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
$111.78
Apple Inc.
-0.87
MSFT
$47.66
Microsoft Corpora
+0.14
GOOG
$516.35
Google Inc.
+5.25

MacTech Search:
Community Search:

Software Updates via MacUpdate

NeoOffice 2014.6 - Mac-tailored, OpenOff...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LibreOffice 4.3.5.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
CleanApp 5.0.0 Beta 5 - Application dein...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
calibre 2.13 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »
Indulge in Japanese cuisine in Cooking F...
Indulge in Japanese cuisine in Cooking Fever’s new sushi-themed update Posted by Simon Reed on December 19th, 2014 [ permalink ] Lithuanian developer Nordcurrent has yet again updated its restaurant simulat | Read more »
Badland Daydream Level Pack Arrives to C...
Badland Daydream Level Pack Arrives to Celebrate 20 Million Downloads Posted by Ellis Spice on December 19th, 2014 [ permalink ] | Read more »
Far Cry 4, Assassin’s Creed Unity, Desti...
Far Cry 4, Assassin’s Creed Unity, Destiny, and Beyond – AppSpy Takes a Look at AAA Companion Apps Posted by Rob Rich on December 19th, 2014 [ permalink ] These day | Read more »
A Bunch of Halfbrick Games Are Going Fre...
A Bunch of Halfbrick Games Are Going Free for the Holidays Posted by Ellis Spice on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

Holiday sale: 13-inch 128GB MacBook Air for $...
 Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
13-inch 2.6GHz Retina MacBook Pro on sale for...
Best Buy has lowered their price on the 2014 13″ 2.6GHz/128GB Retina MacBook Pro to $1149.99 on their online store for a limited time. That’s $150 off MSRP and the lowest price available for this... Read more
Kodak Returns to CES With New Consumer Produ...
Former photography colossus Kodak is returning to CES for the first time in three years where the Kodak booth (#21818 South Hall 1) will showcase a wide range of innovative, imaging-related products... Read more
Invaluable Launches New Eponymously -Named A...
Invaluable, the world’s largest online live auction marketplace, hhas announced the official launch of the Invaluable app for iPad, now available for download in the iTunes App Store. Invaluable... Read more
IDC Reveals Worldwide Mobile Enterprise Appli...
International Data Corporation (IDC) last week hosted the IDC FutureScape: Worldwide Mobile Enterprise Applications and Solutions 2015 Predictions Web conference. The session provided organizations... Read more
Hello Vino Wine App Launches “Safe Ride Home”...
Hello Vino has announced addition of a new “Get a Safe Ride Home” feature in its Food & Drink app with a direct connection to Uber, the technology platform that connects users with rides. The... Read more
DEVON-technologies Releases DEVONthink To Go...
Coeur d’Alene, Idaho based DEVON-technologies, LLC has updated DEVONthink To Go, its mobile companion to DEVONthink, to version 1.5. The update includes an iOS 8 extension, compatibility with the... Read more
The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
It’s 1992 Again At Sony Pictures, Except For...
Techcrunch’s John Biggs interviewed a Sony Pictures Entertainment (SPE) employee, who quite understandably wished to remain anonymous, regarding post-hack conditions in SPE’s L.A office, explaining “... Read more
OtterBox Defender Series Case For iPad mini 3...
With their innovative Touch ID technology and ultrathin profile, the latest tranche of Apple iPads are more desirable than ever, and OtterBox has just announced the Defender Series custom-engineered... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, 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* 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
*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
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.