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

Cyberduck 4.7.1 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
PDFpen 7.2 - Edit and annotate PDFs with...
We're also offering PDFpenPro for 30% off! PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even... Read more
Skype 7.9.746 - Voice-over-internet phon...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
Apple GarageBand 10.1 - Complete recordi...
The new GarageBand is a whole music creation studio right inside your Mac -- complete with keyboard, synths, orchestral and percussion instruments, presets for guitar and voice, an entirely... Read more
Duplicate Annihilator 5.7.7 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator... Read more
OS X Server 4.1.3 - For OS X 10.10 Yosem...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
Firefox 39.0 - Fast, safe Web browser. (...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
pwSafe 4.1 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Kodi 15.0.rc1 - Powerful media center to...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Coda 2.5.11 - One-window Web development...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more

This Week at 148Apps: June 29-July 3, 20...
Into July With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out above the... | Read more »
Sonic Runners Guide
Despite its flaws, Sonic Runners' platforming action is actually quite fun. Though it can be a little more complicated than old-school Sonic games. Here's how to make sure you're up to speed when jumping in. [Read more] | Read more »
Rage of Bahamut is Giving Almost All of...
The App Store isn't what it used to be back in 2012, so it's not unexpected to see some games changing their structures with the times. Now we can add Rage of Bahamut to that list with the recent announcement that the game is severely cutting back... | Read more »
Adventures of Pip (Games)
Adventures of Pip 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** ONE WEEK ONLY — 66% OFF! *** “Adventures of Pip is a delightful little platformer full of charm, challenge and impeccable... | Read more »
Divide By Sheep - Tips, Tricks, and Stre...
Who would have thought splitting up sheep could be so involved? Anyone who’s played Divide by Sheep, that’s who! While we’re not about to give you complete solutions to everything (because that’s just cheating), we will happily give you some... | Read more »
NaturalMotion and Zynga Have Started Tea...
An official sequel to 2012's CSR Racing is officially on the way, with Zynga and NaturalMotion releasing a short teaser trailer to get everyone excited. Well, as excited as one can get from a trailer with no gameplay footage, anyway. [Read more] | Read more »
Grab a Friend and Pick up Overkill 3, Be...
Overkill 3 is a pretty enjoyable third-person shooter that was sort of begging for some online multiplayer. Fortunately the begging can stop, because its newest update has added an online co-op mode. [Read more] | Read more »
Scanner Pro's Newest Update Adds Au...
Scanner Pro is one of the most popular document scanning apps on iOS, thanks in no small part to its near-constant updates, I'm sure. Now we're up to update number six, and it adds some pretty handy new features. [Read more] | Read more »
Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Wars of the Roses (Games)
Wars of the Roses 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple refurbished Mac minis available startin...
The Apple Store has Apple Certified Refurbished 2014 Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off... Read more
College Student Deals: Additional $100 off Ma...
Take an additional $100 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through August 8, 2015. Anyone with a valid .EDU email address can take... Read more
Sky Atlas Star Walk 2 App For iPhone And iPa...
Vito Technology, developer of educational apps for Android and iOS, is for a limited time offering free of charge their Star Walk 2, the next generation of the company’s best-selling stargazing guide... Read more
13-inch 1.6GHz MacBook Air on sale for $849,...
Amazon has the 2015 13″ 1.6GHz/128GB MacBook Air on sale for $849.99 including free shipping. Their price is $150 off MSRP, and it’s the lowest price available for this model. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
Adorama has the 13-inch 2.5GHz MacBook Pro on sale for $999 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has new 2015 15″ Retina MacBook Pros on sale for up to $225 off MSRP including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1819 $180 off - 15″ 2.5GHz Retina... Read more
RamDisk4Mac App Helps Run Your Mac Faster And...
Ever use a RAM disk? If you’ve come to the Mac in the OS X era, likely not. The Classic Mac OS had a RAM disk function built-in, but that was dropped in the conversion to OS X. What is a RAM disk?... Read more
13-inch 1.6GHz MacBook Air on sale for $849,...
Best Buy has the 2015 13″ 1.6GHz/128GB MacBook Air on sale for $849.99 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Apple Refurbished iMacs available for up to $...
The Apple Store has Apple Certified Refurbished iMacs available for up to $380 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 27″ 3.5GHz 5K iMac – $1949 $... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
Senior Payments Security Manager - *Apple*...
**Job Summary** Apple , Inc. is looking for a highly motivated, innovative and hands-on senior payments security manager to join the Apple Pay security team. You will Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Sr. Payment Program Manager, *Apple* Pay -...
**Job Summary** Apple Pay is an exciting environment and a…devices in a simple, private and secure way. The Apple Pay Team is looking for an experienced Senior Read more
Project Manager - *Apple* Pay Security - Ap...
**Job Summary** The Apple Pay Security team is seeking a highly organized, results-driven Project Manager to drive the development of Apple Pay Security. If you are Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.