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.*

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Can PokeMatch help you find love with Po...
The unofficial Pokemon GO companion app space has exploded almost as fast as the game itself over the last few weeks. Aspiring app developers, many of them working solo, have given us apps that locate Pokemon, keep track of the server status, and... | Read more »
How to get started with Prisma
If there's one thing people like to do more than taking pictures with their smartphones, it's tinkering with those photos in some way. Numerous apps have sprung up over the last several years that allow you to use filters and special effects to... | Read more »
6 Pokemon GO updates you can expect, acc...
Pokemon GO had a scheduled appearance at this year's San Diego Comic-Con for a while, but it was only relatively close to the show that it was upgraded to a spot in Hall H. That's the biggest venue at SDCC, one usually reserved for the largest... | Read more »
How to evolve Eevee in Pokemon GO
By now, almost everyone should be hip to how to evolve Pokemon in Pokemon GO (and if not, there's a guide for that). Just gather enough candy of the appropriate type, feed them all to the Pokemon, and evolution happens. It's a miracle that would... | Read more »
CSR Racing 2: Guide to all game modes
It might not seem like there are all that many ways to go fast in a straight line, but CSR Racing 2 begs to differ. [Read more] | Read more »
Bulb Boy (Games)
Bulb Boy 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Multi-award winning 2D point & click horror adventure about a boy with a glowing head. | Read more »
5 top free emoji keyboard apps
If we're not at peak emoji yet as a society, it feels like we definitely should be. The emoji concept has gone far beyond what anyone in Japan could have envisioned when the people there unleashed it on an unsuspecting world, but the West has... | Read more »
How to unlock more characters in Disney...
One of the big charms of Disney Emoji Blitz is seeing a wide variety of beloved Disney and Pixar characters transformed into smiling emojis. Even someone like the sneaky Randall from Monsters Inc., who probably never cracked a smile on film, is... | Read more »
Cubway (Games)
Cubway 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Cubway is a journey with an abstract story of lifecycle of rebirth, called Samsara. Guide the cube through the long way full of dangers... | Read more »
Colorcube (Games)
Colorcube 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Turn pieces and blend colours in this minimal yet visually stunning puzzler.Over 200 handcrafted and challenging levels. Features... | Read more »

Price Scanner via MacPrices.net

External Keyboard Innovations For iPad Pro (1...
I’m an input device aficionado. With non-touchscreen computers, which includes all Macs, the keyboard and mouse or trackpad are the tactile points of interface between user and machine, and the... Read more
GSK Rheumatoid Arthritis Study Leverages iPho...
Global healthcare products company GlaxoSmithKline (GSK) says that since 2014 they have begun transforming the way they conduct research, by leveraging state-of-the-art digital technologies — a... Read more
Clearance 12-inch Retina MacBooks, Apple refu...
Apple has Certified Refurbished 2015 12″ Retina MacBooks available starting at $929. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1199 $100 off MSRP - 13″ 2.7GHz/... Read more
13-inch 1.6GHz/128GB MacBook Air on sale for...
Amazon has the 13″ 1.6GHz/128GB MacBook Air on sale for $200 off MSRP for a limited time. Shipping is free: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $799.99 $200 off MSRP Their price is the... Read more
13-inch 1.6GHz/256GB MacBook Air on sale for...
Amazon has the 13″ 1.6GHz/256GB MacBook Air on sale for $200 off MSRP for a limited time. Shipping is free: - 13″ 1.6GHz/256GB MacBook Air (sku MMGG2LL/A): $999.99 $200 off MSRP Their price is the... Read more
Free iOS Business App notably* Helps Service...
PayStudio Inc. has introduced their new business app notably* 1.0, developed for iPhone and iPod touch. notably* was specifically developed to help service and trade professionals go digital and... Read more
27-inch iMacs on sale for $200 off MSRP
Amazon has 27″ iMacs on sale for $200 off MSRP including free shipping: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1799.99 $200 off MSRP - 27″ 3.2GHz/1TB HD iMac 5K... Read more
Mac Pros on sale for $200 off MSRP
B&H Photo has Mac Pros on sale for $200 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3799, $200... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more

Jobs Board

*Apple* Retail - Multiple Positions, Willow...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Evangelist - JAMF Software (United S...
The Apple Evangelist is responsible for building and cultivating strategic relationships with Apple 's small and mid-market business development field teams. This Read more
*Apple* Solutions Consultant - APPLE (United...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
*Apple* Professional Learning Specialist - A...
Job Summary The Apple Professional Learning Specialist is a full-time position for one year with Apple in the Phoenix, AZ area. This position requires a high Read more
*Apple* Picker - Apple Hill Orchard (United...
Apple Hill Orchard, Co. Rte. 21,Whitehall, NY 9/7/16-10/228/16. Pick fresh market or processing apples Productivity of 60 boxes and 80 boxes processing fruit per Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.