TweetFollow Us on Twitter

Code Mechanic

Volume Number: 13 (1997)
Issue Number: 5
Column Tag: develop

Code Mechanic: Better Than Ever Stress Testing

by Dave Evans

There are few things more frustrating than losing access to your debugging tools due to a freeze, because you can't fix what you can't diagnose. The best course is to stop freezes before they start, so I'd like to share a common cause of freezes I've found. I'll also discuss some of the stress-testing options that are available to help you catch freeze-causing problems you might have missed, including an improved debugging tool.

Veteran readers of develop may notice a new title for this column. The previous title, "Balance of Power," was apt for its time, indicating a focus on PowerPC issues. But now that all new MacOS computers are PowerPC-based, everybody's writing about PowerPC, and my efforts in this area are complete. This new title reflects a focus on the mechanics of code tuning, with tips for improving your application's performance and stability, which I hope you'll find just as useful.

Protect Your Vectors

Even if you use a PowerPC-based MacOS computer, the first 256 bytes of memory are dedicated to 680x0 exception vectors, which the 680x0 software emulator uses to emulate 680x0 exceptions and interrupts. On a 680x0-based computer, these values are read by the processor itself when handling an exception or servicing an interrupt.

Under System 7, these important vectors are not memory protected. Any program can read from or write to them, possibly resulting in a serious failure. While not all of the vectors are used, modifying some of them will cause an immediate freeze, leaving you without access to your debugging tools. You probably don't address these vectors intentionally, but it often occurs accidentally when a nil pointer or empty handle is de-referenced.

Unintentionally reading from these vectors will produce a random result. In most cases the vectors are addresses of special system routines; these vectors can have any value, and they vary significantly from one computer model to another. As an example of how easy it is to cause a problem in this area, take a look at the following C code, similar to that found in some applications:

front_window = FrontWindow();
if (front_window->windowKind < 0)
 MyDeskAccessoryRoutine(front_window);

The developers didn't realize that FrontWindow can return nil when no windows are open. In that case the application de-references the nil pointer and makes a logical decision based on the sign of the half word at $6C in low memory, which is the high half of the interrupt level 3 vector. On most Macintosh computers released before 1995, this vector pointed into ROM starting at address $40800000. Because of this, the applications would test the high half word value of $4080, and they wouldn't run the desk accessory routine. This was the right behavior, but for the wrong reason; disaster was averted by luck.

Beginning with all PCI-based PowerPC computers, ROM starts at location $FFC00000. During the development of these computers, we found that applications with code like the above would crash because they executed unexpected code after comparing the new half word value of $FFC0. We were able to work around their problem by changing the interrupt level 3 vector to point to a routine in RAM. This changed the high half word value to be a small positive number, and the applications behaved as expected. Still, the best case would have been if the problem could have been avoided in the first place. The following code is an example of what would have been a better, crash-free approach:

front_window = FrontWindow();
if (front_window && front_window->windowKind < 0)
 MyDeskAccessoryRoutine(front_window);

Checking for nil pointers or handles is one way you can avoid these crashes in the first place. Checking for empty handles is another necessary step, since unlocked relocatable blocks that are marked purgeable may disappear any time memory can move.

To detect problems with purgeable blocks, you'll need tools to stress test your application. Utilities that display heap zones, allowing you to compact and purge a heap on demand, are a good start. For serious testing, however, you'll need a stress tool that operates all the time. One good tool for this is MemHell, which will compact and purge your heap whenever a Memory Manager routine that might move or purge memory is called. This slows down execution of your tests, but it will flush out problems with purgeable blocks.

So, while accidentally reading from low memory can cause unexpected results, accidentally writing to low memory can be fatal, and this is one of the most common causes of freezes that I've noticed. You may think this could never happen in your code, because none of your blocks are purgeable and you always check errors after allocating pointers. Think again; there are plenty of other opportunities. Do you check for an error after every GetResource call? Getting an unexpected error - from a corrupted resource file, for example - is one way you can end up with a nil handle. Besides diligent review of your code, you need to do stress testing to flush out possible errors, or freezes are likely to result.

Are You Stressed Enough?

There are a number of tools to help add stress to your testing. I've already mentioned MemHell for finding problems with purgeable blocks. You'll similarly need a tool to find reads and writes to the exception vectors.

The simplest choice is the ubiquitous and venerable EvenBetterBusError, written by Greg Marriott. This tool safeguards the first four bytes of memory, which are very often accidentally written over or read from. To detect reads, it places in the first four bytes of memory a value which when de-referenced will cause a crash. If you use a nil pointer or empty handle, the illegal value is likely to be used as data or de-referenced, leading to a crash. To detect writes, it checks periodically to see if the value that it placed has been overwritten; if so, you'll be notified with a DebugStr message. EvenBetterBusError is included as a dcmd in MacsBug beginning with version 6.5.4.

I've extended EvenBetterBusError to be more aggressive. The new version, YetEvenBetterBusError, writes a value over the first 256 bytes of memory which will cause a crash into your debugger when de-referenced. It also checks periodically for writes to these locations, but more frequently than EvenBetterBusError does. Like EvenBetterBusError, upon noticing a write to these locations it will notify you with a DebugStr message. YetEvenBetterBusError can be found at www.mactech.com.

To implement YetEvenBetterBusError, I had to sacrifice some compatibility with existing applications. Any application code that assumes the exception vectors start at address 0 will no longer function correctly. Most applications don't use the exception vectors directly, but some copy protection schemes do modify the vectors.

The correct way to determine the location of the exception vectors is by using the 680x0 instruction MOVEC, which must always be executed in supervisor mode. The location of the first vector is stored in the 680x0 VBR (Vector Base Register). To read the address, you would write the following assembly code:

_EnterSupervisorMode ; old sr result in d0
movec   vbr,a0   ; get the vbr
move.w  d0,sr    ; restore the old sr

Always use the VBR to find these vectors. Although early versions of the MacOS always placed them at location 0, they're now often elsewhere. When virtual memory is turned on, for example, the vectors will actually reside in the system heap, and the VBR will point to them. To maintain compatibility, however, if virtual memory doesn't handle an exception it calls through to the original vector table at location 0. This is why even with virtual memory on, writing over the low-memory exception vectors can still cause a freeze.

YetEvenBetterBusError is able to overwrite and then monitor the first 256 bytes of memory by moving the exception vector table entirely. So, even when virtual memory is on, with YetEvenBetterBusError installed the original low-memory vectors are never called. This is why some existing applications may be incompatible with YetEvenBetterBusError.

A Cure for Test Anxiety

It's true that fully testing your code to reflect all possible configurations and user actions can be a near-impossible task. But the perceived stability of both your application and the computer depends on how well we all write and test our software. To do the best possible job, use the stress-testing tools mentioned in this column or in the article "Squashing Memory Leaks with TidyHeap" in this issue. Do the right thing: stress test, then relax!

Thanks to Pete Gontier, Chris Jalbert, Bo3b Johnson, Dave Lyons, Quinn "The Eskimo!", and Keith Stattenfield for reviewing this column.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Splash Cars guide - How to paint the tow...
Splash Cars is an arcade driving game that feels like a hybrid between Dawn of the Plow and Splatoon. In it, you'll need to drive a car around to repaint areas of a town that have lost all of their color. Check out these tips to help you perform... | Read more »
The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »
Space Grunts guide - How to survive
Space Grunts is a fast-paced roguelike from popular iOS developer, Orange Pixel. While it taps into many of the typical roguelike sensibilities, you might still find yourself caught out by a few things. We delved further to find you some helpful... | Read more »
Dreii guide - How to play well with othe...
Dreii is a rather stylish and wonderful puzzle game that’s reminiscent of cooperative games like Journey. If that sounds immensely appealing, then you should immediately get cracking and give it a whirl. We can offer you some tips and tricks on... | Read more »
Kill the Plumber World guide - How to ou...
You already know how to hop around like Mario, but do you know how to defeat him? Those are your marching orders in Kill the Plumber, and it's not always as easy as it looks. Here are some tips to get you started. This is not a seasoned platform... | Read more »
Planar Conquest (Games)
Planar Conquest 1.0 Device: iOS Universal Category: Games Price: $12.99, Version: 1.0 (iTunes) Description: IMPORTANT: Planar Conquest is compatible only with iPad 3 & newer devices, iPhone 5 & newer. It’s NOT compatible with... | Read more »
We talk to Cheetah Mobile about its plan...
Piano Tiles 2 is a fast-paced rhythm action high score chaser out now on iOS and Android. You have to tap a series of black tiles that appear on the screen in time to the music, being careful not to accidentally hit anywhere else. Do that and it's... | Read more »
Ultimate Briefcase guide - How to dodge...
Ultimate Briefcase is a simple but tricky game that’s highly dependent on how fast you can react. We can still offer you a few tips and tricks on how to survive though. Guess what? That’s exactly what we’re going to do now. Take it easy [Read more... | Read more »

Price Scanner via MacPrices.net

12-inch 1.2GHz Silver Retina MacBook on sale...
B&H Photo has the 12″ 1.2GHz Silver Retina MacBook on sale for $1399 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price for this model from... Read more
iPads on sale at Target: $100 off iPad Air 2,...
Target has WiFi iPad Air 2s and iPad mini 4s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for... Read more
Target offers Apple Watch for $100 off MSRP
Target has Apple Watches on sale for $100 for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
Macs available for up to $300 off MSRP, $20 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Watch Super Bowl 50 Live On Your iPad For Fre...
Watch Super Bowl 50 LIVE on the CBS Sports app for iPad and Apple TV. Get the app and then tune in Sunday, February 7, 2016 at 6:30 PM ET to catch every moment of the big game. The CBS Sports app is... Read more
Two-thirds Of All Smart Watches Shipped In 20...
Apple dominated the smart watch market in 2015, accounting for over 12 million units and two-thirds of all shipments according to Canalys market research analysts’ estimates. Samsung returned to... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H Photo has 12″ 1.2GHz Retina MacBooks on sale for $180 off MSRP. Shipping is free, and B&H charges NY tax only: - 12″ 1.2GHz Gray Retina MacBook: $1499 $100 off MSRP - 12″ 1.2GHz Silver... Read more
12-inch 1.1GHz Gray Retina MacBook on sale fo...
B&H Photo has the 12″ 1.1GHz Gray Retina MacBook on sale for $1199 including free shipping plus NY sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model... Read more
Apple now offering full line of Certified Ref...
Apple now has a full line of 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... Read more

Jobs Board

*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
*Apple* Subject Matter Expert - Experis (Uni...
This position is for an Apple Subject Matter Expert to assist in developing the architecture, support and services for integration of Apple devices into the domain. Read more
*Apple* Macintosh OSX - Net2Source Inc. (Uni...
…: * Work Authorization : * Contact Number(Best time to reach you) : Skills : Apple Macintosh OSX Location : New York, New York. Duartion : 6+ Months The associate would Read more
Computer Operations Technician ll - *Apple*...
# Web Announcement** Apple Technical Liaison**The George Mason University, Information Technology Services (ITS), Technology Support Services, Desktop Support Read more
Restaurant Manager - Apple Gilroy Inc./Apple...
…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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.