TweetFollow Us on Twitter

December 92 - KON & BAL'S PUZZLE PAGE

KON & BAL'S PUZZLE PAGE

A MICRO BUG

KONSTANTIN OTHMER AND BRUCE LEAK

[IMAGE 134-137_Puzzle_Page_rev1.GIF]


KONSTANTIN OTHMER AND BRUCE LEAKSee 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. Try to guess this one before BAL does. To figure out your score, see "Scoring" at the end.


KON Have you heard of Spaceward Ho!?

BAL Yeah, it's that awesome conquer-the-galaxy game from Delta Tao. That game has done more to hurt productivity around here than pinball.

KON After they released it, they got several calls complaining about a crash. They tried to reproduce the crash but couldn't.

BAL They don't have that SADE MultiFinder installed, do they?

KON Very funny.

BAL How is their configuration different from the configuration of customers with the problem?

KON Everyone who complained had a 4-meg IIsi, ci, or fx. And the Delta Tao folks tested those configurations.

BAL Hmmm. How does it crash? Can you get into MacsBug?

KON That's part of the problem, the customers who have the crash aren't programmers and don't have MacsBug. The crash is with an Error 01, a bus error.

BAL Well, find one of the machines it crashes on, install MacsBug, and see what's wrong. How hard can it be?

KON So you fly to Bismarck, North Dakota, and install MacsBug, and it doesn't crash anymore. Pretty hard, I guess.

BAL Hmmm. Just MacsBug? Are there any INITs running?

KON The machine has only MacsBug, nothing else.

BAL And you never set a breakpoint, or an A-trap break, or anything?

KON Nope.

BAL Do you have a FirstTime macro?

KON Nope.

BAL So how could MacsBug be interfering?

KON I can't help you there. It's your puzzle.

BAL Well, MacsBug initializes some low-memory values and rearranges things above BufPtr. Is the app doing anything funny that might depend on some low mems?

KON The app follows every programming convention dictated by Inside Macintosh  and the Developer Support Center. They even follow every human interface guideline and . . .

BAL Yeah, yeah, yeah. Impossible. So MacsBug is installed, but it's never invoked.

KON Yep.

BAL What's the app doing when it crashes?

KON It's in the middle of a bunch of calculations -- you know, how many ships got destroyed in battle, how fast planets' populations are growing, what the computer players are doing, that kind of thing.

BAL Well, MacsBug causes the app to launch in a different place.

KON OK.

BAL MacsBug loads above BufPtr, so everything else loads lower. Maybe the app reads past the end of its heap. When MacsBug is in, it's lower in the heap, so the app reads somewhere in MacsBug territory. When MacsBug is out, the app reads past the end of RAM and causes a bus error.

KON Nice theory. But how do you verify that that's the problem without MacsBug?

BAL Launch another app first.

KON Then the Ho! will load even lower in memory. It won't crash.

BAL Use MicroBug.

KON You mean that thing that comes up when you push the NMI switch and MacsBug isn't installed? Where is that documented?

BAL I don't know. It can't be too hard to figure it out, though.

KON Well, the only command I know is G for "Go." What else will it let me do?

BAL You can look at memory and registers, you can set the PC, and you can even exit to the shell. Let's try a Total Display, TD. MicroBug responds with this:

000C30 0000 0000 0074 0000 FFFF 0100 0000 00C4
000C40 0000 FFFF 0000 0000 00AD E5D7 0074 0000
000C50 006E B2D0 0074 0A80 006E 9EB8 0057 0308
000C60 0000 0000 0074 0BAC 006E 49F8 006E 49E0
000C70 000A D96A 2014 0000 0000 0000 0000 0000
000C80 0000 0000 5444 0020 0020 0020 0020 0020

KON It looks like it's dumping memory from C30.

BAL Yeah, from SysEqu.a we see that C30 is SEVarBase. The system exception vars go up to CBF. I guess that's where the exception vectors dump the processor state when an exception occurs.

KON Since the system sets up the SEVars, they're set up on any exception regardless of the debugging environment. Using MacsBug, we can figure out that the first two lines are registers D0-D7, the next two lines are A0-A7, then the PC, then the status register, then what?

BAL I don't know, but at C84, it looks like what we typed: TD.

KON You could read a book written in ASCII!

BAL Let's try something else, maybe it can do math. Let's try DM PC-10.

KON It works.

BAL Yeah. In addition to the PC, it knows registers as RA0 or RD0 (but you set registers with a line like D0 = 5, not RD0 = 5). You can set memory using SM.

KON Anyway, back to the Ho!

BAL So in the Ho! I can look at the PC and the registers and figure out that it's looking past the end of memory.

KON You can't do an IL or an IP, so you can't prove that bogus values in a register are causing the bus error.

BAL I go into MacsBug on my PowerBook and disassemble the code with the DH command.

KON How do you find the problem code in the source?

BAL I pattern-match using the Find command on the PowerBook. Once I find the problem in MacsBug on the PowerBook, I'm golden.

KON Right! Here's the scoop: One of their pointers got messed up and they were reading off the end of their heap. The value they read had only a minor impact on the calculations, so no one noticed the problem. When MacsBug was in, they were reading in MacsBug's code space, which is a valid address and didn't cause a bus error. The reason it was reported on 4- meg IIsi's, ci's, and fx's is that only '030 or '040 machines that have the ci-class ROM cause bus errors when reading a valid RAM address that doesn't have RAM installed.

BAL And reading off the end of RAM on an 8-meg machine in 24-bit addressing mode just reads the ROM, which is valid.

KON Instead of this MicroBug detour, you could just write a flag value on the screen from various interesting places in the source. The flag value when you crash tells you where you were last.

BAL Yeah, but that's been done before. And it doesn't give us a good excuse to discuss MicroBug.

KON OK, Mr. MicroBug, what's the fewest keystrokes you can use to do an ExitToShell from MicroBug?

BAL Well, ExitToShell is Toolbox trap A9F4. The Toolbox trap table begins at $E00, so you can calculate the address of the trap and then use the G command.

KON Once you have the address, that's a minimum of seven keystrokes. You like to type a lot.

BAL I need some time to think about that one.

KON While you're thinking, how do you restart from MicroBug?

BAL Let's just leave everyone in suspense until next time.

KON Nasty.

BAL Yeah.

KONSTANTIN OTHMER AND BRUCE LEAK are basically slackers who go on way too many vacations. Unfortunately, they write buggy code and there are always a number of bugs that they need to fix on their return. But in true slacker style, they wouldn't think of fixing their own bugs. Enter the Puzzle Page, a sly coverup for getting someone else to solve these problems. Instead of fighting through buggy code with MacsBug, they call each other looking for easy answers. To keep pace with their bugs, they're lobbying the develop  staff to do a whole issue of just Puzzle Pages. *

SCORING

  • If you stick with MacsBug and never even try MicroBug, score 25.
  • If you figured out the bug before BAL did, score 50.
  • If you start to use MicroBug and like it better than MacsBug, score 75.
  • If you start to play Spaceward Ho! regularly, and like it better than MicroBug, score 100. *

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

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple iMovie 10.1.6 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more
TechTool Pro 9.5.1 - Hard drive and syst...
TechTool Pro has long been one of the foremost utilities for keeping your Mac running smoothly and efficiently. With the release of version 9, it has become more proficient than ever. TechTool... Read more
Jamf Pro 9.99.0 - Powerful sysadmin/ente...
Jamf Pro (formerly Casper Suite) is the EMM tool that delights IT pros and the users they support by delivering on the promise of unified endpoint management for Apple devices. At Jamf, connecting... Read more
VueScan 9.5.78 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Adobe Lightroom 6.10.1 - Import, develop...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
iPhoto Library Manager 4.2.7 - Manage mu...
iPhoto Library Manager allows you to organize your photos among multiple iPhoto libraries, rather than having to store all of your photos in one giant library. You can browse the photos in all your... Read more
Smultron 9.4 - Easy-to-use, powerful tex...
Smultron 9 is an elegant and powerful text editor that is easy to use. Use it to create or edit any text document. Everything from a web page, a note or a script to any single piece of text or code.... Read more
TextSoap 8.4 - Automate tedious text doc...
TextSoap can automatically remove unwanted characters, fix up messed up carriage returns, and do pretty much anything else that we can think of to text. Save time and effort. Be more productive. Stop... Read more
Merlin Project 4.2.3 - $349.00
Merlin Project is the leading professional project management software for OS X. If you plan complex projects on your Mac, you won’t get far with a simple list of tasks. Good planning raises... Read more
QuarkXPress 13.0.0.0 - Desktop publishin...
QuarkXPress 2017 is the new version that raises the bar for design and productivity. With non-destructive graphics and image editing directly within your layout, you no longer have to choose between... Read more

Latest Forum Discussions

See All

Goat Simulator PAYDAY (Games)
Goat Simulator PAYDAY 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICES **iPhone 4S, iPad 2, iPod Touch 5 or better Goat Simulator: Payday is the most... | Read more »
Zombie Gunship Survival Beginner's...
The much anticipated Zombie Gunship Survival is here. In this latest entry in the Zombie Gunship franchise, you're tasked with supporting ground troops and protecting your base from the zombie horde. There's a lot of rich base building fun, and... | Read more »
Mordheim: Warband Skirmish (Games)
Mordheim: Warband Skirmish 1.2.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.2.2 (iTunes) Description: Explore the ruins of the City of Mordheim, clash with other scavenging warbands and collect Wyrdstone -... | Read more »
Mordheim: Warband Skirmish brings tablet...
Legendary Games has just launched Mordheim: Warband Skirmish, a new turn-based action game for iOS and Android. | Read more »
Magikarp Jump splashes onto Android worl...
If you're tired ofPokémon GObut still want something to satisfy your mobilePokémon fix,Magikarp Jumpmay just do the trick. It's out now on Android devices the world over. While it looks like a simple arcade jumper, there's quite a bit more to it... | Read more »
Purrfectly charming open-world RPG Cat Q...
Cat Quest, an expansive open-world RPG from former Koei-Tecmo developers, got a new gameplay trailer today. The video showcases the combat and exploration features of this feline-themed RPG. Cat puns abound as you travel across a large map in a... | Read more »
Jaipur: A Card Game of Duels (Games)
Jaipur: A Card Game of Duels 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ** WARNING: iPad 2, iPad Mini 1 & iPhone 4S are NOT compatible. ** *** Special Launch Price for a limited... | Read more »
Subdivision Infinity (Games)
Subdivision Infinity 1.03 Device: iOS Universal Category: Games Price: $2.99, Version: 1.03 (iTunes) Description: Launch sale! 40% Off! Subdivision Infinity is an immersive and pulse pounding sci-fi 3D space shooter. https://www.... | Read more »
Clash of Clans' gets a huge new upd...
Clash of Clans just got a massive new update, and that's not hyperbole. The update easily tacks on a whole new game's worth of content to the hit base building game. In the update, that mysterious boat on the edge of the map has been repaired and... | Read more »
Thimbleweed Park officially headed to iO...
Welp, it's official. Thimbleweed Park will be getting a mobile version. After lots of wondering and speculation, the developers confirmed it today. Thimbleweed Park will be available on both iOS and Android sometime in the near future. There's no... | Read more »

Price Scanner via MacPrices.net

Huawei Unveils New ‘Business-Styled’ MateBook...
Huawei has introduced a trio of new MateBook laptops, expanding its mobile portfolio and building on its success in delivering attractive and powerful high-end devices. The company claims the HUAWEI... Read more
Deal! Gold 12-inch 1.2GHz Retina MacBook for...
Amazon has the 2016 Gold 12″ 1.2GHz Retina MacBook (MLHF2LL/A) on sale for $350 off MSRP for a limited time. Shipping is free: - 12″ 1.2GHz Gold Retina MacBook: $1249.99 $350 off MSRP We expect this... Read more
13-inch 2.0GHz MacBook Pros on sale for $100...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $100 off MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 13″ 2.0GHz MacBook Pro Space... 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
Apple refurbished 9-inch and 12-inch iPad Pro...
Apple has Certified Refurbished 9″ and 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 9″... Read more
Apple Certified Refurbished iMacs available f...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Sale! 15-inch 2.6GHz Silver Touch Bar MacBook...
DataVision has the 15″ 2.6GHz Silver Touch Bar MacBook Pro (MLW72LL/A) on sale for $2199 including free shipping. Their price is $200 off MSRP, and it’s the lowest price available for this model (... Read more
A Kaby Lake Processor Upgrade For The MacBook...
Now they tell me! Well, actually Apple hasn’t said anything official on the subject, but last week Bloomberg News’s Mark Gurman and Alex Webb cited unnamed “people familiar with the matter”... Read more
Kodak’s Camera-First Smartphone EKTRA Launche...
The Eastman Kodak Company and Bullitt Group have announced the availability of a U.S. GSM version of the KODAK EKTRA Smartphone. The U.S. launch coincides with a software update addressing requests... Read more
Apple Launches App Development Curriculum for...
Apple today launched a new app development curriculum designed for students who want to pursue careers in the fast-growing app economy. The curriculum is available as a free download today from Apple... Read more

Jobs Board

*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
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Systems Engineer - California Polyte...
Cal Poly, San Luis Obispo Apple Systems Engineer Department: ITS - Customer & Tech Support (134900) College/Division: Academic Affairs Salary Range: Position Read more
Best Buy *Apple* Computing Master - Best Bu...
**508718BR** **Job Title:** Best Buy Apple Computing Master **Location Number:** 001526-Odessa-Store **Job Description:** **What does a Best Buy Apple Computing Read more
Data Engineer - *Apple* Media Products - Ap...
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 Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.