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

Latest Forum Discussions

See All

Brawl Stars veteran-backed Octopo Studio...
The 2024 PlayX4 B2B is kicking off tomorrow in Ilsan, lasting until the 26th in KINTEX Hall 1, and Octopo Studio has announced that they will be making an appearance. Shooting RPG Bounty Pang will be on show as the developers seek publishing... | Read more »
The Astral Express sets off to bring Hon...
After vanquishing yet another threat and adding a fine hat to their wardrobe, you’d think the crew of the Astral Express would be able to kick back after their adventures in Penacony. Clearly not though, as HoYo has announced the next leg of the... | Read more »
Dragons rein supreme in Pokemon Unite as...
If you were looking for the strongest type in Pokemon and asked the fanbase, I would wager most people's thoughts would turn to Dragon. They hit hard, are often powerful, and get used by some of the toughest trainers. Pokemon Unite is honouring... | Read more »
Players can take a peek into the design...
It doesn’t matter how much effort developers put into their classes, or how many special little mechanics there are; if there is one that wields two blades, I’m ignoring everything else. Diablo Immortal recently announced such a class in the shape... | Read more »
Android users have a new option in the c...
When you are in the thick of a firefight or trying to pull off a mid-combat parkour flip through a squad of foes, sometimes touchscreen control just won’t do it for you. For those intense sessions, you could benefit from a good mobile controller,... | Read more »
Jagex releases the first of three origin...
At this point, I am sure everyone has heard of Runescape, and or Runescape Classic. It has been going strong for 23 years, with constant content and story coming out. Luckily for fans of the game, or fantasy in general, Jagex has announced an... | Read more »
Watcher of Realms unveils new story and...
Watcher of Realms players are in for quite the feast this month, as Moonton release two powerful new heroes, including one that will burst down even the most mighty of foes. Recruit your new friends, and then burn through the Main Quest expansion... | Read more »
Reverse: 1999 continues its trip down un...
The field trip to Australia continues in Reverse: 1999 as Phase 2 of Revival! The Uluru Games kicks off. You will be able to collect new characters, engage with new events, get hordes of free gifts, and follow the story of a mushroom-based... | Read more »
Ride into the zombie apocalypse in style...
Back in the good old days of Flash games, there were a few staples; Happy Wheels, Stick RPG, and of course the apocalyptic driver Earn to Die. Fans of the running over zombies simulator can rejoice, as the sequel to the legendary game, Earn to Die... | Read more »
Top Mobile Game Discounts
Every day, we pick out a curated list of the best mobile discounts on the App Store and post them here. This list won't be comprehensive, but it every game on it is recommended. Feel free to check out the coverage we did on them in the links below... | Read more »

Price Scanner via MacPrices.net

Amazon is blowing out clearance 13-inch M1 Ma...
Amazon has clearance Apple 13″ M1 MacBook Airs on Memorial Day sale for $300 off original MSRP, only $699. Their prices are the lowest available for new MacBooks this Holiday shopping season. Stock... Read more
MacBook Sale! 13-inch M2 MacBook Airs are onl...
Best Buy has Apple 13″ MacBook Airs with M2 CPUs in stock and back on sale this week on their online store for $150 off MSRP in Space Gray, Silver, Starlight, and Midnight colors. Prices start at... Read more
Apple increases iPhone trade-in values ahead...
Get up to $650 on the purchase of new or refurbished iPhone at Apple using their official Trade In program, now with increased values ahead of WWDC 2024. Trade in your old iPhone, and Apple will... Read more
Amazon has Apple’s iPad discounted to a new l...
Amazon has Apple’s 10th-generation WiFi iPads on sale discounted up to $40 off MSRP, starting at only $334. With the discounts, Amazon’s prices are the lowest we’ve seen for these iPads: – 10″ 10th-... Read more
13-inch M3 MacBook Airs on sale for $100 off...
Best Buy has Apple 13″ MacBook Airs with M3 CPUs in stock and on sale this week for $100 off MSRP. Prices start at $999. Their prices, along with Amazon’s, are the lowest currently available for new... Read more
Weekend Deal! Best Buy has Apple Watch Series...
Best Buy has Apple Watch Series 9 models on sale for $100 off MSRP on their online store this weekend. Sale prices available for online orders only, in-store prices may vary. Order online, and choose... Read more
Apple Watch SE on sale starting at $199 this...
Best Buy has all Apple Watch SE models on sale this weekend for $50 off MSRP on their online store. Sale prices available for online orders only, in-store prices may vary. Order online, and choose... Read more
Mac Pro with M2 Ultra CPU on sale for $6699,...
In the market for one of Apple’s Mac Pro towers? B&H Photo has the base Mac Pro with an Apple M2 Ultra CPU, 64GB RAM, and 1TB SSD on sale for $6699 including free 1-2 day shipping to most... Read more
New May Verizon promotion: Switch and get a f...
Red Hot Deal Days at Verizon: Switch to Verizon this month, and get the 256GB iPhone 15 Pro for free, with trade-in, when you add a new line of service. Verizon is also offering a free cellular iPad... Read more
Updated Apple MacBook Price Trackers
Our Apple award-winning MacBook Price Trackers are continually updated with the latest information on prices, bundles, and availability for 16″ and 14″ MacBook Pros along with 13″ and 15″ MacBook... Read more

Jobs Board

Salon Manager BTC - *Apple* Blossom Mall -...
Salon Manager BTC - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/144535/winchester-va-united-states) - Job Read more
Omnichannel Associate - *Apple* Blossom Mal...
Omnichannel Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Beauty Consultant - *Apple* Blossom Mall -...
Beauty Consultant - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Hair Stylist - *Apple* Blossom Mall - JCPen...
Hair Stylist - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Blossom Read more
Operations Associate - *Apple* Blossom Mall...
Operations Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.