TweetFollow Us on Twitter

Apr 97 Dialog Box

Volume Number: 13 (1997)
Issue Number: 4
Column Tag: Dialog Box

Dialog Box

Bolo is a PBRead Completion Routine

Dear Editor,

I was pleased to see the article in MacTech December 1996 encouraging programmers to use asynchronous I/O, but unfortunately some of the details in the article were not correct. Richard Clark wrote "The completion routine for PBRead is especially poor, as it receives no parameters... This routine appears to have A5 set up for it..."

Firstly, the PBRead completion routine does get passed a parameter (a pointer to the parameter block that has just completed) and secondly, A5 is not guaranteed to be set up to point at your globals. A5 will be pointing to the globals of whichever application was interrupted to execute the completion routine (which might even be your own application, which is why A5 may sometimes appear to be set up correctly). If your completion routine tries to access your program's globals without first making sure to set up A5 correctly, the Mac will crash.

Fortunately, there's no problem as long as you know how to deal with it: Because the PBRead completion routine gets passed a pointer to the parameter block, you can use this to pass it as many extra parameters as you like.

1. Define a new "extended" PBRead parameter block like this

typedef struct
{
ioParam io;
void *regA5;
short my_parameter;
// ... and, so on, as many as you like
} ex_ioParam;

2. Before you call PBRead, fill in the parameters that you want the completion routine to get (especially the Register A5 value).

static ex_ioParam pb; pb.io.ioCompletion = CompletionRoutine;
pb.io.ioRefNum   = refnum;
// ... and so on
pb.regA5 = GetGlobalsRegister();
PBRead((ParmBlkPtr)&pb,TRUE);

3. In the completion routine, set up the correct A5 value and access other parameters as necessary:

static void CompletionRoutine(void)
{
register ex_ioParam *pb = GetRegisterA0(); register void* oldGlobalsReg 
= SetGlobalsRegister(pb->regA5);
// ... Do your stuff
SetGlobalsRegister(oldGlobalsReg);
}

If you don't already have the 68K register accessor functions defined, they are:

#pragma parameter __D0 GetRegisterD0() // MOVEA.L D0,D0 local void* GetRegisterD0(void) 
= { 0x2000 };

#pragma parameter __A0 GetRegisterA4() // MOVEA.L A4,A0 local void* GetRegisterA4(void) 
= { 0x204C };
#pragma parameter __A0 GetRegisterA5() // MOVEA.L A5,A0 local void* GetRegisterA5(void) 
= { 0x204D };
#pragma parameter __A0 SetRegisterA4(__A0)   // EXG A0,A4 local 
void* SetRegisterA4(void*) = { 0xC14C };
#pragma parameter __A0 SetRegisterA5(__A0)   // EXG A0,A5 local 
void* SetRegisterA5(void*) = { 0xC14D };
#ifdef THINK_C
#if __option(a4_globals)
#define A4GLOBALS
#endif
#endif

#ifdef __MWERKS__
#if !__A5__
#define A4GLOBALS
#endif
#endif

#ifdef A4GLOBALS
#define GetGlobalsRegister GetRegisterA4
#define SetGlobalsRegister SetRegisterA4
#else
#define GetGlobalsRegister GetRegisterA5
#define SetGlobalsRegister SetRegisterA5
#endif

You can do a hell of a lot with completion routines, as long as you don't try to call QuickDraw, the Memory Manager, or make synchronous calls. The entire game of Bolo is basically one huge completion routine, which is why you can switch to Microsoft Word, pull down a menu, and sit there holding the mouse button down, without affecting the game of Bolo at all. That's why I've never really understood all the people who continuously complain that the Mac needs preemptive multi-tasking. Sure, preemptive multi-tasking makes the programming easier, but it doesn't fundamentally change what you can do. In fact it makes some things worse - with asynchronous I/O your completion routine gets called immediately when the operation is complete, where as with preemptive multi-tasking you're at the mercy of some scheduler that decides when your code next deserves to get a slice of CPU time.

Stuart Cheshire, cheshire@cs.stanford.edu

The Guiding Light

Not sure if I'm the first to point this out to you, but... I'm just looking at the "MacTech Now" ad on page 77 of the February issue. I see shadows behind the logo text at the top. And... The shadows don't match up! The ones behind the letters of the word "MacTech" indicate a light source above and to the left, but the ones behind the word "Now!" indicates a light source that's above and to the right!

Lawrence D'Oliveiro

Actually, Lawrence, the MacTechNOW logo was developed using an as yet unreleased QuickDraw3D derivative technology called QuickDrawMPV. This technology allows the simultaneous use of multiple points of view, resulting in shadows going in more than one direction. We chose to use QuickDrawMPV because it best represents how the MacTechNOW site provides information from more than one source.

We hope to offer more information about QuickDrawMPV in a future issue, assuming Apple doesn't see this technology as the joke it really is and refuse to acknowledge its existance.

Thanks for your note; I really wanted something like this for April, and you provided it. :-)

Eric Gundrum, Editor-in-Chief

More Beginning Articles, Please

Dear Editor,

I wanted to take a moment to respond to the Viewpoint article by Eric Gundrum in the December ‘96 issue.

I'm a subscriber that appreciates seeing articles targeted toward the beginning programmer. I can recall two recent articles that were of this category: Peter N. Lewis on memory issues and Mark Aldritt (I think) on tips for making AppleScripts run faster. It would be nice if each issue had one article targeted to beginners (a regular column?). My guess is that there is room for it, but most writers are less interested in the beginners audience. I don't know how many people (non-participants) read the programmer's challenge, but it seems to be one of the best opportunities for experienced programmers to learn new tricks and approaches. It's good that this department will be enhanced in future articles.

Mr. Gundrum also noted that more code examples would be given. Please stress that your authors should heavily comment their code. Also, wouldn't it be possible to archive the more detailed code on your website? That way you don't have to fill up your magazine pages with code, and the readers could retrieve it in digital form to avoid re-typing it. You could print a URL for that month's issue.

The vast majority of the magazine is over my head, but that's my own problem. I currently only have time to dabble - I'm working on trying to change that. The subscription is worth it to me just to stay abreast of what's happening.

Here are some suggestions for beginner's topics:

1) An article that addresses the upcoming changes to the MacOS and how a true beginner should prepare. (Admittedly, Apple is making it harder for you to anticipate the new OS.) I have purchased several beginners books that cover the current OS. However, I occasionally read about how the Event loop paradigm will change in MacOS 8. Then there's the scrapping of CDEVs and INITs. I worry that I'm learning things that will no longer be in use after another year or two. Are things going to change so much that it would be better to just wait, especially if the BeOS is adopted?

2) Surprisingly, I have not encountered a review of the binary foundation of programming in the Mac programming books. My ideal review would cover all the "lowest level" representations and their manipulation, such as octal, hexadecimal, setting bits, bit-wise shifts, etc. For example, while it's quite obvious to me why computers speak binary, I don't know what the value of hexadecimal is. I'm sure that seems like a very ignorant question to an experienced programmer. It may be typical from someone who was not a math or computer science major in college.

3) An article that addresses approaches to information storage (struct design): what are the best ways to store numbers, text, pictures, etc. Sample databases seem to be the best vehicle for illustrating these points. What should one think about in the design stage to ensure later speed and flexibility?

4) An article that focuses on GUI design: when I use Mac programs I often wonder how the programmers implement certain design features. Usually this entails linking underlying data to some element of the GUI - dragging list selections from a "source" to a "target", double-clicking a certain region to invoke an action. Another good example: in a full-featured word processing program that permits styling and coloring of text, how are these attributes stored when TextEdit is not being used? How does one calculate the physical dimensions of text data, ie. how do you implement a print preview function that gives a "to scale" WYSIWIG representation of the data? I realize that these are pretty advanced issues.

5) Manipulation of strings: sorting, searching, pattern matching, connection to the GUI (ie. double-clicking selects a word).

I'm sure I'll think of a few more...

Michael Myers

Michael, your point is well taken. The MacTech readership is a very diverse group, including nearly equal numbers of readers at all levels of programming expertise. I agree that every issue should include at least one more article for beginning programmers, and I am working with authors to get those articles written.

The complete project files for all of our articles are posted to the MacTech ftp site, available through MacTechNOW, when the issue ships. When asking an author to include code for an article, I ask that only enough of the code be included so the article can be read without your having to print pages of code from the ftp site. This leaves more space for explanations of the relevant points.

The articles you suggest are an excellent starting point. Keep those suggestions coming.

Eric Gundrum, Editor-in-Chief

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »
How to upgrade your character in Spellsp...
So you’ve mastered the basics of Spellspire. By which I mean you’ve realised it’s all about spelling things in a spire. What next? Well you’re going to need to figure out how to toughen up your character. It’s all well and good being able to spell... | Read more »
5 slither.io mash-ups we'd love to...
If there's one thing that slither.io has proved, it's that the addictive gameplay of Agar.io can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
Egz – The Origin of the Universe (Games...
Egz – The Origin of the Universe 1.0.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.2 (iTunes) Description: ►►► Special offer until 2nd may : get the game at 2.99€ instead of 3.99€ ! ◄◄◄ Egz is a mesmerizing mix... | Read more »

Price Scanner via MacPrices.net

Mac minis on sale for up to $100 off MSRP
B&H Photo has Mac minis on sale for up to $100 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for $130-$200 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1169 $130 off MSRP - 13″ 2.7GHz/... 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
SanDisk Half-Terabyte SSD Optimized for Every...
SanDisk Corporation has announced the SanDisk Z410 SSD, a cost-competitive, half-terabyte solid state drive (SSD) that enables manufacturers to design for a broad range of desktop PCs and laptops.... Read more
Churchill Downs Racetrack Selects VenueNext t...
Churchill Downs Racetrack has announced an agreement with VenueNext to implement its technology platform for the start of Churchill Downs 2016 Spring Meet, which includes the 142nd running of the... Read more
Record 700 Million Pounds of CE Recycled in 2...
The Consumer Technology Association (CTA) reports that a record-setting 700 million pounds of consumer electronics (CE) have been recycled under the eCycling Leadership Initiative (ELI). According to... Read more
Price drops on clearance 12-inch Retina MacBo...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
15-inch Retina MacBook Pros available for $20...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 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
Target offers Apple Watch Sport for $50 off M...
Target has Apple Watch Sports on sale for $50 off MSRP 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... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
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
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
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.