TweetFollow Us on Twitter

September 95 - KON & BAL's Puzzle Page: Video Nightmare

KON & BAL's Puzzle Page: Video Nightmare

Ian Hendry and Eric Anderson

See if you can solve this programming puzzle, presented in the form of a dialog between a pseudo KON (Ian Hendry) and BAL (Eric Anderson). 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. Even if you never run into the particular problems being solved here, you'll learn some valuable debugging techniques that will help you solve your own programming conundrums. And you'll also learn interesting Macintosh trivia.

BAL I've got one for you, KON: I just updated to System 7.5 on my 8100/80 AV. Everything seemed OK for a while. I was comparing Scenery Animator to Vistapro and I noticed that my cool new desktop pattern had disappeared. It was there when I booted, but just as the Finder was coming up, the desktop changed to a black-and-white pattern.

KON That's easy. Go back to System 7.1 and the world will be fine again. Next.

BAL Hey, 7.5 is the source of much wonderment. It's really a lot of fun!

KON I don't know that much about 7.5. Metrowerks and THINK C seem to run fine on 7.1. Is this part of that new puzzle CDEV that was added to spruce up the system?

BAL Quit trying to change the subject. My desktop pattern went away and I'm not happy about it.

KON Hmm. Did you change anything on your machine?

BAL I turned on VM for the memory-hungry rendering stuff.

KON So turn off VM and see if the problem goes away.

BAL Wow, that worked great. Now all I have to do is buy $1800 worth of tariff-enhanced RAM so I can render my flyby of the Pentagon.

KON I won't ask what you're up to these days. My recent stock dealings have left me low on bail money. Did you try it on 7.1.2?

BAL It didn't happen on straight 7.1.2, but I installed System Update 3.0 and it happened. VM must have changed in this update.

KON Sounds like a VM problem all right. Paste the older VM resources into the new system. (I love component software.)

BAL The problem is still there. Does this let VM off the hook?

KON VM is never off the hook, but if your only problem is that the desktop pattern is black and white, maybe you should stop whining and do your work.

BAL No, this is more serious. I opened the Monitors control panel and there was no depth list and no monitor tile in the rearrange section.

KON Well, this should be pretty straightforward. Does it happen every time?

BAL No such luck. This one is really evil. I've been trying to get a reproducible case for days. Sometimes it happens right away, sometimes it goes away for hours. Once it starts happening, it seems to keep happening across restarts. It doesn't happen as reliably across shutdowns. It seems to happen more in millions of colors but will happen at other depths too. Switching the display back and forth between a 13-inch and a multiple-scan display sometimes causes the problem to show up. Changing VM and RAM disk settings seems to affect the reproducibility.

KON Cool! The random bugs are always the most fun. Let's get our trusty MacsBug and see if we can find where it's going bad. Look at the video driver and GDevice.

BAL When I try to enter MacsBug, the mouse freezes but MacsBug doesn't come up.

KON Dang, I hate it when the tools don't work.

BAL Perhaps we should devote this column to model rocketry and video games instead.

KON Now there's a thought. If I type Command-G, does the mouse unfreeze, or do I have to reboot?

BAL The machine comes back when you hit Command-G.

KON So MacsBug is there; you just can't see it. I'll use log foo to dump the output to a file named foo, followed by

dm @@thegdevice gdevice

Then I'll use drvr to see if the video driver is alive.

BAL Nice log trick, KON! The GDevice is fine, and the driver looks as if it's loaded and active.

KON Drivers and VM sometimes don't get along. Maybe the driver is doing something wrong. Did you try other video cards?

BAL It seems to happen only on Power Macintosh AV models.

KON It's the nasty "fungus" problem from Issue 17 all over again, or maybe your card has gone bad.

BAL Well, I'm pretty sure there was no "fungus" around when the AV card was developed, so it's probably not that. Besides, we're sticking to production software in this column from now on. Anyway, I thought it might be my card, too, so I borrowed your card. Thanks for the loaner. I turned on VM and it worked like a champ . . . for a while. Now I've got the same problem again.

KON You killed my card?

BAL Admittedly, it's a computer that can do anything. But for the purposes of this column, that idea is pretty far-fetched.

KON Here's what might be happening: Early in the boot process, VM isn't present. For each card, the system calls the card's PrimaryInit code and creates a GDevice. When VM loads, it changes the logical address mappings. When the driver is called again, it assumes a one-to-one logical-to-physical mapping of RAM, so the card starts responding to bogus address cycles. This confuses the card's bus translator, and . . .

BAL Whatever. Any other stabs in the dark? Lose five points and try again.

KON OK. Perhaps there are subtle timing variations when VM is present, and the video card might have borderline hardware that's affected by these timing dependencies. Or maybe the card's controller gets into a state where it no longer responds to its address space.

BAL You're getting desperate. It's not a hardware problem. The declaration ROM is there and everything looks fine. You can't blame this on the hardware. Let's once again follow the software decision tree.

KON Yeah, you're probably right. Now that I think about it, those ideas seem really out there.

So what you're telling me is that the desktop pattern is black and white, MacsBug isn't working, and the Monitors control panel doesn't show depths or a monitor tile. Let's find out when MacsBug stops working.

BAL When the machine boots, MacsBug is working, but by the time you get to the Finder, it's gone. It seems to vanish early in the boot process.

KON See if you get to the first Display Manager call with an atb DisplayDispatch or atb ABEB.

BAL OK. MacsBug is still alive.

KON I'll set a breakpoint just after the first Display Manager call and then go.

BAL Yep. There doesn't seem to be a problem now. But the weird thing is, if you trace over the Display Manager call and then type go, MacsBug will eventually go away.

KON Wacky. Sounds like a Display Manager bug.

BAL Earlier you said it was a VM bug.

KON Both have been convicted criminals in the past, so you can't blame me for thinking they're suspects. I'll bet you a buck it turns out to be neither! Do an atb on the Display Manager call and trace from there until MacsBug goes away; it shouldn't take too long.

BAL Sorry, MacsBug never goes away. The problem isn't reproduced.

KON So what you're telling me is that if I trace over the Display Manager call and then go, I can't get into MacsBug after I'm done booting. But if I keep tracing, the machine boots fine and MacsBug is always available.

BAL That's right. Let me help you along a little bit. There's an SSecondaryInit call (which runs SecondaryInit code for the video cards) just a few 680x0 instructions after the first Display Manager call. Does that help at all?

KON What happens if we do an atb on SSecondaryInit?

BAL I can't reproduce the problem. If I set a breakpoint just after the Display Manager call and go, the problem disappears. If I do an atb on the Display Manager call, and either go from there or trace over it and go, the problem happens. If I trace over it for a few instructions and go, the problem doesn't happen.

KON So, what are the "few" instructions? It looks like they're the ones whacking the video driver.

BAL No, they're just a few MOVE instructions to innocuous RAM locations -- nothing that should touch video.

KON What does this Display Manager call do? Could it be hosing anything in the slots?

BAL I don't think so. I used MacsBug to skip it entirely and the problem still happens.

KON This isn't getting us anywhere. Maybe the desktop pattern problem has some better clues. The General Controls control panel in System 7.5 has an INIT resource that calls HasDepth to decide whether to use the color pattern. It then sets a PRAM bit to remember whether to use a color pattern across restarts. When the desktop pattern is black and white, I'll use the log trick to find out what the HasDepth call is returning.

BAL It returns an error.

KON Aha! Since HasDepth returns an error, the INIT resource thinks it's on a display that can support only one bit per pixel (black and white), so it disables the color desktop pattern and resets the PRAM bit; the color desktop pattern is now gone forever.

BAL OK.

KON Let's trace HasDepth and find out what's wrong.

BAL It looks as if the Slot Manager returns the correct values for the active functional sResource of the card but fails to find the depth. It returns -316, an smInitStatVErr. According to Errors.h, this error indicates that the siInitStatusV field was "negative after primary or secondary init." This means the card's PrimaryInit or SecondaryInit code returned an error.

KON We can bet it's not PrimaryInit, because the GDevice is good. If the error had happened in PrimaryInit, QuickDraw would have gotten an smInitStatVErr when it called the Slot Manager to build the GDevice.

BAL You're finally making some progress!

KON MacsBug also makes Slot Manager calls (when it tries to switch depths), which explains why it fails. That means the problem must be with the SSecondaryInit call. Once the Slot Manager gets this error, most Slot Manager calls return errors.

BAL But this doesn't explain what's causing the Slot Manager to fail to begin with, or why the problem goes away every time we get close to it with MacsBug.

KON Maybe we should try this with a BootBug card. Can you still get one?

BAL Maybe, but we're doing pretty well here. Let's keep going a little longer.

KON Let's try to figure it out by brute logic. What does the SecondaryInit code look like on this card?

BAL MOVE.L A0,A0.

KON That's it? Two bytes? No RTS? Cool! A bug in the AV card ROM! Does this mean we all get new cards with the new 2.0 ROM? Maybe they can simplify that complicated Monitors control panel Options dialog at the same time. How does it boot at all?

BAL Good question. Designing Cards and Drivers for the Macintosh Family says that the SecondaryInit entry on a video card is an SExecBlock, which is a header followed by actual code. The Slot Manager validates the header before it executes the code. The first byte of an SExecBlock is the revision number, and the Slot Manager checks for a revision byte of 0x02. Since MOVE.L A0,A0 is 0x2048 in hex, the first byte of the AV card's SecondaryInit entry is 0x20, which is a bogus entry, and the Slot Manager will never try to execute the SecondaryInit code.

KON So it's pretty lame, but it should work, right?

BAL Yes. Remember, we added SecondaryInit to the boot process because some machines didn't have 32-bit QuickDraw in ROM. On a machine without 32-bit QuickDraw in ROM, video cards have to disable their functional sResources with direct bit depths (16 and 32) in their PrimaryInit code, because the PrimaryInit code runs before the disk is up and the cards can't tell if the system has 32-bit QuickDraw installed. SecondaryInit was added to give these cards a chance to reenable those direct depths after 32-bit QuickDraw was loaded from disk. Power Macs obviously have 32-bit QuickDraw in ROM, and this card only runs on a Power Mac, so it doesn't need SecondaryInit.

KON Let's walk through the SSecondaryInit call and see what it does. Why does VM make a difference? And why is MacsBug causing the problem to go away if you set breakpoints?

BAL You're just full of questions, aren't you? You're supposed to be giving the answers!

KON Let's walk through SSecondaryInit.

BAL For each card, it looks for a SecondaryInit entry in the card's ROM. The entry contains a header followed by the SecondaryInit code. If there's no SecondaryInit entry on the card, SSecondaryInit bails out early. If there is a SecondaryInit on the card, the Slot Manager tries to execute it with SExec and then checks the status from the SExec call. If the status is negative (an error), the Slot Manager marks the slot with that evil -316 error, and the slot is bad from there on out.

KON So who is responsible for setting the error?

BAL The code executed by SExec, in this case the SecondaryInit code, should set the status error. If the header is bad, the code never gets run and the status never gets set.

KON Let me guess: the boot code never initializes the status before calling SExec.

BAL Yep. And it's allocated on the stack as a local variable, which means that the status is set to whatever garbage is left on the stack. At this point in the boot process you're still in supervisor mode, so MacsBug is sharing your stack. When MacsBug is used, it pushes stuff onto the stack and then pops it off when it leaves (changing the garbage below the stack in the process). That's why setting breakpoints and tracing mask the problem. BootBug also uses the stack, so it too would have interfered with the bug.

Between the first Display Manager call and the SSecondaryInit, the system allocates stack space for the SPBlock parameter for the SSecondaryInit call. After the SPBlock is allocated, the stack pointer is very close to where the local variables for SSecondaryInit will be allocated. At this point MacsBug's stack usage will affect those never-initialized local variables.

This is something else to add to your list of gotchas for MacsBug: If you're in supervisor mode (as you are at this point in the boot process) and you set breakpoints, MacsBug is sharing your stack, and its use of the stack may affect uninitialized variables. Later in the boot process, VM switches the machine to user mode; from then on, MacsBug and applications use different stacks and MacsBug will not interfere with uninitialized variables on the stack.

KON The garbage that VM leaves on the stack (sometimes) happens to be negative. When the boot code gets to SecondaryInit and allocates variables on the stack, it happens to use an area of the stack affected by VM.

BAL Well, I never turn VM on, so I'm always in supervisor mode, and MacsBug always shares my stack. But now I've finally found a good use for VM: turn it on when I have a bug that's hard to reproduce when MacsBug gets involved, and see if it becomes reproducible.

KON That'll slow your machine down.

BAL Nasty.

KON Yeah.


SCORING

75-100 Excellent; you probably have a video-in jack built right into your head.

50-70 Maybe we should be working for you.

25-45 Maybe you should be working for us.

5-20 Maybe you should stick to television.*

Thanks to Rich Collyer, Kent Miller, Mike Puckett, John Yen, KON (Konstantin Othmer), and BAL (Bruce Leak) for reviewing this column.*

IAN HENDRY (AppleLink HENDRY; Internet hendry@apple.com) gets paid by Apple to work on video stuff. His hobbies include shipping products and collecting new Engineering managers. He can be found skipping meetings to play Ultimate and working all hours to make up for it. Ian's going to be a dad soon, and though he has been in rigorous sleep-deprivation training for years, he's hoping (but still not certain) that he's ready for what he's gotten himself into.*

ERIC ANDERSON (AppleLink ERIC3) skipped out on the OS Services group at Apple to get away from the chore of working on VM and the Thread Manager. Now he works as Ian's evil twin on video-related Mac OS issues -- and he gets bugs assigned to him that state, "When using a multisync monitor with my threaded test app while VM is on, this funny thing happens." Seeing that there's no escape, Eric wants more than ever to move to Hawaii and build boats.*

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Together 3.6.1 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
Cloud 4.1.1 - 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
OmniFocus 2.7.1 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
CleanApp 5.1.1 - Application deinstaller...
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
ForkLift 3.0 Beta 2 - Powerful file mana...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Sublime Text 3126 - Sophisticated text e...
Sublime Text is a sophisticated text editor for code, markup, and prose. You'll love the slick user interface, extraordinary features, and amazing performance. Features Goto Anything. Use Goto... Read more
1Password 6.3.3 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
WhatsApp 0.2.1880 - Desktop client for W...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
NeoFinder 6.9.3 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Amadeus Pro 2.3.1 - Multitrack sound rec...
Amadeus Pro lets you use your Mac computer for any audio-related task, such as live audio recording, digitizing tapes and records, converting between a variety of sound formats, etc. Thanks to its... Read more

Our 5 Favorite iMessage Sticker Packs
At long last, iMessage joins the ranks of messaging apps the likes of LINE and Whatsapp, adding an impressive collection of stickers. They’re a great way to add a little something extra to your daily conversations. [Read more] | Read more »
How to get past Vulture Island's tr...
Vulture Island is a colorful and quirky mish-mash of platforming and puzzles. It’s creative and fresh, but sometimes the game can throw a curveball at you, leaving you stuck as to how you should progress. These tips will help you explore smoothly... | Read more »
The new Clash of Kings is just for Weste...
If you’ve played the original Clash of Kings, you’ll probably recognise the city building, alliance forging and strategic battles in Clash of Kings: The West. What sets this version apart is that it’s tailor made for a Western audience and the... | Read more »
Frost - Survival card game (Games)
Frost - Survival card game 1.12.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.12.1 (iTunes) Description: *Warning: the game will work on iPhone 5C and above and iPad Pro / 4. Other devices are not supported* | Read more »
How to build and care for your team in D...
Before you hit the trail and become a dog sledding legend, there’s actually a fair bit of prep work to be done. In Dog Sled Saga, you’re not only racing, you’re also building and caring for a team of furry friends. There’s a lot to consider—... | Read more »
How to win every race in Dog Sled Saga
If I had to guess, I’d say Dog Sled Saga is the most adorable racing game on the App Store right now. It’s a dog sled racing sim full of adorable, loyal puppies. Just look at those fluffy little tails wagging. Behind that cute, pixelated facade is... | Read more »
Let the war games commence in Gunship Ba...
Buzz Lightyear famously said, “This isn’t flying, this is falling – with style!” In the case of Gunship Battle: Second War, though, this really is flying - with style! The flight simulator app from Joycity puts you in control of 20 faithfully... | Read more »
How to get a high score in Fired Up
Fired Up is Noodlecake Games’ high score chasing, firefighting adventure. You take control of a wayward firefighter who propels himself up the side of a highrise with blasts of water. Sound silly? It is. It’s also pretty difficult. You can’t... | Read more »
NBA 2K17 (Games)
NBA 2K17 1.0 Device: iOS iPhone Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Following the record-breaking launch of NBA 2K16, the NBA 2K franchise continues to stake its claim as the most authentic sports video... | Read more »
Dog Sled Saga (Games)
Dog Sled Saga 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: A game by Dan + Lisa As a rookie musher, foster a dogsledding team whose skills will grow if they're treated right. Week by... | Read more »

Price Scanner via MacPrices.net

15-inch 2.2GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 15″ 2.2GHz Retina MacBook Pro (MJLQ2LL/A) on sale for $1799, including free shipping plus NY sales tax only. Amazon also has the 2015 15″ 2.2GHz Retina MacBook Pro (... Read more
Toughbook Celebrates 20 Years of Ruggedized M...
Panasonic System Communications Company of North America, Division of Panasonic Corporation of North America (Panasonic) today celebrates the 20th anniversary of its industry-leading Toughbook mobile... Read more
12-inch 1.1GHz Gray Retina MacBook on sale fo...
B&H Photo has the 2016 12″ 1.1GHz Gray Retina MacBook on sale for $1199.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
Save 30% on Camera Guard’s Secure Protection...
To celebrate the release of macOS Sierra, Miami-based security solutions company, ProtectStar has announced a special 30% discount on Camera Guard Professional for Mac 2016. This innovative security... Read more
DVDFab Special Deal – Get a 1-Year Free Licen...
Beijing, China based specialist in the field of DVD, Blu-ray and video backup solutions, Fengtao Software has launched its Autumn Special Deals 2016, giving a 1-year free license of a randomly picked... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379 $120 off MSRP - 21″ 2.8GHz iMac: $1199.99 $100 off MSRP - 21″ 1... Read more
13-inch 2.7GHz/256GB Retina MacBook Pro on sa...
Amazon.com has the 13″ 2.7GHz/256GB Retina Apple MacBook Pro on sale for $151 off MSRP including free shipping: - 13″ 2.7GHz/256GB Retina MacBook Pro (sku MF840LL/A): $1348 $151 off MSRP Read more
Apple TVs on sale for up to $50 off MSRP
Best Buy has 32GB and 64GB Apple TVs on sale for $40-$50 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more

Jobs Board

Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Sr. *Apple* Mac Engineer - Net2Source Inc....
…staffing, training and technology. We have following position open with our client. Sr. Apple Mac Engineer6+ Months CTH Start date : 19th Sept Travelling Job If Read more
*Apple* Retail - Multiple Positions-Norfolk,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 51829230 Detroit, Michigan, United States Posted: Sep. 19, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC is an Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.