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.*

 
AAPL
$100.11
Apple Inc.
+0.00
MSFT
$46.44
Microsoft Corpora
+0.00
GOOG
$576.36
Google Inc.
+0.00

MacTech Search:
Community Search:

Software Updates via MacUpdate

QuickBooks 2015 16.0.0.1352 R1 - Financi...
QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save.... Read more
Mac DVDRipper Pro 5.0.1 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
SyncTwoFolders 2.0.5 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
FinderPop 2.5.7 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
VueScan 9.4.45 - 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
LibreOffice 4.3.2.2 - 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
calibre 2.4 - Complete e-library managem...
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... Read more
Default Folder X 4.6.9b1 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
Beamer 2.0.4 - Stream any movie file fro...
Beamer streams to your Apple TV.... Plays any movie file - Just like the popular desktop movie players, Beamer accepts all common formats, codecs and resolutions. AVI, MKV, MOV, MP4, WMV, FLV. To... Read more
Adobe Premiere Elements 13.0 - Consumer...
Adobe Premiere Elements just got better. Now you can create quick movies from favorite parts of your clips or tell big life stories in full-on productions. Add motion inside movie titles, punch up... Read more

Latest Forum Discussions

See All

iPollute Review
iPollute Review By Nadia Oxford on September 30th, 2014 Our Rating: :: iPOLLUTE, uPOLLUTEUniversal App - Designed for iPhone and iPad iPollute is a delightful-looking pollution simulator, though it’s a bit light on “game” content... | Read more »
This Week at 148Apps: September 22-26, 2...
Your Source For The Latest App Reviews   | Read more »
Kill Shot Review
Kill Shot Review By Jennifer Allen on September 29th, 2014 Our Rating: :: OCCASIONAL MISFIRESUniversal App - Designed for iPhone and iPad Kill Shot is often satisfying, but its more than pushy in-app purchases will leave you... | Read more »
It Came From Canada: Vainglory
I’m going to be straight you with folks. I don’t know much about MOBAs, and I’m certainly no professional. That means I don’t know how well Vainglory, the upcoming multiplayer online battle arena showcased in the most recent Apple keynote, compares... | Read more »
The Tunnel – An Endless Fall Review
The Tunnel – An Endless Fall Review By Nadia Oxford on September 29th, 2014 Our Rating: :: FREE FALLIN' Universal App - Designed for iPhone and iPad The Tunnel – An Endless Fall is a bit harder to get the hang of than most “endless... | Read more »
Ubisoft Launches Just Dance Now in the A...
Ubisoft Launches Just Dance Now in the App Store Worldwide Posted by Jessica Fisher on September 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
For Better, or for Worse? How the New Ap...
With the release of iOS 8 (finally), Apple has made some noticeable changes on the App Store – bundles, videos, and Testflight integration, to name a few. Many of these additions will make things friendlier for App Store shoppers, but how has it... | Read more »
Asphalt Overdrive Review
Asphalt Overdrive Review By Jennifer Allen on September 29th, 2014 Our Rating: :: SHORT LIVED ADRENALINEUniversal App - Designed for iPhone and iPad Asphalt Overdrive is a fairly simple auto-running racing game, but one that’s... | Read more »
Kritika: Wrath of the Pirate King Update...
Kritika: Wrath of the Pirate King Update Introduces New Class: Shadow Mage. Posted by Jessica Fisher on September 29th, 2014 [ permalink ] | Read more »
Vampire Tribunals Review
Vampire Tribunals Review By Campbell Bird on September 29th, 2014 Our Rating: iPhone App - Designed for the iPhone, compatible with the iPad Hunt other vampires in the “real world” in this location-based multiplayer game.   | Read more »

Price Scanner via MacPrices.net

MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only, in-... Read more
Apple Releases OS X Mavericks bash Update 1.0...
Apple has released a patch update for OS X Mavericks users to address the recently-detected “Shellshock” security bug in BSD UNIX’s bash shell. Apple says only a few Mac users who had manually... Read more
Pivotal Payments Ready for Apple Pay – FlexPo...
Pivotal Payments, a provider of merchant services and global payment processing solutions, has announced its proprietary FlexPoint platform will support credit and debit transactions through Apple’s... Read more
iStabilizer Announces Tabarm — First Friction...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, dollies, mounts, and remotes for smartphones, tablets, and cameras, announced today the iStabilizer tabArm, the first... Read more
IStabilizer Flex Smartphone Tripod Wins Usa T...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, and other products for smartphones, tablets, and cameras, has announced today that its iStabilizer Flex smartphone... Read more
13-inch 2.8GHz Retina MacBook Pro on sale for...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels Desktop... Read more
Get Down to Business On Your iPad With Free N...
Utah based Poem LLC has introduced Nexticy 1.0, a full-featured, affordable (Free) forms application for iPad. Nexticy makes it ‘way easy’ to create, store, retrieve and analyze paper-like forms of... Read more
13-inch Retina MacBook Pros on sale for $100...
Best Buy has 13-inch 2.6GHz Retina MacBook Pros on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Prices are for online orders only, in-... Read more
Reports Outline Claimed 12-Inch Macbook Air D...
Blogger Jack March says that according to information he’s received from an anonymous “source familiar with Apple’s plans,” the next-generation MacBook Air will launch in mid-2015 and be available... Read more

Jobs Board

*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
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
*Apple* Solutions Consultant (ASC) - Apple (...
**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 / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.