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Tasks

Priorities - A: high, B: medium, C: low; Status - _: unfinished, X: finished, C: cancelled, P: pending, o: in progress, >: delegated. Covey quadrants - Q1 & Q3: urgent, Q1 & Q2: important
A1XPost announcement about exemptions and juniors {{Tasks:488}} (CS161.Teaching)
A2XGive array exercise {{Tasks:483}} (CS21A.Teaching)
A3XTeach filesystems {{Tasks:475}} (CS161.Teaching)

Schedule

Notes

9. Code for Nethack screenshots in Emacs

Categories: EmacsHacks#11 -- Permalink
(defvar nethack-screenshot-file "~/.nethack-notes" "Filename to store Nethack data in.")
(defun sacha/nethack-take-screenshot (caption)
  (interactive "MCaption: ")
  (save-window-excursion
    (save-excursion
      (find-file nethack-screenshot-file)
      (goto-char (point-min))
      (insert
       ".#1 " caption "\n\n"
       (with-current-buffer (get-buffer "*nethack map*")
         (buffer-substring-no-properties (point-min) (point-max)))
       "\n\n"
       (with-current-buffer (get-buffer "*nethack status*")
         (buffer-substring-no-properties (point-min) (point-max)))
       "\n\n")
       (save-buffer))))

8. CS161 announcements

Categories: CS161#18 -- Permalink
Yes, there are exemptions: 90 and above. This includes the grades from the projects. The list of exempted people will be posted on Monday. <b>Bad news for the CS juniors: you have to take the test also at the same date! (Uh oh...) Thursday, 1:30 - 3:30 PM at C115. (That's Chem 115)</b> AND there's extra work _after_ the finals.

(Apologies for the confusion. Apparently, missed important small text. My bad.)

7. Stuff Eric should check out

Categories: None -- Permalink
- Alain Chesnais (a-LAHN she-NAY) - YT Lee, head of SEAGRAPH - Barbara, in charge of organizing conferences (sponsored and in cooperation of) - CGEMS link from http://www.siggraph.org (Computer Graphics Educational Materials Store, free) - ACM DL - Talk at 3:30 - trends

5. Instructions for wearable chording keyboards

On wear-hard@haven.org, John <nop>McKown said:

http://chordite.com

where you can download free instructions on how to build a wearable, chording keyboard.

E-Mail from John McKown

4. eBay service

Categories: None -- Permalink
I keep getting

Dear eBay Member,

Dear customer, you have been billed for $15.00 recently. Please update your billing information at eBay Billing Center.

This is eBay auto generated message, if you think you received it by mistake or you want to remove these notifications, please update your profile at Billing Center.

in my mail. Now, I don't use eBay at all, so chances are something weird is happening.

E-Mail from eBay Service

3. meetup.com

Categories: None -- Permalink
Jeremy Hogan said on open-source-now-list@redhat.com:

Many of you have probably already heard of Meetup.com due to its prominence in the Democratic primaries. In order to help LUGs, school groups, and advocates to better and more easily co-ordinate gatherings we've started redhat.meetup.com.

You'll see us adding functionality over the coming weeks, and we'll be co-ordinating a special global meetup on April 1, 2004, so I urge you to go ahead and sign up now. If you haven't used it, it's a great way to spin up activity, to meet others interested in Red Hat, Linux, F/OSS face to face in a casual setting.

http://www.meetup.com might be a nice way to organize local meetings.

E-Mail from Jeremy Hogan

2. CS21A: ArrayList

Categories: CS21A#33 -- Permalink
Quick review: arrays

Arrays are a neat way to store a fixed number of items. You can declare and create arrays and loop over them. They're perfect for board games and other applications where you know exactly how many items you'll need to store.

EXERCISE: (Due 11:59:59 PM Sunday)

Download http://sacha.sachachua.com/notebook/cs21a/address.zip and unzip it into a convenient directory. Open the HTML file and run it in JCreator, or open a command prompt and type "appletviewer address.html". (If it gives you "Command not found", try "c:\j2sdk1.4.2_02\bin\appletviewer address.html".

E-mail your answers to me by the specified time.

- Put 0 in the index field and a name in the name field. Press Set.

Put 1 in the index field and a different name in the name field. Press Set. Put 0 in the index field and press Show. What happens? Put 1 in the index field and press Show. What happens? (Note that the data doesn't get saved to the hard disk - you'll learn how to do that in CS21B!)

- What is the largest number that you can use in the index field

without getting an error?

- What is the smallest number that you can use in the index field

without getting an error?

- How many items can this simple address book store?

- As practice in applets, components, layouts and arrays, write your

version of this applet (<nop>ReallySimpleAddressBook). It should look like this applet and behave exactly like this applet. (5 points)

- As practice in objects and arrays of objects, create a Person class.

The Person class should have a name attribute and a phone attribute (both Strings), and it can have any methods you want. Make a <nop>SimpleAddressBook applet that has a "Phone:" field after the Name: field. It should set and show this as well as the name. Instead of having two separate arrays for name and phone (you might forget to update one of them - I do that all the time!), use just one array of Person objects. (5 points)

- Make a <nop>SearchableAddressBook applet. Add a "Search by name" button.

When pressed, you should display the first record whose name equals the one in the name field. NOTE: Use .equals(...) instead of == to compare strings. You can also use .equalsIgnoreCase(...) if you want "ABC" to be considered the same as "abc". (5 points)

You may ask other people for help as long as you type and understand your entire submission.

1. CS161: Notes on file systems

Categories: CS161#17 -- Permalink
I might not be able to give a lecture on filesystems this Friday (or it might whiz past with my mile-a-minute speech), so here are some detailed lecture notes on file systems to help you review for the final exams.

File systems. You know about file systems because that's one of the most obvious things about your computer. You know that your documents are hidden somewhere on your hard disk. You know that programs are stored in another directory. You know that you can make your own directories to organize your programs and your data.

Why are file systems important? Well, remember what happens during brown-outs or power fluctuations? If the power goes off and then on again in a computer lab without uninterruptible power supplies (UPSes), people will lose all their unsaved work. That's because the kind of memory we use loses its data if it doesn't have power. Hard disks, on the other hand, keep data even when the computer is off - so save often!

Attributes

Let's take a look at the data associated with files. Different operating systems keep track of different things, but here's a short list.

- Name: The files in a directory should have unique names. This lets

us refer to that file.

- Type of file: What kind of file is it? Is it an image? A text

document? A webpage? This could help the operating system determine the best way to open this file.

- Location: Remember our discussion about sectors, tracks, and

cylinders? Where is the file on the hard disk? In terms of the logical filesystem, in what directory is the file?

- Size: How large is the file? How much data is in it right now?

- Protection: Can other people edit this file? run this file? read

this file? Who can work on this file?

- Time/date: When was it last modified? (Sometimes the operating

system stores more information, like creation time and access time)

- User identification: Who owns this file? (Windows 98 and below

don't care about this.)

Operations

- Create, write, read, delete: Makes sense.

- Reposition within a file: When you're reading a book and you want

to flip back to a certain place, you don't have to close the book and then open it again. You can just go to a certain position. Same with files. Some files will let you easily jump to a specified position within the file. On the other hand, some files will only let you go forward and not back.

- Truncate: Get rid of everything after a certain position.

Access methods

- Sequential: You can only access it going forward. You can't go back.

To read the 7th line in the file, you have to read the first 6 lines.

- Direct: You can jump around. This is also known as a random-access

file.

- Indexed: This is like the way a dictionary works. You can jump

around, but some places (like the beginning of entries for each letter) are marked so that you can jump to them easily.

Partitions

Your hard disk is divided into partitions (PC term) or volumes (Mac term). These are the "drives" you see on Windows. You can split up one hard disk into a C: drive and a D: drive. If you learn about something called RAID, you can combine two hard disks into just one drive. ("Drive" is a confusing term, though, as you can have C:, D:, E:, F:, etc. on just one hard disk.)

Directories
Single-level directory

Long, long ago, MSDOS didn't have directories. Seriously. Well, there was one directory, and all of your files had to be under it. You can imagine how this sucked, as all of your program files had to be in the same place as your data. Not only that, you were limited to 8 characters for the filename and 3 characters for the extension.

People used clever names like AAAAAAAA.TXT, AAAAAAAB.TXT and AAAAAAAC.TXT for their files. Of course, after six months, who could remember what the contents of each file were?

The diagram shows a single-level directory. The directory entries are "cat", "bo", "a", "test", "data", "mail", "cont", "hex" and "records". All of these entries point to files.

Two-level directory

Now we got to organize them by user, at least. Still sucked because all of your files were in just one directory, but at least you didn't have to worry about other people's naming schemes.

Tree-structured directory

This is the kind of directory tree you got used to in Microsoft Windows. You can create directories (aka folders) inside directories inside directories inside directories.

Links aren't actually real directories. In Microsoft Windows, they're fake - for example, you can't cd into them from the command line.

Acyclic graph directory

Remember the ln command from Unix? This is where links in Unix come in. (They're _real_ links, not like the fake ones in Microsoft Windows. =) )

ln somefile anotherfile

creates a link: anotherfile will refer to the exact same file that somefile refers to on the hard disk. They point to the same place on the hard disk.

This allows you to have acyclic graph directories, because the same file is referred to in two or more places.

General graph directory

ln returns! This time, we use it to make a symbolic link.

ln somefileordir anotherfileordir -s

Symbolic links can point to directories, so it's perfectly acceptable to make a link that points to one of your parent directories and thus get into some kind of loop.

Basically, a general graph directory is anything that could have these loops.

File protection

You don't want just anyone messing around with your files. Remember Unix file permissions and chmod? This slide talks about some of those permissions, although the access groups the slide uses are different from Unix permissions. Under Unix, it's user, group, others. Other operating systems support access control lists (ACLs) - this means that instead of just giving permission to one group, you can specify exactly who gets to do what to the file.

Allocation methods

Here we start looking at how things are physically stored on your hard disk. You can start up defrag to get an idea of what it looks like.

Contiguous allocation

It's like contiguous allocation for memory. All the space the file needs should be in one continuous block. The nice thing about it is that it's easy to figure out where all the data is - just find the starting position and count off so and so many bytes. If you need to allocate space for a new file, try either first-fit or best-fit. Downside of this is that file sizes are fixed, because once it's been allocated and another file has been allocated next to it, the file can't grow.

Linked allocation

The file is treated as a list of blocks, where a block is a fixed-size contiguous collection of bytes on the hard disk. The blocks don't all have to be together on the hard disk - each block in the file points to the next one. Plus side: files can grow, just link in new blocks. Minus: To read the file, you have to hop around the hard disk, plus all of that pointing around wastes space because each block has to refer to the next one. Solution: use groups of blocks (aka clusters), but these might be bigger than you need - if so, then space is wasted.

File allocation table

Form of linked allocation. The links to the next block are kept in one large table in a certain place in the hard disk.

Indexed allocation

Still uses blocks, but instead of each block pointing to the next one, one block has an index that points to all of the blocks. This block is called the inode (index node) under Unix. If the file is too big for one index block, the index block refers to other index blocks.

Free-space management

How your computer can tell how much space you have free.

- Bit vector/map: keep track of every single block! Requires 1 bit per

block (1 if it's occupied, 0 if it's free).

- Linked list: Like the linked allocation scheme (one block points to

the next one) except this keeps track of the free ones.

- Grouping: The first free block has an index of free blocks (as many

as it can). If there are more free blocks than will fit in the index, it just points to another index block.

- Counting: Keep track of the beginning of each set of free blocks

and how many free blocks there are.

Directory implementation

How directories work. That is - how do directories store info about files within them?

One way is to just keep a list of all the filenames in that directory. If you have a million files (and it's happened!), this can get _really_ slow.

Hashtables are supposed to be faster at lookup, so naturally there's a way to use them too.

Consistency checking

Remember the scandisk that shows up when you improperly shutdown your computer? This is what's happening. Your computer's checking if what it thinks your filesystem should be like is different from what it actually is.

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I'd love to hear about any questions, comments, suggestions or links that you might have. Your comments will not be posted on this website immediately, but will be e-mailed to me first. You can use this form to get in touch with me, or e-mail me at sacha@sachachua.com .

Page: 2004.02.20
Updated: 2004-11-2119:44:1419:44:14+0800
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