TweetFollow Us on Twitter

Debugging PPC
Volume Number:10
Issue Number:8
Column Tag:Powering Up

Debugging

A slight detour into the subterranean areas of the PowerMac

By Richard Clark, General Magic & Jordan Mattson, Apple

Since January, we have been demonstrating that programming a Power Macintosh is very similar to programming a 68K-based Macintosh. This month we’ll show you that debugging on a Power Macintosh is quite similar to debugging on a 68K Macintosh, but with a few changes due to Mixed Mode and the new runtime architecture. (Astute readers might notice that we promised to talk about Human Interface programming this month. We’ve had to take a slight detour away from this plan, due to technical problems with our Human Interface demo code and a new job, but we’ll be back on track next month.)

We’ll begin by examining the available tools, then review the most common new problems a Power Macintosh programmer faces.

Choose Your Tool

We’ll begin with some good news - debugging an emulated application on the Power Macintosh is the same as debugging that application on a 68K Macintosh. You can use MacsBug, SADE, Jasik’s “The Debugger,” TMON, the THINK C debugger, and anything else you use normally. In fact, MacsBug just thinks it’s running on a 68020.

However, since you’re reading this column you probably want to debug a PowerPC application on your Power Macintosh. If you like using source-code debuggers, you’ll need to use a different set of tools: either Apple’s “Macintosh Debugger for PowerPC”, or Metrowerks’ “MW Debug.” Three tools support assembly-level debugging on PowerPC: Apple’s “Macintosh Debugger”, Jasik’s “The Debugger”, and MacsBug.

Looking At The New Debuggers

The new Power Macintosh source-level debuggers come in two parts - the debugger application, and a low-level “nub” which does most of the work of handling breakpoints and reading/writing memory. (Apple supplies the nubs for both its own debugger and the Metrowerks debugger.) You could think of the nub as a version of Macsbug which depends on an application to provide the interface.

Like Macsbug, the nub used with Apple’s “Macintosh Debugger” installs itself at the lowest level of the operating system and completely takes over the machine when active. (The nub actually disables interrupts while doing its work.) Since the machine is completely frozen when the nub has control, Apple’s debugger requires two machines: a Power Macintosh with the nub and the program being tested, and a 68K Macintosh or Power Macintosh running the debugging application. These two machines are connected by a serial cable which carries commands and data to the nub, and data back from the nub.

The two-machine mode provides some real advantages. If something happens to the machine running the debugging application, you can restart it and continue as if nothing happened. Also, using two machines for debugging keeps the user interface that you’re testing separate from the interface on the test tools. (Anybody’s who’s tried to track down redrawing problems with a single-machine source level debugger can appreciate that!)

Metrowerks’ debugger uses a different version of Apple’s nub which doesn’t take over the machine completely and which doesn’t use the serial port directly. This version of the nub communicates with the debugging application through the Program-to-Program Communication toolbox, which allows the nub and the application to be on the same machine or connected via the Macintosh’s built-in networking.

This nub has both advantages and disadvantages. The first disadvantage: since the nub is built on the PPC toolbox, much of the system has to be functioning correctly for the nub to communicate with the debugging application, so a crash could also kill your debugging tools. Disadvantage 2: the Process Manager gets involved when you send a message via the PPC toolbox, which limits the top speed of the connection. The final disadvantage is this: since this nub isn’t installed automatically as an extension, so you have remember to start it running before you run the debugger application, or place it in your startup folder. In this nub’s favor, you can debug with one machine, which saves money, desk space, and keeps you from trying to remember which mouse does which!

Both of these debuggers work with .SYM/.xSYM files - known generically as “symbol files” - which are generated by the PowerPC linker. (The only difference between a .SYM file and an .xSYM file is the name - Apple’s engineers adopted the convention of ending PowerPC symbol files with .xSYM so that developers who build “fat binaries” could have symbol files for both the PowerPC and 68K code at the same time.) Symbol files match the start of each line of source code to the corresponding location in the compiled program. Symbol files also list each of the variables in the application, and where each variable is located. Using this information, debuggers can determine which line of source the current program counter points to, let you set breakpoints at the beginning of a current line (and single-step through the source code), and examine and modify variables.

Without a symbol file, the debuggers can only show you machine-language instructions, and let you modify locations in memory directly.

Teaching An Old Debugger New Tricks

The other two PowerPC debuggers - MacsBug and Steve Jasik’s “The Debugger” are 68K debuggers with some additional commands for the Power Macintosh.

MacsBug is still a 68K debugger at its heart: it is completely emulated and all of the register displays, breakpoints, stack crawls, and code displays operate as if running on a 68020. The largest single change to MacsBug is that it now knows about the “Modern” Memory Manager introduced with the Power Macintosh. The new memory manager uses different algorithms and different structures internally than the “classic” memory manager; this causes older versions of MacsBug to claim that the heap is corrupted when using the new memory manager. (MacsBug 6.5 contains the changes to work with the new memory manager.)

If MacsBug is still a 68K debugger, how can you debug PowerPC code with it? Apple has released an unsupported set of debugger extensions (known as dcmds, pronounced dee-commands) for PowerPC debugging. These commands include a disassembler, register display, stack crawl, and breakpoint setter for PowerPC code. (Setting breakpoints requires that you have one of the debugging nubs installed, so the breakpoint will be handled in the source-level debugger.)

The new MacsBug and the dcmds were shipped with the March, 1994 issue of develop magazine (issue 17), on the CD-ROM which accompanied the magazine. The dcmds don’t appear to be available anywhere else. However, the new MacsBug is available on the Internet via anonymous ftp. Connect to ftp.apple.com, look in the /dts/mac/tools/macsbug directory, and download macsbug-6-5d6.hqx. (If there’s a later version in the directory, take that instead.)

On the other hand, Steve Jasik’s “The Debugger” has been completely revised to include PowerPC debugging support. It supports source-level and assembly-level debugging, including breakpoints, stack traces, and disassembling PowerPC code. The Debugger isn’t a complete solution for PowerPC debugging yet, since single-stepping through source code, A-Trap breaks, and the “step spy” command don’t work for PowerPC code. (They should be working by the time you read this column.)

One interesting feature of The Debugger is the way it annotates PowerPC disassemblies. Every line is labeled with the name of the instruction, which helps if you’re learning PowerPC assembly language.


/* 1 */
;#  void MainLoop()
;#  {
;#    EventRecordtheEvent;
;#  
                     .MainLoop
 0:mfsprr0,LR  ; Move From Special Purpose Reg
 4:stmw r28,-16(SP); Store Multiple Word
 8:stw  r0,8(SP) ; Store Word
 C:stwu SP,-96(SP) ; Store Word with Update
 10:    lar29,gDone(RTOC) ; Load Address
;#    while (!gDone) {
 14:    jplad_2  ; Branch
;#      WaitNextEvent(everyEvent, &theEvent, 1L, NULL);
;#  
;#      // The external code didn't handle the event, so we will
 18:  lad_1 li r3,-1 ; Load Immed
 1C:    lar4,theEvent(SP) ; Load Address
 20:    lir5,1 ; Load Immed
 24:    lir6,0 ; Load Immed
 28:    bl.WaitNextEvent  ; Branch, set LR
 2C:    lwz RTOC,20(SP) ; Load Word and Zero

The Debugger is completely self-contained, i.e. it doesn’t require a separate nub, and requires a symbol file to enable its source-level debugging abilities.

Are There Any New Bugs On The Power Mac?

The first thing to remember when debugging a Power Macintosh program is that you’re still using a Macintosh. The same toolbox managers are present on the Power Macintosh, and much of the Toolbox and Operating System code is emulated. This means that most crashes into MacsBug (or another debugger) happen for the same reasons as on a 68K Macintosh: the program either passed a bad value to the toolbox, or corrupted a block in the heap, or grew the stack into the heap, and so on.

Still, the Power Macintosh can crash in some unique ways, due to the presence of Mixed Mode and the limited memory protection afforded by the new runtime architecture.

Using Mixed Mode incorrectly can cause crashes into 68K code. The most common problem in all newly ported PowerPC code comes from forgetting to pass a Universal Procedure Pointer:


/* 2 */
// Oops! We’re passing the address of a procedure directly
partCode = TrackControl(myControl, startPoint, myActionProc);

This causes a crash into the debugger with an “illegal instruction error” on the 68K side. You can diagnose this error in several different ways:

• Use the PowerPC disassembler (or the dis dcmd in MacsBug) to look at the address where the crash occurred. If you see reasonable PowerPC instructions, you probably forgot to pass a Universal Procedure Pointer.

• Think about what your code is doing. Did you click on a control, is a dialog being drawn, or is an Apple event being handled? These all involve callback procedures, so you may have forgotten a UPP.

• Set a break on the Mixed Mode A-trap. (This only works in MacsBug.) Using the ATVB (A-Trap Vector Break) dcmd, you can set a break on _MixedModeDispatch (0xAAFE). (The normal A-Trap Break command won’t work because the Power Macintosh doesn’t run all traps through the same trap dispatcher as the 68K Macintosh.) This lets you look at every mixed mode switch

The Power Macintosh’s limited memory protection can uncover bugs you didn’t know you had. If Virtual Memory is turned on, your code is loaded into a read-only area, so attempting to overwrite your code will result in an Access Violation. Even if VM is off, attempts to write to location 0 usually trigger access violations, as do all attempts to overwrite the ROM.

Data structure alignment presents some of the nastiest bugs. Remember that the PowerPC prefers to have all 2-byte values begin on even addresses, and 4-byte values on addresses which are multiples of 4. The compiler will help maintain this alignment in data structures by inserting “padding” in the appropriate places. Naturally, if you weren’t expecting this padding, your program might behave in unexpected ways.

Diagnosing this bug is easy: try re-compiling your code with the “use 68K alignment” option to your compiler, and see if the problem goes away. If it does, you probably forgot to install alignment directives on one or more of your structures.

Next Month In Powering Up

Next month, we’ll return with an “all-code” column exploring some Human Interface techniques you can apply on the Power Macintosh, and talk about a few nifty PowerPC hacks.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

calibre 2.69.0 - Complete e-book library...
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 librarian... Read more
Evernote 6.9.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
jAlbum Pro 13.5 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
jAlbum 13.5 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Google Chrome 53.0.2785.143 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
Chromium 53.0.2785.143 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 53.0.2785.143: [Security Fix] High CVE-2016-... Read more
QuickBooks 2015 16.1.7.1524 R8 - Financi...
Save 20% on QuickBooks Pro for Mac today through this special discount link QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track... Read more
Sierra Cache Cleaner 11.0.1 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Default Folder X 5.0.7 - Enhances Open a...
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 on... Read more
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

Pumped BMX 3: Beginner tips and tricks
There’s a whole lot more to Pumped BMX 3 than meets the eye. Your goal is to perform a wide array of sweet flips and tricks, but that’s easier said than done. It takes well practiced timing and coordination, and the game doesn’t really explain that... | Read more »
Cybird’s latest release - BFB Champions...
Launched in the UK in early September, BFB Champions’ newest update is loaded with great new features, and looks set to outshine the original version by taking it out of soft launch and giving it a new lease of life. | Read more »
3 apps to boost your focus
As someone who works from home, my workspace is a minefield of distraction. Cats, tasty snacks, the wind blowing past my window, that cleaning that I suddenly can’t put off any longer. If I let distraction takes its course, I find that soon half... | Read more »
Pumped BMX 3 (Games)
Pumped BMX 3 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: The final instalment of the smash hit #1 rated BMX game is here! Following on from the insane success of Pumped BMX 2, Pumped 3... | Read more »
4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »
The beginner's guide to destroying...
Age of Heroes: Conquest is 5th Planet Games’ all new turn-based multiplayer RPG, full of fantasy exploration, guild building, and treasure hunting. It’s pretty user-friendly as far as these games go, but when you really get down to it, you’ll find... | Read more »

Price Scanner via MacPrices.net

CAZE Annouces New Zero 5 Case for Jet Black i...
Hong Kong basd CAZE has announced Zero 5 case for iPhone 7/ 7 Plus, one of the world’s thinnest clear hard cases, measuring just 0.5 millimeters. CAZE has been producing and improving the Zero 5... Read more
Nest Egg Inventory App for iOS Offers Conven...
Campbell, California based Winprogger LLC has announced the release and immediate availability of Nest Egg – Inventory 4.1.22, an important update to their easy-to-use, yet comprehensive inventory... Read more
Factor4, LLC Launches Apple iOS and Android G...
Factor4, LLC, which offers gift and loyalty services to the SMB marketplace, has released free mobile applications that enable merchants to process via all Apple and Android devices. The Apple and... Read more
15-inch Retina MacBook Pros on sale for $200...
B&H Photo has 15″ Retina Apple MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Apple refurbished iMacs available for up to $...
Apple has 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 available: - 21″ 3.... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Apple refurbished 2016 13-inch MacBook Airs a...
Apple has Certified Refurbished 2016 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB MacBook... Read more
1.4GHz Mac mini on sale for $449, save $50
Adorama has the 1.4GHz Mac mini on sale for $50 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini (Apple sku# MGEM2LL/A): $449 $50 off MSRP To purchase a mini at... Read more
Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more

Jobs Board

*Apple* Retail - Multiple Positions- Akron,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Hardware Design Validation Engineer - *Apple...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
Systems Architecture Prototyping - *Apple*...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more Read more
*Apple* Retail - Multiple Positions- South B...
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.