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
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

VueScan 9.4.41 - 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
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - 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
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
BBEdit 10.5.12 - Powerful text and HTML...
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

Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »
Baseball Battle Review
Baseball Battle Review By Jennifer Allen on August 29th, 2014 Our Rating: :: SIMPLE HITTINGUniversal App - Designed for iPhone and iPad Simple and cute, Baseball Battle is a fairly fun baseball game for those looking for something... | Read more »
Checkmark 2.1 Update Released, and it’s...
Checkmark 2.1 Update Released, and it’s on Sale for a Limited Time Posted by Jessica Fisher on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Labor Day Weekend iPad mini sale; $50 to $100...
Best Buy has the iPad mini with Retina Display (WiFi models) on sale for $50 off MSRP on their online store for Labor Day Weekend. Choose free shipping or free local store pick up. Price is for... Read more
13-inch 1.4GHz MacBook Air on sale for $899,...
Adorama has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. Read more
It’s Official: Apple Issues Invitations To Se...
Apple has issued one of its characteristically cryptic press invitations for a special event to be held at the Flint Center for the Performing Arts in hometown Cupertino on Sept. 9, 2014 at 10:00 am... Read more
Tablet Shipments To See First On-year Decline...
TrendForce analyst Caroline Chen notes that when the iPad launched in 2010, it was an instant hit and spurred a tablet PC revolution, with tablets so popular that that notebook PC sales stagnated and... Read more
SOBERLINK Releases Apple iOS Compatible Handh...
Cypress, California based SOBERLINK, Inc., creator of the first handheld Breathalyzer designed to improve recovery outcomes, continues to show prominence in the mobile alcohol monitoring space with... Read more
New 21″ 1.4GHz iMac on sale again for $999, s...
Best Buy has the new 21″ 1.4GHz iMac on sale for $999.99 on their online store. Their price is $100 off MSRP. Choose free shipping or free local store pick up. Price is for online orders only, in-... Read more
Smartphone Outlook Remains Strong for 2014, U...
According to a new mobile phone forecast from the International Data Corporation (IDC) Worldwide Quarterly Mobile Phone Tracker, more than 1.25 billion smartphones will be shipped worldwide in 2014,... Read more
Save up to $60 with Apple refurbished iPod to...
The Apple Store has Apple Certified Refurbished 5th generation iPod touches available starting at $149. Apple’s one-year warranty is included with each model, and shipping is free. Many, but not all... Read more
12-Inch MacBook Air Coming in 4Q14 or 2015 –...
Digitimes’ Aaron Lee and Joseph Tsai report that according to Taiwan-based upstream supply chain insiders, Apple plans to launch a thinner MacBook model either at year end 2014 or in 2015, and that... Read more
Sapphire Screen “Most Wanted” iPhone 6 New Fe...
According to the ‘uSell.com iPhone Most Wanted Survey’ — a representative survey of 1,000 U.S. smartphone users conducted by used iPhone marketplace uSell.com — close to half of all smartphone users... 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...
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...
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.