed
- 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