TweetFollow Us on Twitter

Winter 92 - KON & BAL'S PUZZLE PAGE

KON & BAL'S PUZZLE PAGE

KONSTANTIN OTHMER AND BRUCE LEAK

[IMAGE 103-105_Puzzle_Page_rev1.GIF]

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.

KON I wrote this program that crashes with a bus error and I can't figure out what's going wrong.

BAL If it's crashing with a bus error, that's easy: just figure out where the bogus bus error address came from.

110 KON Well, that's the problem. I look around and there's not a bad address anywhere.

BAL Let me see.

105 KON OK.

Bus Error at 1B586
1B582	BFEXTU (A3){D6; $00}, D0
1B586	*ADDQ.W	#$4,A3
1B588	MOVE.L(A2)+,D1

Register A3 has $70E368, and A2 has $70DEDC. So that's your puzzle; what do you do now?

BAL Hmmm. Suppose I trace a few times.

100 KON No problem; everything seems to work OK.

BAL What if I type "Go" and hope for the best?

95 KON You crash immediately with an address error at $1B5A4. The code is trying to do an RTS, but the stack is trashed.

BAL Well that's just a little ways down from where I was before. Anything funny happening in between?

90 KON Nope, just a loop that doesn't touch the stack or anything.

BAL Was the stack OK when I crashed the first time?

85 KON No, the top address on the stack was garbage.

BAL OK, so I run the program again and break just before this routine gets called and check the stack.

80 KON The stack is OK. Everything looks fine. You're at a JSR (A0).

BAL Is A0 OK?

75 KON Yep, it points to the code you were looking at before--at $1B582.

BAL So I step in and look at the stack.

70 KON You crash immediately with a bus error at $82.

BAL Huh? Sounds like someone jumped to $0.

65 KON A0 looked OK. It was just doing a JSR (A0).

BAL Some weird MacsBug bug?

60 KON In this case, no.

BAL All I did was step into a subroutine and I crash somewhere totally different?

55 KON Pretty cool, huh?

BAL Are the registers OK? What does the stack look like?

50

KON Garbage everywhere. The stack has all kinds of noise on it, and the registers seem pretty fragged.

BAL Hmmm. I try it again; this time I set a breakpoint a few instructions before the JSR (A0).

KON OK.

BAL Is everything OK?

45 KON The registers, stack, and code look OK.

BAL So I trace a few instructions, up to the JSR.

40 KON You crash immediately after the first trace with a bus error at $0104B0CA.

BAL Well that address is garbage. Was it in any register or on the stack before I traced?

35 KON Nope.

BAL What happened to the stack?

30 KON There's 56 extra bytes on it now.

BAL What? Is my machine possessed?

KON It's just a computer.

BAL Some interrupt nastiness happening?

25 KON When you crash, you're at interrupt level 1.

BAL Now we're getting somewhere. Does MacsBug enable interrupts when I trace?

20 KON Yes.

BAL So where's the level 1 interrupt vector kept?

15 KON $64.

BAL I DM it and see if it's OK.

10 KON It's $104B07C. Pretty close to where you crashed.

BAL So it sounds like someone is trashing the interrupt vectors, and all the interrupts are held pending when I'm in MacsBug. As soon as I do anything that returns control to the Macintosh, I blow up. So I step spy on $64 and see who trashes it.

0 KON It's a routine that assumed a buffer was being allocated but wasn't. So the buffer pointer was NIL, and the routine wrote all over low memory, including the exception vectors.

BAL Nasty.

KON Yeah. So how could you catch this before the vectors get trashed?

BAL Doesn't EvenBetterBusError catch writes to NIL?

KON Only at VBL time, and the chances of a VBL interrupt happening before the VBL vector gets trashed are mighty slim.

BAL You could initialize pointer variables to a bus error number like $50FFC001 instead of NIL. If you did that, you'd crash at the write and know immediately what was wrong.

KON Cool.


KONSTANTIN OTHMER AND BRUCE LEAK As a mere lad, Bruce pulled the programmer's key out of the stone and swore allegiance to the Lady of the Leak. Years later, while good King Bruce was doing penance for his wandering ways, young KON of Locksley had to defend the crown against the Mongol hordes attempting to draw directly to the screen. Upon his return, King Bruce declared, "Let there be time," and there was. Today young KON and his sovereign do battle against the forces of evil lurking in your local heap.*

SCORING

  • 100-110 Be honest.
  • 75-95 Next time we find a bug, we're  calling you .
  • 50-70 So this has happened to you!
  • 25-45 No doubt about it, these puzzles are tough.
  • 0-20 Well, maybe next time . . . *

Thanks to Scott Douglass for reviewing this column.*

 
AAPL
$116.31
Apple Inc.
+1.64
MSFT
$48.70
Microsoft Corpora
+0.48
GOOG
$534.83
Google Inc.
-2.16

MacTech Search:
Community Search:

Software Updates via MacUpdate

Herald 5.0.1 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
Firetask 3.7 - Innovative task managemen...
Firetask uniquely combines the advantages of classical priority-and-due-date-based task management with GTD. Stay focused and on top of your commitments - Firetask's "Today" view shows all relevant... Read more
TechTool Pro 7.0.6 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PhotoDesk 3.0.1 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos! (NO Uploads! / Image Posting! Instagram forbids that! AND you *need* an *existing* Instagram account). But you can do... Read more
SuperDuper! 2.7.3 - Advanced disk clonin...
SuperDuper! is an advanced, yet easy to use disk copying program. It can, of course, make a straight copy, or "clone" -- useful when you want to move all your data from one machine to another, or do... Read more
MacJournal 6.1.5 - Create, maintain, and...
MacJournal is the world's most popular journaling software for the Mac. MacJournal 6 adds a calendar mode that show entries from any journal, geolocation, word count, and progress tracking, as well... Read more
Skim 1.4.10 - PDF Reader and note-taker...
Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file. Skim includes many features and has a... Read more
FontExplorer X Pro 4.2.2 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
SoftRAID 5.0.5 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID's intuitive interface and powerful feature set makes this utility a must have for any Mac OS X... Read more
DEVONthink Pro 2.8.2 - Knowledge base, i...
Save 10% with our exclusive coupon code: MACUPDATE10 DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research... Read more

Latest Forum Discussions

See All

Weather or Not - Reports and Forecasts...
Weather or Not - Reports and Forecasts for your Calendar 1.0.0 Device: iOS iPhone Category: Weather Price: $2.99, Version: 1.0.0 (iTunes) Description: Weather or Not is a beautiful and intuitive way to check the weather and... | Read more »
Sago Mini Road Trip (Education)
Sago Mini Road Trip 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Go for a fun-filled drive with Jinja the cat. Pick a destination, select a vehicle and hit the road. What will Jinja... | Read more »
New Tower Defense Game, Kingdom Rush: Or...
New Tower Defense Game, Kingdom Rush: Origins, is Available Today Posted by Jessica Fisher on November 20th, 2014 [ permalink ] iPad Only App - Designed for the iPad | Read more »
Sunburn! (Games)
Sunburn! 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Your ship is gone. Your crew is scattered. One option remains. Gather your crew... and jump into the sun. Reunite your... | Read more »
Tapventures Review
Tapventures Review By Jennifer Allen on November 20th, 2014 Our Rating: :: ODDLY COMPELLINGUniversal App - Designed for iPhone and iPad Tapventures is an increasingly hands-off one-tap RPG, but expect it to hook you despite your... | Read more »
Who Wore it Best? The Hunger Games: Girl...
With The Hunger Games: Mockingjay Part 1 out this weekend, Who Wore it Best? pits two Hunger Games tie-ins, Girl on Fire and Panem Run, against each other in a brutal and pointless fight to the death. I wonder where we got that idea from? | Read more »
Ironkill Review
Ironkill Review By Jennifer Allen on November 20th, 2014 Our Rating: :: LACKLUSTER PUNCHINGUniversal App - Designed for iPhone and iPad Ironkill is a freemium focused fighting game that doesn’t offer particularly thrilling fights... | Read more »
Real-Time Multiplayer Match-3 RPG Crusad...
Real-Time Multiplayer Match-3 RPG Crusaders Quest Set to Launch Next Month Posted by Ellis Spice on November 20th, 2014 [ permalink ] | Read more »
Checkpoint Champion Review
Checkpoint Champion Review By Jennifer Allen on November 20th, 2014 Our Rating: :: SPEEDY DRIFTINGUniversal App - Designed for iPhone and iPad Checkpoint Champion is a drift-focused racing game that’s ideal for short but fun gaming... | Read more »
MediaFire iOS 8 Native Update Brings New...
MediaFire iOS 8 Native Update Brings New “Power Upload” Feature to iPad and iPhone Posted by Jessica Fisher on November 20th, 2014 [ | Read more »

Price Scanner via MacPrices.net

64GB iPod touch on sale for $249, save $50
Best Buy has the 64GB iPod touch on sale for $249 on their online store for a limited time. Their price is $50 off MSRP. Choose free shipping or free local store pickup (if available). Sale price for... Read more
15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale for $1799.99 for a limited time. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of... Read more
New Logitech AnyAngle Case/Stand Brings Flexi...
Logitec has announced the newest addition to its suite of tablet products — the Logitech AnyAngle. A protective case with an any-angle stand for iPad Air 2 and all iPad mini models, AnyAngle is the... Read more
2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1499 including free shipping plus NY sales tax only. Their price is $500 off original MSRP. B&H will... Read more
16GB Retina iPad mini on sale today for $199,...
 Staples has 2nd generation 16GB Retina iPad minis on sale for $199 on their online store for a limited time. Their price is $100 off MSRP. Choose free shipping or free local store pickup (if... Read more
Developers Start Designing Apps for Apple Wat...
Apple has announced the availability of WatchKit, software that gives developers a set of tools to easily create experiences designed specifically for Apple Watch. Apple’s developer community can now... Read more
C Spire Launches iPad Air 2 and iPad Mini 3 o...
C Spire has announced that iPad Air 2 with Wi-Fi + Cellular and iPad mini 3 with Wi-Fi + Cellular are now available on its 4G LTE network. C Spire offers both new iPads with a range of data plans... Read more
Are You On Your Last PC? – The ‘Book Mystique
Will your current PC be your last? Quite possibly so if you define “personal computer” as a traditional desktop or laptop form factor machine according to some commentators. So then, the upshot that... Read more
Save up to $180 on MacBook Airs with Apple re...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
16GB iPad mini available for $219, save $30
Walmart has 16GB iPad minis (1st generation) available for $219 on their online store. Their price is $30 off MSRP. Choose free shipping or free store pickup (if available). Price for online orders... Read more

Jobs Board

*Apple* Solutions Consultant (ASC)- Retail S...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Store Leader Program - College Gradu...
Job Description: Job Summary As an Apple Store Leader Program agent, you can continue your education as you major in the art of leadership at the Apple Store. You'll 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
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.