TweetFollow Us on Twitter

May 92 - KON & BAL'S PUZZLE PAGE

KON & BAL'S PUZZLE PAGE

SLEEPING BEAUTY

KONSTANTIN OTHMER AND BRUCE LEAK

See if you can solve this programming puzzle, presented in the form of a dialog between Konstantin Othmer (KON) and Bruce Leak (BAL). The dialog gives clues to help you. Keep guessing until you're done; your score is the number to the left of the clue that gave you the correct answer. These problems are supposed to be tough. If you don't get a high score, at least you'll learn interesting Macintosh trivia.


During the development of QuickTime, a number of interesting bugs reared their ugly heads. Try to figure out this one before KON does.

120 BAL Here's the problem: When you play a movie on a Macintosh IIfx the machine hangs after about six hours. If you turn off the sound, or try it on any Macintosh other than an fx, it doesn't hang.

KON How does it hang? Is the Macintosh locked up?

115 BAL Well, the movies aren't playing. They just all freeze about halfway through. Menus still work, though. You can even switch to the Finder and click between windows and move icons around, but when you launch an application or open a folder, the Finder draws one of the zoom rectangles and then hangs.

KON Like a time bomb. Can you get into MacsBug?

110 BAL Yeah.

KON Whew. For a second I thought this was going to be really tough. You've got MacsBug, so what's the problem?

BAL I don't know, you tell me.

KON How do movies get time? SystemTask or something?

105 BAL No, you have to call MoviesTask.

KON Figures. So I set a break on MoviesTask; is it getting called?

100 BAL Nope.

KON The application is supposed to call it?

95 BAL Yep.

KON So is WaitNextEvent getting called?

90 BAL Nope.

KON But menus work???

85 BAL Yep.

KON WaitNextEvent is being called with a bogus sleep time. I set a breakpoint on it, pull down a menu, and see what the sleep time parameter is.

80 BAL Now you're thinking, Kon! But no. The sleep time is 1, just as it's supposed to be.

KON Hmmm. So I trace 50 times and see where I'm spinning.

75 BAL There's a bunch of stuff going on; it's not just some simple loop.

KON So I record A-traps.

70 BAL There are three traps getting called: ABF7 from inside MF, A0DD (PPCToolbox) from a big block in the system heap that's not QuickTime, and A030 (EventAvail) from a 'scod' resource that's different from the block calling ABF7.

KON There are no null events coming through to the application, so MoviesTask never gets called. The sleep time must never be expiring. Are ticks running?

BAL How do you figure that out?

KON I DM ticks, continue executing, and then DM ticks again.

65 BAL Ticks doesn't change.

KON It's updated by some hardware mechanism at interrupt time, right?

BAL Sure, a hardware mechanism . Is that what they teach you at Caltech?

KON OK, OK. There's a heartbeat task that generates a level-1 interrupt that updates ticks, right?

60 BAL Yeah.

KON So is the level-1 interrupt happening?

BAL How do you check that?

KON I know from reading this cool develop  column that the level-1 interrupt vector is at location $64, so I set a break there and see if it's firing.

55 BAL Wait a second. That's the way ticks are updated on every Macintosh except the fx. This problem happens only on an fx.

KON Why is the fx different?

50 BAL The guy that designed the hardware assumed that the heartbeat task--and thus ticks-- was supposed to happen every 60th of a second. Unfortunately it's supposed to be every 60.14th of a second or something, so Gary Davidian fixed it by installing a Time Manager task that updates ticks. The extended Time Manager, found in System 6.0.4 and later, adds a drift-free mode for Time Manager callbacks. This allows for accurate scheduling of periodic events without long-term drift. With the extended Time Manager, the next callback is scheduled with respect to when the current callback should have fired, rather than the current time. So as of System 6.0.4 there are two types of Time Manager tasks: the regular ones and the drift-free ones. Ticks are updated via a drift-free task, so they advance accurately over time.

KON Hmmm. So is the Time Manager task that updates ticks getting called?

45 BAL Obviously not; ticks aren't changing.

KON How are the tasks in the queue organized?

40 BAL They're kept in the order that they fire in.

KON So where is the ticks task in the queue?

35 BAL Well, it's not the first one, and the first one is scheduled for sometime tomorrow.

KON Fine. I leave and come back tomorrow. Does my movie start playing?

30 BAL It could be. But we're shipping before then.

KON So the first element is getting messed up and never completes. Then none of the other items in the queue get executed because they're all deltas off the first element and the movies hang.

25 BAL Now we're getting somewhere.

KON So how is that first element getting confused?

BAL Whose problem is this?

KON OK. Who owns the first item?

BAL How do you figure that out?

KON I do an IL to see what traps it's calling. I see where it is in the heap. I set a breakpoint on it, force it to fire, and see what it does.

20 BAL You're knee deep in spaghetti. You could probably figure it out this way but it's pretty nasty.

KON OK. I break on InsTime to see who installs it. I break on PrimeTime to see who starts it up. I figure out whether it's using InsTime or InsXTime.

15 BAL The PrimeTime comes from an 'snth' resource. It was installed with InsXTime.

KON Aha! The Sound Manager. That's why it works when the sound is turned off.

BAL They don't pay you enough, Kon.

KON So how does this element get updated in the queue? Don't Time Manager tasks call PrimeTime to reschedule themselves?

10 BAL Yeah.

KON So is someone calling PrimeTime on the Sound Manager task with a bogus value?

5 BAL Not really. It always calls PrimeTime with a value of 0, indicating that it wants to be called right away. The Sound Manager does this since it's not reentrant. By scheduling a task, it knows it won't be called until it's finished servicing the current interrupt, avoiding reentrancy problems.

KON I'm sure everyone that's reading this has figured it out by now. I know I have.

BAL You're bluffing again, Kon.

KON It's easy. Since the drift-free Time Manager schedules tasks based on when they should occur, and since this sound task is using a count of 0 when calling PrimeTime, its backlog gets bigger and bigger. So each PrimeTime call schedules an event that should have occurred further and further in the past. The task is executed immediately, of course, but the backlog builds. It always puts this element at the head of the queue, but eventually it overflows the long that contains the backlog, and the scheduling time becomes incredibly large. There are about 4 billion microseconds in a long, which is about an hour. The Time Manager counts in units that are about 20 microseconds, so it would take about 20 hours to overflow. Take away a bit for signed math and another because it's calculating scheduling times, and I would guess it should hang about once every 5 hours. It must have taken forever to find that one.

BAL Yeah, it was a drag. We fixed it before we shipped QuickTime, of course. But it's just the kind of thing that makes scheduling software projects so hard.

KON Nasty.

BAL Yeah.

KONSTANTIN OTHMER AND BRUCE LEAK have been puzzling about reality, life, the universe, and even computers for a long time. Since the great success of their Graphics '90 World Tour, which included peace-keeping, hostage-freeing, and wall-smashing, they settled down, shipped a few QuickDraw packages, and cleaned out their closets. Then came the coup: da division of da Union. Konstantin got QuickDraw, 200 rubles, and a guaranteed spot at the front of the bread line. Official party line on Bruce: "vacationing in the Crimea." Bruce was actually working on the first QuickDraw spinoff. To provide a seamless upgrade path, and to leverage off of brand awareness, he decided to call this project QuickTime. *

SCORING

  • 100-120 Members of the QuickTime team and their immediate family aren't eligible.
  • 75-95 Scores count only on the first reading!
  • 50-70 Not bad--buy yourself an ice cream.
  • 25-45 The next one will be easier.
  • 5-20 Stick to word searches.*

Thanks to Gary Davidian and Jean-Charles Mourey for reviewing this column.*

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Spotify 1.0.53.758. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
beaTunes 4.6.14 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
Vienna 3.1.10 :d05d7a5d: - RSS and Atom...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
Day One 2.1.8 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more
iDefrag 5.1.8 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
Arq 5.7.9 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
VirtualBox 5.1.20 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
beaTunes 4.6.14 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
iDefrag 5.1.8 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
Day One 2.1.8 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more

Latest Forum Discussions

See All

Blizzard is looking to hire a mobile dev...
A new thread on the popular video game rumor forum, NeoGAF, uncovered an interesting job listing over at Blizzard Entertainment. It appears the studio behindStarCraft, World of WarCraft, Hearthstone,andOverwatch is looking to bring on a new hire... | Read more »
Legend of Zelda meets Cooking Mama in ne...
Dungeon Chef is what happens when you mix the RPG elements (and style) of a Legend of Zelda game, with cooking elements. Although, now that The Legend of Zelda: Breath of the Wild also has cookingelements, so maybe the gameplay is not so novel.... | Read more »
ChordFlow (Music)
ChordFlow 1.0.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0.0 (iTunes) Description: ChordFlow is a chord sequencer with a unique 4-track polyphonic arpeggiator, extensive chord library, MIDI out and Ableton Link... | Read more »
The Walking Dead: A New Frontier is out...
The newest season of Telltale Games'The Walking Dead is well underway. After the release of the third episode, "Above the Law" about a month ago, episode four, "Thicker Than Water" is hot and ready for more zombies and gut-wrenching emotional... | Read more »
Best games we played this week
Another week, another new wave of mobile games do dive into. We've dug through the list of apps that came out this week to tell you which apps are worth your sweet time. And while there weren't too many games this week, there were some big ones.... | Read more »
Vignettes (Games)
Vignettes 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: Vignettes is a casual but unique exploration game without text or characters, where objects shapeshift as you spin them around... | Read more »
Get Me Outta Here is an 80s retro shoote...
Are you ready to fight some aliens? Because Crescent Moon Games has released the retro shooter Get Me Outta Here on iOS devices today. [Read more] | Read more »
Get a bunch of Apple productivity apps f...
If you're an Apple Mac owner, you're probably aware of the host of Apple productivity apps the company includes in all new Mac purchases. Apps like iMovie, Keynote, and of course, GarageBand. While you used to be able to also buy these apps... | Read more »
Terra Mystica (Games)
Terra Mystica 1.03 Device: iOS Universal Category: Games Price: $9.99, Version: 1.03 (iTunes) Description: Short Summary:≈≈≈≈≈≈≈≈≈≈≈≈≈ | Read more »
Ms. Spell (Games)
Ms. Spell 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cast spells and battle monsters in this turn based game, that has you delving into ever the changing Dreadwood to retrieve the lost... | Read more »

Price Scanner via MacPrices.net

15-inch Touch Bar MacBook Pros, Apple refurbi...
Apple is offering Certified Refurbished 2016 15″ Touch Bar MacBook Pros for $360 to $420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
13-inch MacBook Airs on sale for up to $150 o...
Overstock.com has 13″ MacBook Airs on sale for up to $150 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $869.99 $130 off MSRP - 13″ 1.6GHz/256GB MacBook Air (sku... Read more
15-inch Touch Bar MacBook Pros on sale for $1...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
15-inch 2.7GHz Touch Bar MacBook Pros on sale...
Amazon has 2016 15″ 2.7GHz Apple Touch Bar MacBook Pros in stock today and on sale for $150-$200 off MSRP. Shipping is free: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray (sku MLH42LL/A): $2599 $200... Read more
Apple now offering Certified Refurbished 13-i...
Apple is now offering Certified Refurbished 2016 13″ Touch Bar MacBook Pros for $270-$300 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.9GHz... Read more
MyGiHealth Digestive Symptom Tracker Version...
My Total Health, Inc. has announced the release of MyGiHealth 2.1, an important update to their digestive symptom tracker developed exclusively for iPhone, iPad and iPod touch devices. MyGiHealth is... Read more
Galaxy S8 Materials Costs Highest by Far Comp...
The new Samsung Galaxy S8 equipped with 64 gigabytes (GB) of NAND flash memory carries a bill of materials (BOM) cost that comes out to US$301.60, much higher than for previous versions of the... Read more
iCarMode 4.0 Car Dashboard App For iOS Integr...
Indie developer Diego Resnik has announced the release of iCarMode 4.0, an update to his productivity app developed for iOS devices. iCarMode has positioned itself as a true car dashboard app,... Read more
How to save $150+ on Apple’s 13-inch 2.0GHz n...
Apple Authorized Reseller B&H Photo has non-Touch Bar 13″ 2.0GHz MacBook Pros on sale for $150 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more

Jobs Board

*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Music Marketing Lead, iTunes & *Apple*...
# Music Marketing Lead, iTunes & Apple Music Job Number: 56868140 Culver City, California, United States Posted: Apr. 17, 2017 Weekly Hours: 40.00 **Job Summary** Read more
*Apple* Media Products - Commerce Engineerin...
Apple Media Products - Commerce Engineering Manager Job Number: 57037480 Santa Clara Valley, California, United States Posted: Apr. 18, 2017 Weekly Hours: 40.00 Job Read more
*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.