TweetFollow Us on Twitter

June 95 - KON & BAL'S PUZZLE PAGE

KON & BAL'S PUZZLE PAGE

A Branch Too Far

Chris YERGA

[IMAGE 124-129_Puzzle_Page_http1.GIF]

See if you can solve this programming puzzle, presented in the form of a dialog between Konstantin Othmer and guest puzzler Chris Yerga. 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 please, make KON & BAL's day by submitting a puzzle of your own to AppleLink DEVELOP.

Chris I have a piece of code that runs fine on my Quadra, but when I run it on a plain old 68000 it crashes.

KON A 68000? So you're still trying to get GX to run on that Mac Portable in Cary's office, huh? How does it crash?

Chris With an address error.

KON What's hard about that? Your code is doing a 2- or 4-byte access to an odd address, which is OK on a 68040 but not on a 68000. It's a trivial problem.

Chris That's what I thought, but the address it's accessing appears to be uninitialized data. The code is simply allocating a block and then storing a pointer in the block, but the store never seems to occur, because afterward the block has random data in it. And of course the block itself is long-aligned, because it came from the Memory Manager, so there's no problem there.

KON Maybe on your Quadra, but on a Mac Plus the Memory Manager allocates blocks that are word-aligned.

Chris Thanks for the history lesson, chief, but this isn't a Mac Plus and the Memory Manager on this machine is much different -- much simpler, actually. It just so happens that our Memory Manager always long-aligns blocks.

KON Since when are you writing new Memory Managers? I thought you wrote graphics code.

Chris Yes. But the cornerstone of any decent graphics system is its Memory Manager -- I wouldn't expect a "QuickDraw classic" guy like you to understand.

KON I understand memory management just fine, Jackson. Show me where you're setting up this data.

100 Chris This is the interesting section:

NewDMAQueueEntry
...
+0030 0020C5D6  MOVE.L  D3,D0
+0036 0020C5DC  _NewPtr
+0038 0020C5DE  MOVE.L  A0,-$0008(A6)
+003C 0020C5E2  MOVE.L  -$000C(A6),-(A7)
+0040 0020C5E6  MOVEA.W -$000E(A6),A1
+0044 0020C5EA  MOVE.L  A1,-(A7)
+0046 0020C5EC  JSR     *+$53A4 ; 00211990
+004A 0020C5F0  MOVE.L  D0,-$000C(A6)
+004E 0020C5F4  MOVE.L  A2,(A0,D0.L)

It makes the _NewPtr call, makes some other function call, and then stores the pointer in A2 into our newly allocated buffer.

KON What's the other function call doing?

95 Chris I'm not sure, actually. The C source doesn't indicate that a function call should be happening:

buffer = NewPtr(totalSize);
count = count * size;
*(long *) (buffer + count) = (long) handlerProc;

The only thing I see happening in the source code is a multiply. You don't need a function call for that.

KON Could this be some wacky C++ operator overloading nonsense? C++ is very good at generating extra function calls. I think we'd better bring a SmartFriend in on this one.

Chris Don't bring out the big guns just yet. The code is written completely in plain old C: no C++ and no CFront.

KON It's got to be code you've written, because it's a PC-relative JSR. Since it's not an A5- relative JSR, it's not going through the jump table, so it couldn't have been linked in from a library or something external to your program. It looks like a call to a static function.

Chris Actually, all the JSR instructions in this code are PC-relative. I wrote a tool that transforms all JSRs that go through the jump table to PC-relative JSRs. You may want to sit down for this next part -- it's a little tricky.

KON That's why it ended up in the Puzzle Page. Let's hear it.

90 Chris All the code is being built into a ROM; however, our development system of choice only allows us to build our code as a Mac application. So I created a custom tool that postprocesses the application and turns it into something that will run out of ROM. Basically we use the CODE 0 jump table to link everything together.

KON I see. You know where the code will reside in ROM, so you fake out the jump table to make it look as if all the segments are loaded. Since nothing will ever move or unload, your ROM jump table entries never need to change. Pretty tricky.

Chris But not tricky enough. The scheme you describe will work, but it has two problems. First, our ROM can be mapped into different addresses, so the code must be completely relocatable. A Mac jump table contains JMP instructions with fixed long addresses (for example, JMP $4083143A), which are not relocatable. Second, as a cycle counter like you should know, the jump table is superfluous here, because no code will ever move or unload while it's running. You're doing a JSR to the JMP in the jump table -- the extra JMP is unnecessary and wastes cycles.

KON I didn't know you GX guys counted cycles. How do you get away with removing the extra JMP?

85 Chris Our tool scans all the object code for all instructions that reference the jump table. They are JSR xx(A5) (function call), PEA xx(A5) (pass a function pointer on the stack), and LEA xx(A5),Ax (get a function pointer in a local variable). When it finds one of these instructions, it looks up the function in the jump table and changes the instruction to a PC-relative version that simply references the address of the target function directly. Everything is PC-relative, so it relocates correctly, and there are no extra instructions.

KON But the PC-relative addressing mode has only a 16-bit offset, so you can only reference functions within 32K of the PC. Is your ROM that small?

80 Chris Are you kidding? The pictures for the About box are bigger than that. When we need to reference a function that's beyond 32K, we create a "jump island" that's still PC- relative but allows us a greater reach.

0020122A:   JSR     $00201FA8       ;go to jump island
0020122E:   MOVE.W  D0,(A3)
...
00201FA8:   LEA     *+0,A0          ;get pc
00201FAC:   ADDA.L  #$00014B02,A0   ;add long offset
00201FB2:   JMP (A0)                ;jump to destination

KON That looks suspicious to me. You're messing with A0 in your jump island.

75 Chris But that should be OK because the whole thing is written in C, which never passes a parameter in A0 and never expects A0 to be preserved.

KON I can't find a specific problem, but I'm still a little suspicious of all this OS code you're writing. You said that this code runs fine on your Quadra. Does the Quadra version undergo the jump table transformation process?

Chris No. The Quadra version is run just like a Mac application out of RAM.

KON Tell me all the differences between the two environments.

70 Chris The 68000 version of the software is different in two ways. First, it's generated with the exact same compiler, but without 68020 code generation enabled. Then, I run it through my BuildROM tool, which transforms all the jump table references to PC- relative references. The 68000 target hardware is a diskless system; the only way to get code into it is through ROM cartridges, so we can't try to run a version of the software that hasn't undergone the BuildROM step.

KON So the bug may have nothing to do with the differences between the 68040 and 68000 processors -- it may be the BuildROM process. Let's sum up what we know: First we saw a piece of code that allocates a block of memory and stores a value into that block, but the store never seems to happen. Near that code is an unexplained JSR which we believe is a function call. Finally, we know that a critical difference in the environment is that you alter the code path of function calls. It's really starting to smell like your BuildROM tool.

Chris The evidence is all circumstantial, counselor. The only questionable part of the BuildROM process we've seen is my usage of A0 in the jump islands, but I can't see a case where a C function call takes a parameter in A0 or assumes A0 is preserved.

KON We should trace through the NewDMAQueueEntry routine and see what that mysterious function call is doing.

65 Chris I put in a breakpoint. But when I run the program again, I drop into the debugger with a debug message complaining that some kind of parameter is out of range.

KON You mean the bug isn't reproducible?

Chris After many tries, this is the first time it's failed this way.

KON Hmm. What parameter is out of range?

Chris The debug message doesn't say, exactly. The value it's complaining about is in D2. The value is $00004E56.

KON That's a funny-looking number. It looks like an opcode to me. dh 4E56 tells me it's a LINK A6,#xx instruction. Where did the value in D2 come from?

60 Chris From a MOVE.W (A0),D2 instruction. The code seems to think A0 points to some data.

KON But it looks as if it points to code instead. Where is A0?

Chris It points to the start of a routine called VBLHandler.

KON VBLHandler sure sounds like an interrupt service routine to me, which would explain the various failure modes. Is the routine written in C? C routines don't bother to preserve A0, so your interrupt routine is trashing a register!

55 Chris There's some inline assembly code to save all the registers. Take a look:

 void VBLHandler(void)
{
asm {
    MOVEM.L     A0-A6/D0-D7, -(SP)
};

FlushQueue();
HandleVBLTasks();

asm {
    MOVEM.L     (SP)+, A0-A6/D0-D7
};
}

KON It seems quite suspicious to me that A0 points to the beginning of this routine. Sounds like your jump islands are at work. Look at the interrupt vector for your VBL handler.

50 Chris On this particular machine the VBL is handled as a level-6 interrupt. The level-6 vector is at $0078. It points to a jump island entry like the one above.

KON And the first thing it does is trash A0 before your inline assembly gets a chance to save it! Maybe you should stick to drawing bitmaps and leave the OS work to someone else.

45 Chris The problem is that the code that installed the interrupt handler is more than 32K away from the handler itself. So when it did an LEA xx(A5),Ax to get the address of the interrupt handler, the ROM builder tool needed to stick a jump island in there. Nasty. But I still need a way to do PC-relative jump islands.

KON Use the stack, son. Try this:

PEA     * ;push the pc
ADD.L   #xxxxxx,(sp) ;add a long offset
RTS     ;jump to the destination

Chris All this and you can draw bitmaps too! I'll fix the ROM builder tool to do this. But somehow I doubt that a VBL interrupt was hitting us at the same spot in my NewDMAQueueEntry routine every time. There must be another problem there.

KON We still have the breakpoint there; before we recompile and fix the bug, let's run it again and see if we can get the original failure mode to happen again.

40 Chris This time we hit the breakpoint. We trace over the NewPtr and see that it returns a valid pointer in A0. We step into the JSR at $20C5EC and it takes us to one of my jump islands.

KON Which alters A0, and then jumps to the function being called. What is the function doing?

35 Chris It's a very short routine; it just takes some parameters off the stack and does a few multiplies. It returns the result in D0 just like any normal C function would.

KON But is it using the trashed A0 for anything?

Chris No. In fact, it's not using any address registers at all. It only uses data registers for the multiplies. I repeat: no C function would ever take a parameter in A0.

KON So we return to the NewDMAQueueEntry routine, with a return value in D0. We save D0 in a local variable on the stack frame, and then hit the instruction at $20C5F4, which stores a value in the buffer pointed to by A0.

30 Chris But A0 still has the result of the jump island calculation in it. The code didn't set up A0 to point to anything!

KON Look at the listing of NewDMAQueueEntry again. The code gets the result of the NewPtr call in A0, makes the function call, and then assumes that A0 still has the valid pointer in it. But meanwhile, some wannabe OS programmer has gotten in there and hosed A0 on us!

20 Chris That function call seems to be doing the multiply. It must be some runtime math library that the compiler uses.

KON I'll bet one of your variables is a long word. With 68020 code generation turned on, the compiler was able to generate a long multiply instruction, but the 68000 doesn't have a long multiply instruction, so it calls the math library.

10 Chris I see. Since the math library was written by the same people who wrote the compiler, their code generator knows that A0 won't get trashed, so it doesn't bother to save and restore it around the call to the long multiply routine. Pretty sneaky.

KON But nice. You want the person writing your code generator to be the ultimate cycle counter. Since the Mac Segment Loader implementation doesn't trash A0, it's a worthwhile optimization for them to make.

Chris Except in this case, the code that performs a multiply is more than 32K away from where the math library resides in the ROM, so it hits a jump island and loses the value of A0.

KON Even a lowly register like A0 is sacred sometimes.

Chris So it appears.

KON Nasty.

Chris Yeah.

SCORING

  • 80-100 Please fax your resumé to Catapult Entertainment, Inc., (408)366-2471.
  • 60-75 We also have junior positions available.
  • 40-55 Don't worry; just let CopyBits do the tricky stuff.
  • 10-35 I see you've done your share of long multiplies. *

Chris YERGA During his four years working on QuickDraw GX, Chris learned a lot about graphics systems and large projects. He's currently employed by Catapult Entertainment, where he learned that a Sega is kinda like a Macintosh, a SNES is kinda like an Apple II GS, and carbon dioxide can be explosive. *

Thanks to Josh Horwich,

KON (Konstantin Othmer), and BAL (Bruce Leak) for reviewing this column. *

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Bookends 12.5.8 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Chromium 44.0.2403.125 - 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 44.0.2403.125: This release contains a number... Read more
iMazing 1.2.2 - Complete iOS device mana...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
Audio Hijack 3.2.0 - Record and enhance...
Audio Hijack (was Audio Hijack Pro) drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio... Read more
FontExplorer X Pro 5.0.1 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
Calcbot 1.0.2 - Intelligent calculator a...
Calcbot is an intelligent calculator and unit converter for the rest of us. Featuring an easy-to-read history tape, expression view, intuitive conversion, and much more! Features History Tape -... Read more
MTR 5.0.0.1 - The Mac's oldest and...
MTR (was MacTheRipper)--the Mac's oldest and smartest DVD-backup app--is now updated to version 5.001 MTR -- the complete toolbox, not a one-trick, point-and-click extractor. MTR is intended for... Read more
LibreOffice 4.4.5.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
Adobe Lightroom 6.1.1 - Import, develop,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
File Juicer 4.41 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. It finds and... Read more

Pac-Man Championship Edition DX has brou...
Bandai Namco has released Pac-Man Championship Edition DX on iOS and Android, which features the classic arcade gameplay that we've all grown to love. Pac-Man Championship Edition DX can be enjoyed in much shorter bursts than the arcade versions... | Read more »
Cosmonautica (Games)
Cosmonautica 1.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.1 (iTunes) Description: Cast off! Are you ready for some hilarious adventures in outer space? | Read more »
Rescue humanity from a Demon horde in An...
Angel Stone is Fincon's follow up to the massively successful Hello Hero and is out now on iOS and Android. You play as a member of The Resistance, a group of mighty human warriors who have risen up in defiance of the Demon horde threatening to... | Read more »
Gallery Doctor (Photography)
Gallery Doctor 1.0 Device: iOS iPhone Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Free up valuable iCloud and iPhone storage with Gallery Doctor, the only iPhone cleaner that automatically identifies the... | Read more »
You Against Me (Games)
You Against Me 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: A simple game… You. Me. Claim, steal, lock, score, win! | Read more »
Yep, it's True - Angry Birds 2 is O...
The not exactly rumors were true and the birds are back. Angry Birds 2 has come to the App Store and the world will... well I suppose it'll still be the same, but now we have more bird-flinging options! [Read more] | Read more »
You Could Design Your Own Card for Chain...
If you've ever wanted to create your own item, weapon, trap, or even monster for Chainsaw Warrior: Lords of the Night, this is your chance. Auroch Digital is currently holding a contest so that fans can fight to the death (not really) to see which... | Read more »
Bitcoin Billionaire is Going Back in Tim...
If you thought you managed to buy everything there is to buy in Bitcoin Billionaire and make all the money, well you though wrong. Those of you who made it far enough might remember investing in time travel - and it looks like that investment is... | Read more »
Domino Drop (Games)
Domino Drop 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Domino Drop is a delightful new puzzle game with dominos and gravity!Learn how to play it in a minute, master it day by day.Your... | Read more »
OPERATION DRACULA (Games)
OPERATION DRACULA 1.0.1 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0.1 (iTunes) Description: 25% off launch sale!!! 'Could prove to be one of the most accurate representations of the Japanese bullet hell shmup... | Read more »

Price Scanner via MacPrices.net

Sale! 13-inch MacBook Pros on sale for $100 o...
B&H Photo has 13″ MacBook Pros on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.5GHz/500GB MacBook Pro: $999.99 save $100 - 13″ 2.7GHz/128GB Retina... Read more
Sale! Save $100 on 13-inch MacBook Airs this...
B&H Photo has the 13″ 1.6GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model.... Read more
Worldwide Tablet Market Decline Continues, Ap...
The worldwide tablet market declined -7.0% year-over-year in the second quarter of 2015 (2Q15) with shipments totaling 44.7 million units according to preliminary data from the International Data... Read more
TP-LINK TL-PA8030P KIT Powerline Featuring Ho...
Consumer and business networking products provider TP-LINK is now shipping its TL-PA8030P KIT AV1200 3-Port Gigabit Passthrough Powerline Starter Kit that expands your home’s network over its... Read more
Apple refurbished iPad Air 2s available for u...
The Apple Store has Apple Certified Refurbished iPad Air 2s available for up to $140 off the price of new models. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB... Read more
Updated Apple iPad Price Trackers
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers. Read more
Apple refurbished 2014 13-inch 128GB MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.4GHz/128GB... Read more
Apple’s Education discount saves up to $300 o...
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
Save up to $600 with Apple refurbished Mac Pr...
The Apple Store has Apple 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... Read more
Mac Pros on sale for up to $260 off MSRP
B&H Photo has Mac Pros on sale for up to $260 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: $3719.99... Read more

Jobs Board

Executive Administrative Assistant, *Apple*...
…supporting presentation development for senior leadership. * User experience with Apple hardware and software is preferred. Additional Requirements The following list Read more
*Apple* Bus Company is now hirin - Apple Bus...
Apple Bus Company is now hiring school bus drivers in the Pettis County area. Class B CDL preferred. Free training provided. No nights or weekends required. Flexible Read more
*Apple* Certified Mac Technician - Updated 6...
…and friendly, hands-on technical support to customers troubleshooting and repairing Apple /Mac products with courtesy, speed and skill. Use your problem-solving skills Read more
Infrastructure Engineer - *Apple* /Mac Envir...
…a part of a team Requires proven problem solving skills Preferred Additional Apple Certified System Administrator (ACSA) Apple Certified Technical Coordinator (ACTC) 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.