A Review of Gnome Shell from my Perspective (and a Comparison with Compiz)

I tried out Gnome Shell today. (And it didn’t break everything! I followed their instructions it build and ran fine, and when I killed it, my normal environment with the normal Gnome Panel and Compiz… went back to normal.) Its shaping together nicely, there are many good things and I think its a great effort by everyone behind it. (But just a warning I don’t know all the technical things behind everything here, so please excuse me if I miss something or don’t use the correct terminology. This review is just from my perspective/my view. It is not a proper usability evaluation, nor have I looked and which is better engineered or anything too technical.)

My Desktop Running Gnome Shell

My Desktop Running Gnome

The obvious difference is there is no bottom panel in Gnome Shell and the top panel is different (but its still in development of course so in a later version they may make more use of it).

My Current Work-flow

Window Management

Normally in Gnome I use Compiz a lot to help me manage my open windows. Compiz/Compiz Fusion has a lot of plugins, but over time I’ve found a few which I really like and I use all the time.

If I have a bunch of windows in one workspace and I want to switch to another I usually use Scale (shortcut of Super + Tab), although I still sometimes use the bottom taskbar, and I always use that taskbar when the window is minimised (Because Compiz can’t access minimised windows pixmaps so they don’t appear in Scale unfortunately. This is a real killer.). I can also right click on a window in this view to close it. This makes it really easy and fast to kill a heap of windows that I have finished with. This makes my search space when changing windows much less and hence much easier.

The Compiz Fusion Scale Plugin

To change workspaces I use Expo (shortcut of Super + E). But I don’t actually use more than I workspace all that often, even though I think I should be. The other great thing is I can drag windows from one workspace to another while in Expo.

Compiz Expo Plugin

Some other shortcuts I use for window management very frequently are,

• Alt + Left Mouse to move a window (with the great wobbly windows effect)
• Alt + Middle Mouse to resize a window.
• Alt + Right Mouse to close a window.
• Super + Scroll to zoom in.
• Ctrl + Alt + (1-9) on the keypad to place a window in a grid. This is great for getting say a terminal to run your program next to the editor with the code. This gives me the benefits of a tiling window manager such as xmonad (although changing the window focus between two side by side windows is not as easy as it would be in xmonad)/an arrangement similar to what you can get using Terminator.
• Super + Shift + (Up Arrow/Right Arrow) to extend a window to the maximum extents in a vertical/horizontal direction.

I keep making refinements to this, but it works very well for me as it is.

Application Starting

In Gnome I use the Panels Run Application dialogue (see pic) (with the shortcut Alt + `) and the terminal (with the shortcut Ctrl + `) to start new applications. Those shortcuts really make things easier and faster.

The Panel's Run Application Dialogue

The run dialogue is good. I can run programs like firefox, gedit just as you would in the terminal but it means I don’t have to have a terminal open or open one first (its all amount maximising efficiency, so I can get to where I want to be as fast as possible). Also I can enter locations such as /etc/whatever and Nautilus will be opened to that location. That text box has tab completion (and it actually shows the suggestions) which makes things easier and faster.

In Gnome Shell

Gnome Shell Activity Mode (sorry, I'm not sure what its actually called)

Window Management

In Gnome Shell (it uses Metacity not Compiz) you can do all your window management and application starting through the Activities mode. Which can be started either by the Super key, clicking Activities, or dragging the mouse to the top left edge (although it seems I must go to the exact 0,0 pixel not 0,1 or 1,0 which is a bit annoying). This is good it gives the user some choice they may happen to have their hand near Super so they use that, or they may only using the mouse so they can use that (actually I will set up Compiz Scale to work with both Super Tab and a top left mouse move).

On the down side, Gnome Shell did not seem to be as fast and responsive as similar Compiz tools. What I mean is that on my system where the Scale tool is fast, as in the windows move smoothly and quickly, when I go into the Activities mode its has a small delay (less than a second, but its still annoying) and its seems a bit jumpy and jerky when everything is moving. But of course its still in development so I’m not going to criticise this. Apart from this, it seems just like Compiz Expo + Scale together. This activity mode window management is good, but there are some small things like I can’t seem to close windows from this activity mode (like I can in Compiz’s Scale), but I can move windows from one workspace to another in Gnome Shell just like in Compiz’s Expo. Also it can also be annoying to have Scale and Expo mixed together (of course I can just just Alt + Tab or move windows around so I can focus on another, but I don’t really like that idea).

Unlike Compiz/Gnome’s multiple workspaces, in Gnome shell you can add these dynamically. Which I think is a better idea than the static type that normal Gnome/Compiz uses.

Gnome Shell allows you to dynamically add/remove workspaces

Things seems to be shifting towards emphasising multiple workspaces. What I need to try to remember to do is USE these multiple workspaces, grouping windows together where they group nicely, instead of just putting everything in one workspace. Window managers could help me with this, like they could remember that I often have Firefox on workspace 2, so when I run it automatically put it there and switch to workspace 2. I haven’t tried this, so I don’t know if it would help me, or just frustrate me by doing what I don’t want every time. I’m not even sure if Compiz can do this anyway.

I’m not sure where dock’s like Avant or Cairo fit in, but I never really found them to make things easier.

Application Starting

The other noticeable thing in Gnome Shell is that bar on the left. In normal Gnome you have your menu bar which has Applications, Places and System (which I wish I could easily shorten to Apps, Places and Sys to save space). Given I have this new user thing on the right where I can shutdown/logout/suspend/hibernate… from the only real thing I use System for is the Preferences and Administration. Yet I can never remember if what I want is in admin or pref. I recently discovered this system preferences thing which just puts it all in one window categorised into appropriate groups. I’m sure some find the two lists easier and some find the single window easier. When I scan with my eyes in a list I just go up/down, but when I scan a grid my eyes wander all over the place with no apparent system. As such its probably a more random search than a well defined one. There is heaps of things you could test out (we looked at some in my HCI course) to try to make the grid layout faster but nonetheless I think I like the grid better.

I use the Places bar often, and I think the Gnome Shell implementation makes things easier as they are listed in two columns, unlike traditionally where the number of bookmarks is limited and I need to navigate to a sub-menu to show them all. It seems I can’t change the size proportions of those three sections on the side, but again its still in development. You could look at this a number of ways but because the panels are gone, if you are using a full screen application you can focus on that, with nothing cluttering the edge or distracting you from your task at hand. Traditionally everything in layered down, you have panels, then window decorations then menu bars, status bars, tabs (in Firefox), removing all that so that you just have the task at hand in your vision can be a great thing (yes I know there is a full screen feature in Firefox, and you can set Gnome panels to hide). When you are working in a browser its up to the web site (unless you have the time to do some Greasemonkey scripts) to allow you to again remove outside clutter, yet many application-like web sites allow you to do this (Alt + Shift + G when editing in WordPress, u in Google Reader (to some degree)). Anyway that is moving away a bit from the topic of this post.

At the bottom of the left bar, you have recent documents. I use recent documents very very rarely (as in the shortcuts to them, not the documents themselves). Although I still think that a well designed system for access to recent documents integrated with some kind of search capability would be very useful for me, and I would use it often. However I am yet to find such a system that I like. The concept in my mind is something like the Lifestream design that Wei Zhou blogged about. An interface where time is on the horizontal axis, where you could change the scale and location of this view easily, view related things such as the weather for that particular time, your location if you have a GPS enabled laptop, etc. Also it should be integrated with a good filter feature (anything such as file type, file size, location, tags…) that lets you narrow down your search space. Something like that is what I have in my mind as a great use of a “recent documents” feature. GNOME Zeitgeist looks like it may address some of this.

Lastly the top section is the application launcher.

Gnome Shell Menu

The actual menu in some ways is much better than the normal Gnome menu. Larger icons and a short description of the application are good. When I open the Gnome menu bar, I never need to see what’s on my screen in order to make my selection from the menu bar (and if I forgot what I wanted to start I can always close it then open it again). You have the whole screen so you may as well use it, and Gnome Shell seems better in this respect. The bad thing is I don’t like the use of pages. If not everything will fit on one column, you have to change the pages at the bottom. Instead you should be able to scroll through the options with the mouse wheel, or the ones that don’t fit go in another column to the right (like Windows XP can do, and yes I used to use Windows XP).

The search box above this doesn’t behave like the traditional Gnome Panel’s Run Application dialogue. For example I can’t type a file path, and tying gedit then enter won’t take me where I want to go (gedit). Instead it takes me to some other entry I have defined in the menu bar. Now I can see some reasons why this could be better. Really I want to launch any executable files in my $PATH, but a user who doesn’t use the terminal probably doesn’t want this. An option so that the user can choose how they want it to behave would be better, I think.

Gnome Shell's search box doesn't behave as I expected.

Having all my icon application starters in the top Gnome Panel was nice but there is no reason those can’t be added to Gnome Shell, but again it’s still in development. Although now that I’ve been using the interface for an hour or so, I think that they may create more clutter. Actually I think I would prefer that that top panel bar in Gnome shell would only appear in the Activity mode (but still recognise the top left mouse gesture). Although this may be scary for newbie’s (hey I got intimidated the first time I used Blackbox, I couldn’t work out that right clicking on the desktop gave me a menu) so an option would be much better.

Anything thing I wanted to mention was, I use Firefox a lot, and a lot of the concepts and issues with window management can be applied to tabs in a browser. The folks over at Mozilla are working on this so I’m eager to see what they come up with, but as more and more things are done through HTML web pages, it just means I’m going to have more and more tabs open that I need to manage, and navigate. Like starting a new application in a desktop environment you often start a new task (web page/tab) in a web browser. I’ve been using Ubiquity for a while now at I find it really good. Although they are up to release 0.5, I’m still using 0.1.9rc6. Although I can think of many improvements, its still really efficient at starting new tasks.

Oh an in case you were wondering from my Screenshots there, I’m using the orange-theme (orange-theme – 1.3.0.jaunty.ppa2+nmu1) from https://launchpad.net/~bisigi/+archive/ppa/+packages.

SBS Playlist to RSS Feed Perl Script v2

I have made some changes to my original script. This new perl script will scrape info from sbs.com.au and give an RSS feed of the items in the specified playlist. I only know of two playlists (94 = Latest Full Episodes, 95 = Preview Clips). Only one line needs to be changed to use the script to give the RSS feed of a different playlist. The major improvement is the items that are only available over RTMP now have the correct URL which was previously incorrect (but now the script runs slower as it has to grab more pages from the web). I use the url, http://player.sbs.com.au/video/smil/index/standalone/$item_code/ to find out the url details.

FLVStreamer appears to do a good job of downloading media over the RTMP protocol. Just use ./flvstreamer -r rtmp://file.flv > file.flv. Mozilla has an article on how to add protocol’s to firefox here. But I didn’t bother with that as the command is simple as it is, and building an app with a save as dialogue is beyond me for now, but I hope to learn that soon.

[Update: It seems that you also need to have the --swfUrl argument set ('http://player.sbs.com.au/web/flash/standalone_video_player_application.swf' works.). Also the perl script doesn't get the file name correctly (it uses the thumbnail image url, rather it should be using the url's given at the /video/smil/index pages).]

For local use the current format will probably be what you want, but in a production environment you probably want to have the script save the RSS file to disk and have people hit that RSS file with the requests. Just set the perl script to run every now and then. Unfortunately I can’t seem to upload .pl files to WordPress (I’ve put a link, but that will expire eventually)… I really need to get my own site.. There is so much customisation I would like to do and many experiments to try out on a live server, but the $$$’s are too much…

On another note I tried out EPIC (Eclipse Perl Integration), which was fairly simple to install. It seems much nicer than using a plain text editor and command line, especially the debugging abilities that it adds.

SBSPlaylistToRSSv0.2.1.pl

Deque Implementation Design

For Assignment 1 of COMP2911 we got the task of implementing a deque, using arrays and linked lists (in Java). Here is the design I used for the implementation.

ArrayDeque

This was quite a challenge.

One approach was to have an array of size capacity, and also store some left and right index pointers to know where the deque ranges in the array.

My first idea (actually inspired by my tutor) was basically, to add to the right of the deque you start at the left and push right, to add to the left of the deque you start at the right and push left. When the two ends collide, you make a new array larger keeping the left and right parts of the deque at the leftmost and rightmost parts of the array.

This turned out to have some problems. All worked well, except for a couple of things. Such as show here,

, because now you are left in the middle and your not just pushing straight from one side to another.

This next situation below was also troublesome because you have to be careful where you store your left/right index pointers. You need to carefully think of what if they overlap? And if they are equal or overlap, is your arrayDeque full or empty?

So what I did is look at the different ways to store the left/right index pointers. In all these diagrams orange means the location of the left index pointer, and the violet is the location of the right index pointer.

a) In this diagram I point to the next available space.

b) In this diagram I point to the location of the most recently added item. At the very beginning I loop them over.

All this just lead to much confusion for me and many bugs and problems. So I had a look at an entirely new approach. It turned out much better, and I learnt that if things aren’t working out and you just keep getting lots of bugs then sometimes trying another approach can really help out. I also learnt that its really hard to know the best way to do something right from the start you really have to try something and if its not working to well than just try something else and see how it goes. So here is what I finally did.

Initially I just ignored the fact that I had a limited size array, and that things are actually mod array size. I just had two index pointers (shown in orange and blue) which pointed to the next free space at the left and right ends of the deque. I kept these as ints and they I don’t think of them as wrapping round. If you forget about the array based implementation and go back to the original problem of a deque you see that really all you need is a big long line with a left end and right end pointer. Now all you have to do is remember that whenever you are dealing with implementation level things such as accessing the array, to use index mod capacity, rather than the raw index value (which may be out of the array range). That and you need to have a check to know when to resize your array.

Under this scheme the number of items in your deque is rightIndex – leftIndex – 1, therefore your array is full if and only if (rightIndex – leftIndex – 1) >= capacity. (Where capacity is the size of the array). If this is true then you need to resize your array.

The method I choose was to simply shift the deque along (either direction) so that the leftIndex is at -1.

LinkedDeque

This was much simpler. Basically I made a link object that stored an item and a left and a right pointer (to other link objects). I would store the leftmost and rightmost link items for the deque and that is all. I guess I could have stored size as part of the deque object and updated it whenever new link items were added or removed to the deque, but as we were given no requirements to do it one way or another I made it so that the size method would go though the whole deque and do a summation every time it was called.

The only thing I really had to watch was to ensure that I kept the left and right pointers for each link item up to date with changes, and this was my primary source of bugs.

Very Useful Ubuntu/Unix Commands

Just some Ubuntu/Unix commands that I seem to find very useful but can be hard to remember at times.

PDF Concatenation

pdftk in1.pdf in2.pdf cat output out1.pdf

Its annoying that nautilus doesn’t integrate this by default, so I could select some PDF’s then right click and choose merge. Luckily I can do this with the fairly simple Nautilus Actions Configuration app. But this takes time and the average user doesn’t have the time to research these things on how to do it themselves. Of course this could be extended further to match what you can do from the command line, such as choose some method of ordering the files, or choose to do weird things such as rotate every second page. Sure you want to keep things simple, so as a start it would be great to see something basic. There are probably even nautilus scripts in the repositories out there that do this exact thing, but you spend half your time finding them and installing them. I think this should be enabled by default, or put in some options page somewhere.

Truncate an mp3 file

mpgsplit input.mp3 [00:00:20-00:00:58] -o output.mp3

It appears that this doesn’t re-encode so it seems to run very fast.

Mount an ISO

sudo mount -o loop ~/disk.iso /media/cdrom0

Strip audio from a video file

ffmpeg -ab 320k -i in.mov outaudio.mp3

Unfortunately when I left out the bit rate, it defaulted to 14K or something much lower than what the source file was using.

Search all files in the current directory for some string

find . -exec grep "searchthisstring" '{}' \; -print
find . -exec grep -q "searchthisstring" '{}' \; -print

Trim and nup pdfs

pdfnup --nup 2x2 --pages 26-140 --trim "1cm 1cm 1cm 1cm" infile.pdf

Wake from Suspend

sudo rtcwake -u -s 12600 -m on

… then you need to manually put the computer in suspend and 12600 seconds later the computer will resume.

Susped

pm-suspend

Shutdown

sudo shutdown -h now
sudo shutdown -r now

… the -h means halt (shutdown), -r means reboot.

Tar and Gz all the files in a directory

tar -cvfz file.tar.gz *

Get file info from an HTTP server without downloading

wget -S --spider http://www.site.com/file

COMP2121 Quick Summary on Particular Aspects

Von-Newman vs. Harvard Architecture.

Von-Newman has a single memory space, share for data and program instructions. Harvard Architecture has separate memory spaces for data and instructions (so you cannot execute from the data memory).

2’s compliment

It is important to know that the hardware does all arithmetic in 2’s compliment. It is up to the programmer to interpret the number as signed or unsigned.

To convert a number from 2’s compliment, for example -45 in 2’s compliment is 11010011, we can do something like this,

To go the other way from say -1 to the 2’s compliment form 11111111 we use that formula. I’m not exactly sure how its supposed to work so I’ve hacked it to make it work.

If the number you wish to convert is negative, let , so that X is positive then take where p is the number of bits you are using (say 8), then subtract X. If the number to convert is less than (where p is the number of bits, say 8 ) then leave it as is and that in your 2’s compliment.

Now that was complicated. But its the only way I can get that advertised formula to work with the given set of sample data (as in that table above).

Sign Extension

Why do we need sign extension? We need it in order to do operations on numbers than have different bit lengths (the number of bits used to represent the number).

Decimal to Binary

From a human kind of approach to convert 221 to binary, we see that , that is 7 is the largest power of 2 less than 221, so we have . That gives us 128, so we still have 93 (221-128) to go. We try , this is less than 93. So far we have . 29 left now, but is greater than 29, so we put a zero in that digit, ie. . If we go on we get . Taking the coefficients of the terms we get the number 221 in binary, 11011101.

We can convert hexadecimal to binary by going from hex to decimal then decimal to binary. For hex to decimal,

(where F23AC is in hex and 992172 is in decimal)

Operations on Signed and Unsigned Multi byte Numbers

add al, bl
adc ah, bh
does a + b, result in is a.

There are 3 multiplication operations, MUL (Multiply Unsigned), MULS (Multiply Signed) and MULSU (Multiply Signed with Unsigned). They each do this. Notice the result is stored in r1:r0.

Thus to do n*m = r where n is 2 bytes unsigned and m is 1 byte signed,
mulsu nl, m ;nl * (signed)m
movw rh:rl, r1:r0
mulsu nh, m ;(signed)nh * m
add rh, r0

We can also do 16bit * 16bit,

;* From AVR Instruction Set Guide, pg 99-100.
;* Signed multiply of two 16-bit numbers with 32-bit result.
;* r19:r18:r17:r16 = r23:r22 * r21:r20
muls16x16_32:
clr r2
muls r23, r21 ;(signed)ah * (signed)bh
movw r19:r18, r1:r0
mul r22, r20 ;al * bl
movw r17:r16, r1:r0
mulsu r23, r20 ;(signed)ah * bl
sbc r19, r2
add r17, r0
adc r18, r1
adc r19, r2
mulsu r21, r22 ;(signed)bh * al
sbc r19, r2
add r17, r0
adc r18, r1
adc r19, r2
ret

brge and brsh

• brge is Branch if Greater or Equal, Signed.
  if () then branch.
  When you do cp Rd, Rr then brge, the branch will be taken if Rd Rr, where Rd and Rr are taken to be signed numbers.
• brsh is Branch if Same or Higher.
  if () then branch.
  When you do cp Rd, Rr then brsh, the branch will be taken if Rd Rr, where Rd and Rr are taken to be unsigned numbers.

Calculating Total Stack Space Needed

Draw a call tree, find the path with the most total weight, that total weight is the total stack size needed. Here is the sample question,

A C program consists of five functions. Their calling relations are shown as follows (the arguments and irrelevant C statements are omitted).

int main(void) {
  …
  func1(…);
  func2(…);
  …
}

int func1(…) {
  …
  func1(…);
  …
}

int func2(…) {
  …
  func3(…);
  func4(…);
  …
}

func1() is a recursive function and calls itself 15 times for the actual parameters given in main(). Both func3() and func4() do not call any function. The sizes of all stack frames are shown as follows.

main(): 200 bytes.
func1(): 100 bytes.
func2(): 400 bytes.
func3(): 1,400 bytes.
func4(): 300 bytes.

How much stack space is needed to execute this program correctly? (3 marks)

There are three paths,

main() func1() func1() x 15 200+100+15×100 =1800

main() func2() func3() 200+400+1400 =2000

main() func2() func4() 200+400+300 =900

The path with the most total weight is main() > func2() > func3(), so this is the total stack space needed.

Nested Interrupts

(Source: Hui Wu’s Lecture Notes)

Keypads with ‘abc’ ‘def’ … buttons

These keypads where to enter b you need to press the abc button twice in succession, but wait to long at it will chose a. Here is a psudo algorithm that seemed to fit this,

.def reg = rN
.def reg = rM
.def count = rX

//passvalue means that we register the given value ie. abc abc wait > b

setup:
clr reg (to some value that is != to a key value) ;set to default
clr count
rjmp keyloop

keyloop:
  check pins for a key
  if no key pressed rjmp keyloop, else continue

  //key was pressed, and value is stored in key
  reset someTimeCounter
  if (key == reg) {
     inc count
     if (count == 3)
        passvalue(reg,count)
  }else{
     if (reg != default) ;so we don't initially passvalue
        passvalue(reg,count) ;send the last value
     reg = key ;store the new one
     count = 1
  }

rjmp keyloop

if someTimeCounter expires and count != 0 //(count up, so expires after time to wait for anymore keypresses) (check count != 0, because if its 0 then we never had any key pressed that we need to send)
   passvalue(reg,count)
   reg = default

Switch Bounce Software Solution

When a switch makes contact, its mechanical springiness will cause the contact to bounce, or make and break, for a few millisecond (typically 5 to 10 ms). Two software solutions are wait and see and counter-based.

1. If we detect it as closed, wait for a little bit and check again.
2. Poll the switch constantly. For each poll if the switch is closed increment the counter. If we reach a certain value in a certain time then the switch was closed (or button pressed).

Serial Communication (Start and Stop bit)

“[The] start bit is used to indicate the start of a frame. Without the start bit, the receiver cannot distinguish between the idle line and the 1 bit because both are logical one. A stop bit is used to allow the receiver to transfer the data from the receive buffer to the memory.” (Wu, Homework 6 Solutions)

UART

(Source: Hui Wu, Lecture Notes)

Sample Q3a

(This code probably won’t work and probably has errors (and maybe not just simple ones, but serious ones that mean that the logic is wrong))

.dseg
A: .byte 20 ;array of size 10, element size 2 bytes

.cseg
ldi XL, low(A)
ldi XH, high(A)

;add the contents of the array.
store 0
store 1
store 2
store 3
store 4
store 5
store 6
store 7
store 8
store 9

;find the largest value
ldi XL, low(A)
ldi XH, high(A)

;start with the 1st element of the array
ld r25, X+
ld r26, X+

ldi r20, 10 ;size of array
loop:
 cpi r20, 0
 breq endloop

 ld r21, X+
 ld r22, X+

 cp r25, r21
 cpc r26, c22
 brlo lowerthan
 ;we have a new max
 mov r25, r21
 mov r26, r22

 lowerthan:

 inc r20
 rjmp loop
endloop: rjmp endloop

.macro store
ldi r16, low(@0)
ldi r17, high(@0)

st X+, r16
st X+, r17
.endmacro

For some reason in my lecture notes I have “eg. fine 2nd or 3rd smallest or largest” so here is a modification to do something like that.

.dseg
A: .byte 20 ;array of size 10, element size 2 bytes

.cseg
ldi XL, low(A)
ldi XH, high(A)

;add the contents of the array.
store 0
store 1
store 2
store 3
store 4
store 5
store 6
store 7
store 8
store 9

;sort into accending
loopthough for the length of array, (by then we can be sure its sorted)
ldi r23, 10
largeloop:
 cpi r23, 0
 breq endlargeloop

 ;point X to the start of A
 ldi XL, low(A)
 ldi XH, high(A)

 ;start with the 1st element of the array
 ld r25, X+
 ld r26, X+

 ldi r20, 10 ;size of array
 loop:
 cpi r20, 0
 breq endloop

 ;the next value
 ld r21, X+
 ld r22, X+

 cp r25, r21
 cpc r26, c22
 brge gethan
 ;r22:r21 < r26:r25
 ;swap the order
 st -X, r26
 st -X, r25
 st -X, r22
 st -X, r21

 ld r24, X+ ;to change the X pointer
 ld r24, X+

 ld r25, X+
 ld r26, X+

 gethan:

 inc r20
 rjmp loop
 endloop:
endlargeloop:

inf: rjmp inf

.macro store
ldi r16, low(@0)
ldi r17, high(@0)

st X+, r16
st X+, r17
.endmacro

COMP2121 – Wk05 – Interrupts

To explain interrupts, Wu used an example of a network card that is downloading a file. The network card has a buffer, and only once this buffer is full (or data stream is complete) should the CPU then copy the contents from the buffer to the RAM. So how does the CPU know when the network card’s buffer is full and when to execute the copy? He described two ways here interrupt and polling.

Polling involves the CPU periodically asking the network card, are you full? Two problems with this method are a) there may be a delay as you have to wait for the poll request to be made and b) it wastes a lot of CPU time. Polling is implemented in software, not hardware.

An alternative to polling is using interrupts whereby the network card will send an interrupt signal to the CPU to get its attention. This needs special hardware to implement, however it is very efficient compared with polling.

An interrupt system must (among other things),

• Wait for the current instruction to finish before taking care of the interrupt.
• Return to the interrupted program at the point where it was interrupted.
• Signal the interrupting device with an acknowledge signal when the interrupt has been recognised.

IRQ is an interrupt request signal.

A daisy chain arrangement (as seen below) allows multiple devices to send and IRQ. However the CPU cannot determine from the IRQ line which device sent the interrupt. So in a daisy chain system when the CPU receives an IRQ, it will send a signal to IO1 asking “did you send the IRQ?” if IO1 sent the request it will reply “yes”, if not it will pass the question on to the next IO device and so on. The response is passed back in the same way.

Reset is an interrupt in AVR, and in the AVR Mega64 there are five different sources of reset. There is a flag in the MCU Control Register for each of these and can be used to determine the source of a reset interrupt. The watchdog timer is one source of reset.

Watchdog Timer

The watchdog timer is used to try to reset the system if an error such hang occurs. The watchdog timer in AVR can be enabled or disabled.

If the Watchdog timer is enabled, it needs to be periodically reset using the wdr instruction. When (if) the Watchdog times out, it will generate a short reset pulse.

PostgreSQL Very Basic Cheatsheet

(Wrote this a few weeks ago when I knew nothing. Indented into my brain now. Should have published earlier or just trashed the post as it seems too simple now. So instead I’ll update it when I find out some new neat tricks.)

List of databases:
$ psql -l

To open one of them,
$ psql MyDatabase

To see what is in the database (list of relations),
mydb=# \d

To examine a specific table,
mydb=# \d TableName

Can execute SQL,
mydb=# select * from Table;

Can edit SQL in an editor from within PSQL,
mydb=# \e

To quit,
mydb=# \q

To load a schema from a file
$ psql mydb -f /home/foo/bar

Also from the shell,
$ pg_dump -O myDB > file
(-O means no ownership information is outputed)

On my server configuration (default for ubuntu) you can restart the PostgreSQL service using,
$ sudo /etc/init.d/postgresql-8.3 restart

COMP3311 – Week 7 Notes

Its pointless repeating what John put in the lecture slides, so this is just my additions that he mentioned but are not in the slides.

Aggregates

Initial condition is defaulted to NULL. So sometimes you will need to define,

  initcond = '';

This is different to NULL, because,

  null || 'abc' --> null

(where || is append) but

  '' || 'abc' --> 'abc'

PHP

$x = 2;
myFunc() {
  global $x;
}

If we omit the global $x, then any references to $x in myFunc will refer to a new local x, not the first x that is set to 2. To avoid this and force any references to x inside myFunc to refer to the first x that is equal to 2, we need this global $x line.

strpos

1. $i = strpos('abc', 'a') --> 0
2. $i = strpos('abc', 'b') --> 1
3. $i = strpos('abc', 'z') --> false

if($i) will only be true in case 2 (false in case 1 and 3). So if we want to test if the second string was at all in the first string we must use,

if($i !== false)

This one will be true in case 1 and 2, but not 3.

COMP2121 – Wk03/04 – Data Transfer/Functions

[Updated: Clarified what I wrote about the way stacks in AVR work, in the Push/Pop section.]

Data Transfer Instructions

(Credit: Hui Wu/COMP2121 Lecture Notes)

Load Direct (ld):

ld Rd, v
Rd  {r0, r1, ..., r31} and v  {x, x+, -x, y, y+, -y, z, z+, -z}

(remember the X, Y, Z pointers)

Load Program Memory (lpm):

Can take on three forms,

Syntax	Operation
lpm	R0⟵(Z)
lpm Rd, Z	R0⟵(Z)
lpm Rd, Z+	Rd⟵(Z) Z⟵Z+1

Z contains the byte address while the program flash memory uses word addressing. Therefore the word address must be converted into a byte address before having access to the data on the program flash memory. Example usage, (Table_1<<1 converts the word address into a byte address)

   ldi zh, high(Table_1<<1) ; Initialise Z pointer
   ldi zl, low(Table_1<<1)
   lpm r16, z+ ; r16=0x76
   lpm r17, z ; r17=0x58
   ...
Table_1: .dw 0x5876
   ..

IN/OUT:

AVR has 64 IO registers. Example,

in Rd, A

out A, Rd

where 0 ≤ A ≤ 63.

Push/Pop:

The stack pointer (implemented as two 8-bit registers in the I/O space) points to the next free space in RAM above/below (depends how you look at it) the stack. The stack in AVR is part of the SRAM space, and the stack (in AVR) grows from higher memory locations to lower memory locations. I’m talking about the actual value of the address, so 0xFFFF is a higher address than 0xDDDD. This got me a little confused at one stage because if you draw a picture of memory with 0×0000 at the top of the diagram, and 0×0001 below it and so on then in reference to the diagram a stack that is getting larger with PUSH operations is growing upwards (you usually associate higher to lower with down, this is why I got confused).

So the first thing you must do is initialise the stack pointer (RAMEND is a good starting location).

So a push operation,

push Rr

will push Rr onto the stack (ie. put the contents of Rr into the location that the SP points to), and then decrement the SP by 1. Pop has a similar opposite effect.

Shift Instructions

Logical shift left

lsl Rd

Rotate Left Through Carry

rol Rd

Both operation change some status register flags.

Functions

We dabbed into this in first year but just to revise and extend a little I’ll try to reiterate this here.

The heap is used for dynamic memory applications (eg. malloc()).

The stack is used to store return addresses, parameters, conflict registers and local variables and other things.

In passing parameters in WINAVR (C compiler for AVR) for say a function call they are passed by value for scalar variables (eg. char, int, float) and passed by reference for non-scalar variables (eg. array, struct).

Rules are needed between the caller and the callee to resolve issues such as,

• how to pass values and references to a function?
• where to get the return value?
• how to handle register conflicts? (if a function wants to use a register that was previously in use)
• how to allocate and deallocate stack memory to and from local variables?

If a register is used in both caller and callee and the caller needs its old value after the return from the callee, then a register conflict occurs. Either the compiler or the assembly programmer needs to check for this. The work around is to save the conflict registers on the stack.

The return value of a function needs to be stored in designated registers. WINAVR uses r25:r24 for this.

A stack consists of stack frames. A stack frame is created whenever a function is called, and it is freed whenever the function returns.

(Credit: Hui Wu/COMP2121 Lecture Notes)

Macros

The AVR assembler offers macros. A macro is just a segment of code that you define and can then use by just calling the macro. Basically the macro name is just a place holder for the macro code. When the program is assembled the macro name will be replaced by the code that macro defines. This defines a macro named mymacro,

.macro mymacro
 lds r2, @0
 lds r3, @1
 sts @1, r2
 sts @0, r3
.endmacro

We can then invoke this macro with,

mymacro p, q

The p, q are used like arguments. So @0 will be replaced with p and @1 will be replaced by q. In AVR you can used @0 up to @9 in the macro body.

Assembly Process

The AVR assembler uses a two pass process.

Pass One:

• Lexical and syntax analysis: checking for syntax errors.
• Record all symbols (labels…) in a symbol table.
• Expand macro calls.

Pass Two:

• Use symbol table to substitute the values for the symbols and evaluate functions (eg. low(-13167)).
• Assemble each instruction (ie. generate machine code). For example add Rd, Rr is encoded as machine code as 0000 11rd dddd rrrr.

References

Wu, Hui. COMP2121 09s1 Lecture Notes.

COMP2121 – ADD and ADC

Just some notes on using ADD and ADC (and also a not on compare and branch instructions at the end). (Thanks also to my lab partner who helped me with some of this stuff.)

The registers for AVR are 8 bits long, as noted if I want to store a 16 bit number I need to use two registers, one will store the low byte and the other will store the high byte. eg the 16 bit binary number a = 1111111100000000 (as big endian) would be stored as a_high = 11111111, a_low = 00000000.

Another thing that I’ve learnt is how addition of multi byte numbers in AVR works. AVR has the instructions ADD (add without carry) and ADC (add with carry). Lets look at a simple case first.

ldi r16, 0b10101010 //the 0b indicates binary ($ or 0x for hex, nothing for decimal, 0 for octal). loads the binary number 10101010 into register 16.
ldi r17, 0b10101010
add r16, r17

What happens here is,

  10101010
+ 10101010
  01010100
  c c c c

The carry bit is set, and hence and overflow has occurred. The actual answer is 101010100 but this has 9 bits and cannot fix in the 8 bits of space we have for the result to be stored into r16. Here we used the ADD instruction this means (at least to the best of my understanding, please correct me if I’m wrong) that we ignore what is originally in the carry flag (part of the status register). If we used ADC and the carry flag was set to 1 then when we add the right most bit as above 0+0 = 0 but we have a carry so we would get a 1 in that last bit. We can use this to do multi byte arithmetic.

Say we have,

//a is a 16 bit number 1010101010101010
ldi al, 0b10101010
ldi ah, 0b10101010
//b is a 16 bit number 1010101010101010
ldi bl, 0b10101010
ldi bh, 0b10101010

//to do a+b (and store the result in a) we do
add al, bl
adc ah, bh

We can then extend this to as many bytes as we want. Say we have a 3 byte number spread over the registers of a_0, a_1, a_2 and another 3 byte number in the registers b_0, b_1, b_2. We could add them like this,

add a_0, b_0
adc a_1, b_1
adc a_2, b_2

The result would be spread over a_0, a_1 and a_2 (remember though that we can still overflow this).

It was also enlightening (though also a nice reminder) to see how this low level addition works. (when doing 1bit addition (without carry), a + b, the sum = a xor b and the carry = a and b.) (see picture below)

(Credit: Annie Guo, Basics of Digital Systems, COMP2121 Notes)

This concept can similarly be applied to other instructions. For example,

cp a, b
breq label

will compare a and be and branch to the label label if a is equal to b (and if not continue on executing the subsequent instructions). However what if you wanted to compare if a two byte number was equal to a two byte number? Sure you could just repeat the code one time for the low byte and another for the high byte but you can also do this,

cp al, bl
cpc ah, bh
breq label

These cp and cpc instructions use some trickery (well not really but I didn’t initially see how this worked, though its quite obvious now) where they modify some flags based on the result of the comparison and then the branch instructions (breq, brne, brlo…) look at the respective flag to see if they should branch or not.