TweetFollow Us on Twitter

Executing Code
Volume Number:10
Issue Number:4
Column Tag:Powering UP

Executing Code
On A Power Macintosh

PEF files are more than just object code; at last you get initialization routines and global data

By Richard Clark, Apple Computer, Inc.

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

This month’s Powering Up takes a different approach than previous columns - in the past, we told you about moving existing code to the Power Macintosh. Today, we’ll tell you some things about using the new capabilities of the Power Macintosh runtime to enhance new and existing applications.

The greatest advantages on the Power Macintosh come from its speed and a new runtime architecture. This architecture was designed from the ground up for operation on a RISC processor; in fact, most of the new runtime was derived from IBM’s runtime model for their RISC systems. The new runtime architecture supports:

• Executable code that may be stored anywhere (i.e. in ROM, in a resource, or in the data fork of a file)

• Executable code which may have its own static data (including global variables)

• Executable code which may export code and data, and which can import code and data automatically from other executables, and

• Register-based calling conventions which execute quickly and efficiently

The structure of executable code

On a Power Macintosh, all native code is packaged up as “fragments.” An application consists of one large fragment (stored in the data fork of the application file), with supporting resources such as windows and menus in the resource fork. Other kinds of PowerPC code, such as INITs and CDEVs, may be stored as fragments in a resource or in the data fork of the file. Finally, the ROMs on a Power Macintosh include fragments which implement parts of the System software.

To support such a wide range of uses, each fragment has some important common capabilities: each has its own static data, references to imported code or data, and pointers to functions and variables that it exports. Each fragment is stored in a “PEF container”, which holds code in the Preferred Executable Format (PEF). Each PEF container consists of three parts: a Data Segment, a Code Segment, and a Loader Segment. Each of these segments is used when loading a fragment into memory:

• The Code Segment holds the executable code. The system treats code as read only, and may load the code anywhere in memory. As a result, all branches in the code have to be either relative to the current location, or go through pointers.

• The Data Segment holds the static data for the code, and a special table called the “Table of Contents” or TOC. The TOC contains pointers to each global variable used by the program, as well as pointers to code which cannot be accessed through a PC-relative branch or which exist in other code fragments.

• The Loader Segment contains tables which list each import and export for the current fragment, and other information as required to load a fragment.

PEF containers always retain the same format no matter where they are stored.

When a fragment is first loaded, the Code and Data segments are copied into memory (where they become Code and Data sections.) If a fragment uses code or data from other fragments, the Code Fragment Manager (CFM) locates and loads the referenced fragments. The CFM then sets up the static data for each loaded fragment; this involves filling in pointers in the Table Of Contents, expanding initialized data values, and calling the fragment’s optional “initialization routine.” Once this is done, the CFM returns the fragment’s “main” address and a unique “connection ID.”

If you try to load an already loaded fragment, the code is not loaded twice, but a second copy of the data might be loaded. Fragments support three types of data sharing:

• Global data sharing uses only one data section no matter now many times the code is loaded.

• Per-context data sharing - the default setting - loads the code once, but allocates a different data section for each “context.” (A “context” is a name space associated with each application, so all of the fragments used by an application (and all of the fragments they use) fall into the same context.) This setting allows a fragment to treat each application which calls it as a separate entity.

• Per-load data sharing allocates a data section each time the fragment is loaded, even if multiple load requests come from the same context. You might use this feature to implement a “network communications” fragment where each time you load the fragment you get another connection to the network. Each connection would get its own copy of the static data.

The Fragment Loading API

Fragments are loaded through calls to the Code Fragment Manager, as documented in FragLoad.h. The FragLoad API consists of 7 calls for loading and unloading fragments, as well as looking up function and data pointers in a loaded fragment:

• GetDiskFragment is commonly used for loading “plug-in additions” - it takes an FSSpec for a file and loads the container contained in the data fork.

• GetMemFragment “prepares” a fragment that was loaded into memory without using the Code Fragment Manager.

• GetSharedLibrary locates an existing Shared Library on the disk and loads it. This call is normally used to get a connection ID for one of the standard shared libraries (such as the System software “InterfaceLib”) before looking up the address of an exported function.

• CloseConnection is used to unload a fragment. This function takes the Connection ID returned by one of the above calls, checks a reference count which is maintained by the CFM, and unloads the code and/or data sections for the given fragment.

• FindSymbol looks up a symbol (code or data pointer) by name given a string and a connection ID. If your application supports drop-in additions similar to HyperCard XCMDs, you could use this to look up the address of an optional function in an addition. As we will see later, this function plays a key role in linking 68K code to a PowerPC application.

• CountSymbols counts the total number of symbols exported from a fragment. Like FindSymbol, this call also requires a connection ID.

• GetIndSymbol can be used to index through all of the symbols exported from a fragment. It takes a connection ID and an index number, and returns the symbol’s name and address.

The API calls in action

We now know enough to start loading and executing code contained in a PEF container. The following code loads a fragment from the data fork of a file and prepares it for execution. (This code is part of “SimpleApp”, which is located on the MacTech source code disks or from the MacTech forums on line.)


/* 1 */
FSSpec  fileSpec;
ProcPtr mainAddr;
ConnectionIDconnID;
OSErr err;
Str255  errName;
err = GetDiskFragment ( &fileSpec, 0, 0, fileSpec.name,
 kLoadNewCopy, &mainAddr, (Ptr*)&gMain,
 errName);

That’s all there is to it - once this call is made, “mainAddr” contains a pointer into the fragment (usually the entry point for the code, though the fragment could put a pointer to some static data there if it wanted to export a table of procedure pointers, for example.) The returned connection ID can be used to look up other exports from the fragment, or to unload the fragment via a call to CloseConnection:

CloseConnection(&connID);

Loading a resource-based fragment is a little more difficult: it’s your responsibility to get the fragment into memory, then you have to ask the CFM to “prepare” the fragment for execution:


/* 2 */
FSSpec  fileSpec;
ProcPtr mainAddr;
ConnectionIDconnID;
short refNum;
Handle  codeResource;
OSErr err;
Str255  errName;

refNum = FSpOpenResFile(&fileSpec, fsCurPerm);
if (ResError() == noErr) {
  codeResource = Get1IndResource('PEF ', 1);
  if (codeResource != NULL) {
    DetachResource(codeResource );
    HLock(codeResource );
    // We have the code, but it's not ready to use yet
    // Ask the Code Fragment Manager to "prepare" the code for execution
    err = GetMemFragment (*codeResource , 0, fileSpec.name,
 kLoadNewCopy, &connID, (Ptr*)&mainAddr,
 errName);
  }
  CloseResFile(refNum);

In both of these cases, if the load fails, err will return an appropriate error code and errName will contain the name of the offending fragment.

Using global variables in fragments -
the Initialization routine

One of the more interesting aspects of fragments is how global variables are supported. Every fragment may have its own static data, including global variables, automatically, and this data can be initialized to set values by the Code Fragment Manager. Thus, if you wrote:

 static int x = 1234;
 static ProcPtr y = &z;

x would receive the value 1234 (stored in the container’s data segment), and y would receive the address of function “z”. (This is actually the address of a Transition Vector, as described in “How TOC switches occur” below, and is computed at load time.)

However, the CFM allows fragments to go beyond these simple initializations. Each fragment may designate optional “Initialization” and optional “Termination” routines which will be called when the fragment’s data section is being set up and torn down. The Initialization routine receives a pointer to a block of useful information, including a FSSpec for the fragment’s file, and can return an error value to stop the fragment from loading. (Otherwise, the Initialization routine must return noErr.) The termination routine takes no parameters and returns non values, and release memory, close files, and otherwise undo whatever the initialization routine did.

The initialization and termination routines could be used to implement fully self-initializing Macintosh managers, for instance, or C++ classes complete with their own constructors and destructors. In the sample application, the initialization routine is used to get the FSSpec for a drop-in addition file from within that file’s code. This allows the addition to access its own resources.


/* 3 */
OSErr OurInitRoutine (InitBlockPtr initBlkPtr)
{
 OSErr  err = noErr;
 short  refNum = -1;
 
 // Make sure this code is coming from the data fork of a file
 if (initBlkPtr->fragLocator.where == kOnDiskFlat) {
 refNum = FSpOpenResFile(
 initBlkPtr->fragLocator.u.onDisk.fileSpec,
 fsCurPerm);
 // and so on 

Next month in Powering Up

Next month’s column takes a look at the Power Macintosh calling conventions, and at how that affects debugging.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

iClock 5.5 - Customizable menu bar clock...
iClock replaces the old Apple's default menu bar clock with more features, customization and increases your productivity. Features: Have your Apple or Google calendar instantly available from the... Read more
Garmin Express 6.18.0.0 - Manage your Ga...
Garmin Express is your essential tool for managing your Garmin devices. Update maps, golf courses and device software. You can even register your device. Update maps Update software Register your... Read more
MarsEdit 4.3.5 - Quick and convenient bl...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
Xcode 11.0 - Integrated development envi...
Xcode includes everything developers need to create great applications for Mac, iPhone, iPad, and Apple Watch. Xcode provides developers a unified workflow for user interface design, coding, testing... Read more
DaisyDisk 4.8 - $9.99
DaisyDisk allows you to visualize your disk usage and free up disk space by quickly finding and deleting big unused files. The program scans your disk and displays its content as a sector diagram... Read more
VMware Fusion 11.5.0 - Run Windows apps...
VMware Fusion and Fusion Pro - virtualization software for running Windows, Linux, and other systems on a Mac without rebooting. The latest version includes full support for Windows 10, macOS Mojave... Read more
Apple Configurator 2.10 - Configure and...
Apple Configurator makes it easy to deploy iPad, iPhone, iPod touch, and Apple TV devices in your school or business. Use Apple Configurator to quickly configure large numbers of devices connected to... Read more
Spotify 1.1.15.448. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
MenuMeters 1.9.8 - CPU, memory, disk, an...
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking... Read more
Blocks 3.5.2 - RapidWeaver plug-in; divi...
Blocks is a plugin for RapidWeaver. It allows you to place blocks of text, graphics, and even raw HTML onto your page wherever you like. Drag blocks around, resize them, and even overlap content. All... Read more

Latest Forum Discussions

See All

Marvel Strike Force is adding Agent Coul...
Marvel Strike Force, the popular squad-based RPG, is set to receive a bunch of new content over the next few weeks. [Read more] | Read more »
Lots of premium games are going free (so...
You may have seen over the past couple weeks a that a bunch of premium games have suddenly become free. This isn’t a mistake, nor is it some last hurrah before Apple Arcade hits, and it’s important to know that these games aren’t actually becoming... | Read more »
Yoozoo Games launches Saint Seiya Awaken...
If you’re into your anime, you’ve probably seen or heard of Saint Seiya. Based on a shonen manga by Masami Kurumada, the series was massively popular in the 1980s – especially in its native Japan. Since then, it’s grown into a franchise of all... | Read more »
Five Nights at Freddy's AR: Special...
Five Nights at Freddy's AR: Special Delivery is a terrifying new nightmare from developer Illumix. Last week, FNAF fans were sent into a frenzy by a short teaser for what we now know to be Special Delivery. Those in the comments were quick to... | Read more »
Rush Rally 3's new live events are...
Last week, Rush Rally 3 got updated with live events, and it’s one of the best things to happen to racing games on mobile. Prior to this update, the game already had multiplayer, but live events are more convenient in the sense that it’s somewhat... | Read more »
Why your free-to-play racer sucks
It’s been this way for a while now, but playing Hot Wheels Infinite Loop really highlights a big issue with free-to-play mobile racing games: They suck. It doesn’t matter if you’re trying going for realism, cart racing, or arcade nonsense, they’re... | Read more »
Steam Link Spotlight - The Banner Saga 3
Steam Link Spotlight is a new feature where we take a look at PC games that play exceptionally well using the Steam Link app. Our last entry talked about Terry Cavanaugh’s incredible Dicey Dungeons. Read about how it’s a great mobile experience... | Read more »
Combo Quest (Games)
Combo Quest 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Combo Quest is an epic, time tap role-playing adventure. In this unique masterpiece, you are a knight on a heroic quest to retrieve... | Read more »
Hero Emblems (Games)
Hero Emblems 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: ** 25% OFF for a limited time to celebrate the release ** ** Note for iPhone 6 user: If it doesn't run fullscreen on your device... | Read more »
Puzzle Blitz (Games)
Puzzle Blitz 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Puzzle Blitz is a frantic puzzle solving race against the clock! Solve as many puzzles as you can, before time runs out! You have... | Read more »

Price Scanner via MacPrices.net

Get one of Apple’s new 2019 iPhone 11 models...
Boost Mobile is offering the new 2019 Apple iPhone 11, iPhone 11 Pro, and 11 Pro Max for $100 off MSRP. Their discount reduces the cost of an iPhone 11 to $599 for the 64GB models, $899 for the 64GB... Read more
13″ 1.4GHz Silver MacBook Pros on sale for $1...
B&H Photo has new 2019 13″ 1.4GHz 4-Core Touch Bar Silver MacBook Pros on sale for $100 off Apple’s MSRP. Overnight shipping is free to many addresses in the US. These are the same MacBook Pros... Read more
4-core and 6-core 2018 Mac minis available at...
Apple has Certified Refurbished 2018 Mac minis available on their online store for $120-$170 off the cost of new models. Each mini comes with a new outer case plus a standard Apple one-year warranty... Read more
$250 prepaid Visa card with any Apple iPhone,...
Xfinity Mobile will include a free $250 prepaid Visa card with the purchase of any new iPhone, new line activation, and transfer of phone number to Xfinity Mobile. Offer is valid through October 27,... Read more
Sprint is offering the 64GB Apple iPhone 11 P...
Sprint has the new 64GB iPhone 11 Pro available for $12.50 per month for new customers with an eligible trade-in in of iPhone 7 or newer. That’s down from their standard monthly lease of $41.67. The... Read more
Final week: Apple’s 2019 Back to School Promo...
Purchase a new Mac using Apple’s Education discount, and take up to $400 off MSRP. All teachers, students, and staff of any educational institution with a .edu email address qualify for the discount... Read more
Save $30 on Apple’s AirPods at these reseller...
Amazon is offering discounts on new 2019 Apple AirPods ranging up to $30 off MSRP as part of their Labor Day sale. Shipping is free: – AirPods with Charging Case: $144.95 $15 off MSRP – AirPods with... Read more
Preorder your Apple Watch Series 5 today at A...
Amazon has Apple Watch Series 5 GPS models available for preorder and on sale today for $15 off Apple’s MSRP. Shipping is free and starts on September 20th: – 40mm Apple Watch Series 5 GPS: $384.99 $... Read more
21″ iMacs on sale for $100 off Apple’s MSRP,...
B&H Photo has new 21″ Apple iMacs on sale for $100 off MSRP with models available starting at $999. These are the same iMacs offered by Apple in their retail and online stores. Overnight shipping... Read more
2018 4 and 6-Core Mac minis on sale today for...
Apple resellers are offering new 2018 4-Core and 6-Core Mac minis for $100-$150 off MSRP for a limited time. B&H Photo has the new 2018 4-Core and 6-Core Mac minis on sale for up to $150 off... Read more

Jobs Board

Geek Squad *Apple* Master Consultation Agen...
**734380BR** **Job Title:** Geek Squad Apple Master Consultation Agent **Job Category:** Services/Installation/Repair **Location Number:** 000134-Dublin-Store **Job Read more
*Apple* Mobile Master - Best Buy (United Sta...
**727032BR** **Job Title:** Apple Mobile Master **Job Category:** Store Associates **Location Number:** 000356-Manassas-Store **Job Description:** **What does a Best Read more
*Apple* Mobility Pro - Best Buy (United Stat...
**733802BR** **Job Title:** Apple Mobility Pro **Job Category:** Store Associates **Location Number:** 000122-Las Vegas II-Store **Job Description:** At Best Buy, Read more
*Apple* Mobility Pro - Best Buy (United Stat...
**733541BR** **Job Title:** Apple Mobility Pro **Job Category:** Store Associates **Location Number:** 001180-Northridge-Store **Job Description:** At Best Buy, our Read more
*Apple* Mobile Master - Best Buy (United Sta...
**732324BR** **Job Title:** Apple Mobile Master **Job Category:** Store Associates **Location Number:** 000013-Fargo-Store **Job Description:** **What does a Best Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.