TweetFollow Us on Twitter





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. *


  • 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

Square Rave guide - How to grab those te...
Square Rave is an awesome little music-oriented puzzle game that smacks of games like Lumines, but with its own unique sense of gameplay. To help wrap your head around the game, keep the following tips and tricks in mind. [Read more] | Read more »
Snowboard Party 2 (Games)
Snowboard Party 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Crowned the best snowboarding game available on the market, Snowboard Party is back to fulfill all your adrenaline needs in... | Read more »
The best games like Animal Crossing on m...
Animal Crossing amiibo Festival is out right now for the Wii U, reminding us of just how much fun that world can be. Or at least to go back and check in on our villages once in a while. [Read more] | Read more »
Between 2 Taps - Tap for Tap interview M...
Hello, and welcome back to Between 2 Taps, Tap for Tap’s Indie Dev interview series. [Read more] | Read more »
Facility 47 (Games)
Facility 47 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: You wake up alone and freezing in an icy cell. You try the cell door but it’s locked, it seems that you are stuck with no... | Read more »
The best Photoshop alternative on iPad
Instagram and Lightroom are great and all, but sometimes people need to get extra creative with their image editing.Like, Photoshop creative. If you're one of these people, take a look at our pick for the best mobile Photoshop experience on iPad... | Read more »
The Walking Dead: No Man’s Land guide -...
A new update for The Walking Dead: No Man’s Land was released last week, making it the perfect time for you to head back to your base and take out some walkers. Here’s the lowdown on what’s new to the game, and how to take advantage. [Read more] | Read more »
Goat Rider guide - Tips and tricks to st...
We've all been there. One second, we're riding high on a crazed goat, and the next, we've been tossed off it like someone who's no good at goat ridin'. [Read more] | Read more »
Real Boxing 2 CREED: How to become a gre...
Just in time for Rocky fans who can’t wait to see CREED, the latest movie, we have the official tie-in game,Real Boxing 2 CREED. It builds on the success of its predecessor and there’s lots to take in so we at 148apps thought we’d run you through... | Read more »
CoinOp Heroes 2 guide - How to build an...
CoinOp Heroes 2 justlaunched and, like all clickers, it's dangerously addictive stuff. You have to furiously tap your screen to defeat wave after wave of foes and earn an insane amount of cash to spend on character upgrades and an army of minions... | Read more »

Price Scanner via

Best Buy Black Friday deals: Up to $200 off M...
Best Buy has posted their Black Friday sale prices for 2015. Save on MacBook Pros, MacBooks, MacBook Airs, iMacs, iPads, and Apple Watches. Choose free shipping or free local store pickup (if... Read more
Save $30-$40 on new Apple TVs after rebate
Adorama has new Apple TVs on sale for up to $40 off MSRP after mail-in rebate, good through December 15th. Shipping is free, and Adorama charges NY & NJ sales tax only: - 32GB Apple TV: $119.99... Read more
13-Inch Haswell MacBook Air At Two Years – Th...
The 13-inch mid-2013 “Haswell” MacBook Air I ordered in Apple’s November 2013 Black Friday sale was my first new Mac in four and a half years — the longest interval I’ve gone between system upgrades... Read more
Target Black Friday Early Access deals: $100...
Target is offering early access to their Black Friday deals on Apple products on their online store for today, the 25th, only. Choose free shipping or free local store pickup (if available): - Apple... Read more
BlackBerry Q3 Mobility Index Report Finds iOS...
BlackBerry has announced results of its thirteenth Good Mobility Index Report, showing that organizations are increasingly building custom secure apps. Among Good Powered by BlackBerry (formerly Good... Read more
Wednesday roundup of early Black Friday Mac s...
Save up to $500 on a new Mac with these early Black Friday deals from Apple resellers, currently the lowest prices available for these models: (1) B&H Photo has all new Macs on sale for up to $... Read more
iPod nano on sale for $119, $30 off MSRP
Walmart has the 16GB iPod nano (various colors) on sale for $119.20 on their online store for a limited time. That’s $30 off MSRP. Choose free shipping or free local store pickup (if available). Sale... Read more
Adorama Black Friday deals: Up to $400 off Ma...
Adorama has released their Black Friday deals for 2015. Save up to $400 on MacBook Pros, $200 on MacBooks and MacBook Airs, and $270 on iMacs. Use code RYBFDEAL during checkout to see these prices.... Read more
B&H Photo Deals: $200 off 12-inch 1.2GHz...
In addition to the B&H Photo Black Friday week sales we posted yesterday, B&H has lowered their price on two products to $200 off MSRP: - 12″ 1.2GHz Gray Retina MacBook: $1399 save $200 - 13... Read more
Best Buy Early Access: Today only, Up to $125...
Best Buy has iPad Air 2s on sale for up to $125 off MSRP and Apple Watch models on sale on their online store for up to $100 off MSRP with special codes through midnight CT tonight. Choose free... Read more

Jobs Board

Hardware Systems Integration Engineer - *App...
# Hardware Systems Integration Engineer - Apple Watch Job Number: 39380139 Santa Clara Valley, Califo ia, United States Posted: Apr. 23, 2015 Weekly Hours: **Job Read more
Sr. Technical/Project Manager, *Apple* Educ...
# Sr. Technical/Project Manager, Apple Education Job Number: 36588557 New York City, New York, United States Posted: Jul. 30, 2015 Weekly Hours: 40.00 **Job Summary** Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Merchant Operations Manager: *Apple* Pay -...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
*Apple* Pay QA Manager - Apple Inc. (United...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.