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.

 
AAPL
$111.78
Apple Inc.
-0.87
MSFT
$47.66
Microsoft Corpora
+0.14
GOOG
$516.35
Google Inc.
+5.25

MacTech Search:
Community Search:

Software Updates via MacUpdate

Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
calibre 2.13 - Complete e-library manage...
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
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
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
ExpanDrive 4.2.1 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Adobe After Effects CC 2014 13.2 - Creat...
After Effects CC 2014 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). After Effects CS6 is still available... Read more
Evernote 6.0.5 - 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

Latest Forum Discussions

See All

Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »
Indulge in Japanese cuisine in Cooking F...
Indulge in Japanese cuisine in Cooking Fever’s new sushi-themed update Posted by Simon Reed on December 19th, 2014 [ permalink ] Lithuanian developer Nordcurrent has yet again updated its restaurant simulat | Read more »
Badland Daydream Level Pack Arrives to C...
Badland Daydream Level Pack Arrives to Celebrate 20 Million Downloads Posted by Ellis Spice on December 19th, 2014 [ permalink ] | Read more »
Far Cry 4, Assassin’s Creed Unity, Desti...
Far Cry 4, Assassin’s Creed Unity, Destiny, and Beyond – AppSpy Takes a Look at AAA Companion Apps Posted by Rob Rich on December 19th, 2014 [ permalink ] These day | Read more »
A Bunch of Halfbrick Games Are Going Fre...
A Bunch of Halfbrick Games Are Going Free for the Holidays Posted by Ellis Spice on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
It’s 1992 Again At Sony Pictures, Except For...
Techcrunch’s John Biggs interviewed a Sony Pictures Entertainment (SPE) employee, who quite understandably wished to remain anonymous, regarding post-hack conditions in SPE’s L.A office, explaining “... Read more
Holiday sales this weekend: MacBook Pros for...
 B&H Photo has new MacBook Pros on sale for up to $300 off MSRP as part of their Holiday pricing. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1699... Read more
Holiday sales this weekend: MacBook Airs for...
B&H Photo has 2014 MacBook Airs on sale for up to $120 off MSRP, for a limited time, for the Thanksgiving/Christmas Holiday shopping season. Shipping is free, and B&H charges NY sales tax... Read more
Holiday sales this weekend: iMacs for up to $...
B&H Photo has 21″ and 27″ iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only. B&H will also include a free copy of Parallels Desktop software: - 21″ 1.4GHz... Read more
Holiday sales this weekend: Mac minis availab...
B&H Photo has new 2014 Mac minis on sale for up to $80 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 1.4GHz Mac mini: $459 $40 off MSRP - 2.6GHz Mac mini: $629 $70 off MSRP... Read more
Holiday sales this weekend: Mac Pros for up t...
B&H Photo has Mac Pros on sale for up to $500 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2599, $400 off MSRP - 3.5GHz 6-core Mac Pro: $3499, $... Read more
Save up to $400 on MacBooks with Apple Certif...
The Apple Store has Apple Certified Refurbished 2014 MacBook Pros and MacBook Airs available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Save up to $300 on Macs, $30 on iPads with Ap...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
iOS and Android OS Targeted by Man-in-the-Mid...
Cloud services security provider Akamai Technologies, Inc. has released, through the company’s Prolexic Security Engineering & Research Team (PLXsert), a new cybersecurity threat advisory. The... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and 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...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.