TweetFollow Us on Twitter

June 91 - Children of the DogCow - Segments from Outer Space

Children of the DogCow - Segments from Outer Space

Kent Sandvik

Greetings! This is Worldwide Developer Conference week, and DTS is busy working at the debugging lab. Thanks to a great manager, I am able to stay home half time, check out my new kid, and at the same time write sample code and another article for FrameWorks.

In this column, I'm taking a closer look at segmentation: MacApp segmentation strategies, virtual memory possibilities, and other issues related to segmentation. Some parts will be elementary for Veterans of the MacApp Psychic Wars, but I hope the science fiction references in some of the headings will keep such readers awake. Onward!

THE BASICS OF SEGMENTATION

First, you need to specify the segment where each method (or member function, in C++) should reside. This is done using the notation {$S SegmentName} in Object Pascal, and #pragma segment SegmentName in C++. If you forget to place this segment compiler directive in your methods, it will inherit the earlier directive (in C++ as well as Object Pascal) all the way to the end of the file, so suddenly you'll find a lot of methods inside one of your segments. Methods without any defined segment go into the Main segment, which could get really crowded after a while. So, check your segmentation directive for each method. MacBrowse has a neat function for doing this that shows what each segment contains.

The Segmentation Makers sidebar provides guidelines on organizing your methods into segments.

THE TRUE NATURE OF SEGMENTS

Segments are really CODE resources in disguise, so MacApp is able to control the purge and lock bits on segments just as it does on handles or objects (as in TObject's Lock and UnLock methods).

A locked handle is also unpurgeable, so you don't need to worry about purging once you have locked the object in memory. MacApp's global function DBUnloadSeg makes handles, or CODE segments, unlocked-which makes the resource available for purging as well.

Methods are the actual routines that are stored in the CODE resources; data is stored either on the stack, in the heap or in the A5-world, depending. In many cases, calling a method whose segment is not currently stored in memory causes a segment load to occur that might have to move heap blocks in order to locate a place to put the new segment. This is one reason why calling a new method can suddenly make dereferencing bugs pop up.

THE FALL OF UNLOADALLSEGMENTS

One key to the mysteries of MacApp segment loading and unloading is the global function UnloadAllSegments. UnloadAllSegments is called when the application starts, and also when the application has too little memory to do its duties. UnloadAllSegments purges all code segments from memory except resident segments.

It's UnloadAllSegments that displays "I really don't think that you want to unload a segment into which you are going to return!" in the debug window. This happens when MacApp has determined that you are about to unload a segment containing a method that you will need to return to later. (In other words, your stack currently references a method contained in the segment that is about to be unloaded.)

If you do a "find references to UnloadAllSegments" in MacBrowse, you'll find that this function is called from many places; from the initialization phase of the application, and here and there from within various loops.

GOOD NEWS FROM DEBUGGER SPACE

The MacApp debugger can list all the segments that are currently loaded in application heap memory. When you drop down to the debugger level, type 'H' for the segment and heap subsection. Inside this level, to show the segments, type either 'S' (segments currently in memory) or 'ß' (all segments, whether or not they are currently in memory). This shows the segment names, sizes, states (loaded, resident), and so on.

You can do additional resource manipulation from the debugger with the Toggle, or 'X' flag. Inside this level, type 'S' so that each time a segment load occurs, MacApp will break into the debugger and print the routine name that triggered the segment load.

The 'U' flag turns off the automatic segment unloading done by the UnloadAllSegments routine. This is handy for finding out if your program's crashes have been due to mysterious jumps into unexpected routines: if a pointer to another method was suddenly being made invalid as that method's segment was unloaded, the stage is set for a healthy crash. Your code may stop crashing when you use the 'U' flag to turn automatic segment unloading off; if so, that's a good hint to look for problems of this kind.

The 'R' flag checks to see if the total size of the currently loaded resource exceeds the maximum. You can also set the maximum to a new value.

SERVANTS OF THE LIB

Sometimes we want to move a separate segment back into the Main segment, in order to avoid too many segments-a condition that can lead to heap fragmentation. Link can remap the segmentation with routines to other segment names. The syntax looks like this:
Link [options...] objectfile… ≥ progress.file
-sn=oldSegName1=newSegName1
-sn=oldSegName2=newSegName2

This is useful when using MPW 3.2 with MacApp 2.0 or 2.0.1 to remap segments back to the Main segment. The story is, some of the standard functions in libraries in MPW 3.2 have been split from the Main segment. This causes serious heap fragmentation in your MacApp application-for example, when you try to call SetHandleSize(). To avoid this, make the following modifications in the Basic Definitions file:

SegmentMappings = ð
SegmentMappings = ð
#-- insert here
-sn PASLIB=Main ð
-sn STDCLIB=Main ð
#-- end of insertion

This causes the errant routines in the Pascal and Standard C libraries to be remapped back into the Main segment. Also, change the lines in the MacApp.r file as shown in the MacApp.r changes sidebar.

Another solution is to use the linker to mark code resources from the libraries that were once in main as locked. These segments will then be loaded into memory and placed with the main segment, avoiding fragmentation problems. To do this, modify the user variable OtherLinkOptions in the Basic Definitions file:

OtherLinkOptions = ð
-ra PASLIB=resLocked ð
-ra STDCLIB=resLocked

You can also use this technique of locking code resources into memory in your MAMake files (OtherLinkOptions=)-but be careful with these experiments. Finally, you can use the linker to merge old segments into new segments with the -sg option:

-sg newSeg[=old[,old]…] # merge old segments into newSeg

The MPW Lib tool also contains options for changing segment names and merging segments into a segment, which is useful for cases where you only have access to the object code library.

THE RES! AND SEG! OF ETERNITY

Sometimes there is confusion about res! and seg! resources. The seg! resource defines those segments that are loaded to memory when the program is making maximum use of memory. MacApp uses this information when keeping track of the code reserve in order to ensure there is room for the seg! code segments at the maximum point of memory use.

The res! resource defines those segments that are always resident in the heap (segments are made permanently resident via a global function called SetResidentSegment). Note that even if you define a segment in the res! resource, because it's a handle it will still float around in memory.

One use for making segments permanently resident is for time-critical functions that are grouped together in a special segment; thus, loading the segment doesn't require overhead if the method is suddenly needed. For example, this could be used to reduce overhead for time-critical communication methods. Here's an example of a res! resource defined in the resource file:

resource 'res!' (kMyMacApp,    purgeable) {
{   "AWriteConn";
    "AReadConn";
    "APoll";
#if qInspector && !qDebug
    "GDebugConn";
#endif
#if qPerform
    "GPerformanceComms";
#endif
};
};

VIRTUAL MEMORY ARRIVES AT EASTERWINE

One reason other operating systems don't require programmers to specify segmentation is the underlying mechanism of virtual memory and page segmentation, which makes the issue of loading code on demand an automatic process.

With System 7, the Macintosh operating system now has virtual memory. However, there is still need for the programmer to specify segments in the code.

The basics of virtual memory

In a virtual memory system the whole memory is divided into pages. These pages can be loaded into memory or reside on the hard disk at any time. When the program refers to a page that isn't in physical memory, the system generates a page fault, and the memory management unit (MMU) loads the page from the backing store on the hard disk.

When the page fault occurs, the memory manager (with the help of the MMU) first frees up physical memory so it can load the needed page by selecting unused page frames and writing them to the backing store. Then it reads the page data for the needed frame. Thus, pages that aren't needed are usually residing on the hard disk. This event, usually called page-fault handling, requires special hardware in most cases.

Apple's VM scheme for the Mac

Apple's virtual memory only runs on Macintosh computers with a MMU unit. It requires the virtual memory mode to be on (it is not always on by default). The Macintosh implementation of virtual memory is not based on the so-called copy-on-demand, where page after page is loaded in per demand. In the Mac's VM scheme, the Segment Loader has to load in the whole segment. And the Mac's virtual memory model is single-virtual (not multiple-virtual), which means the applications share the address space with other applications and the system.

Global-replacement algorithm

Apple's implementation is a global-replacement algorithm. It keeps a queue of physical page frames in memory, and the frames in this queue are accessed sequentially by page- frame numbers. Each page frame in memory is automatically marked by the MMU according to whether the frame has been recently accessed by the CPU. Mac VM keeps a pointer to the last page frame that has been replaced. When a page-fault occurs, this pointer looks in the queue for a page frame that hasn't been modified since it was read from the disk, and that also has not been accessed recently (an "old" frame). When it find such a beast, it returns information about it to the memory manager. This page can now be "stolen" to use for other purposes.

If the VM does not find such a frame, it looks for a page that has been modified but not replaced recently. If this doesn't work, the VM tries to find the first frame that doesn't contain a page held in physical memory.

This algorithm is simple and fast. It doesn't need to know about application states, and it's space efficient. For more on this algorithm, read the article in the November 1989 issue of Byte, "Mac VM Revealed" by Phil Goldman.

Backing store management

In the Mac's VM implementation, the pages are locked to 4096 byte sizes (this is also the minimal optimal transfer size for the SCSI Manager). The complete virtual memory image is kept on the backing store, instead of pages. This means the end user uses more disk space, but that the page swapping speed is probably improved. If the whole virtual image is not on the disk, then every time a page is replaced, it must be written to the backing store, even if it has not been modified. The biggest burden with virtual memory is the transfer time to and from the hard disk; with the whole virtual disk image on the backing store, the system does not need to write pages that are not modified back to the disk.

VM & segmentation

Automatic segmentation handling is impractical for VM's current implementation. Note that even UNIX™ and other modern (UNIX modern? well…) operating systems still have problems with code that generates a lot of page faults (by loading in a lot of page frames for each function), which leads to unnecessary paging, which leads to swapping (where whole processes are swapped out of physical memory), which leads to thrashing (when all the operating system does is swap processes in and out, 24 hours a day). So, some of these systems have automatic tools that analyze the segmentation strategies and, if possible, move code segments around to avoid paging (which is evil after all).

The best solution is a combination of both virtual memory and segmentation. VM allows the user to run larger programs than would otherwise be possible, and if the developer organizes segments intelligently, excess paging is avoided.

There is still a need for some smart segmentation analysis tool which could produce segmentation directives by analyzing each function in order to figure out how to produce a segment organization such that methods are grouped together for maximum efficiency. VACUUM JUMP TABLES There is a known relation between jump table sizes and segmentation. For normal procedures and functions, a jump table entry is not needed if all calls to the routine are from the same segment. But if there are calls to other segments from the routine, jump table entries are needed. Examine the segmentation of your code; you might find places where a change in segmentation would eliminate jump table entries. The linkmap output (using the MABuild -LinkMap option) shows what each segment contains. With some effort you may shrink big jump tables and improve the performance of your whole application.

Some people worry that many Get and Set methods will increase the jump table entries considerably, but you can avoid this by using clever segmentation strategies or by using C++'s inline functions. Anyway, if your classes are infested with millions of Get and Set methods, perhaps it is time to examine the object. Is it really a structure in disguise?

Caching of results inside the class decreases the need for Get and Set calls. Plus, the major parts of an object can be placed inside one single segment for another performance improvement.You can use dumpobj to dump the object file and find information about each segment.

The Segment Loader has to fill the jump table with the right addresses when the segments are loaded in. When the segment is unloaded, the jump table has to be reset with information about the missing segment. MacApp has to make sure that memory is always available for data and unloaded segments. All this takes time, so clever segmentation does improve performance. For example, if important functions are in the same segment, you eliminate other segment loading events, and when MacApp calls UnloadAllSegments, a place is created for the next suite of segments needed.

STRANGE MPW 3.2 TOYS

For a long time segments were restricted to 32k sizes due to the A5-relative data referencing with 16 bit offsets, but MPW 3.2 eliminates this 32k limit on segment size via new switches to the compilers and the linker.

The 68020 introduced 32-bit PC-relative branching (BSR.L statements), but that didn't help the Classic and other 68000-based Macintosh computers. Instead, MPW 3.2 makes use of branch islands. This simple, elegant concept is based on the implementation of PC-relative code-to-code references. The linker splits a large code segment up into smaller 32k areas by inserting branch islands. These branch islands serve as intermediate points that are within range of PC-relative jumps, thus making it possible to make a call across a segment that would otherwise result in a larger-than-32k jump.

Another new feature is "32-bit everything," which transparently removes the infamous limitations on code segment sizes, jump table sizes and the size of the global data areas. The drawback is a larger code size footprint and some slowdown due to increased load time for the larger code segments. But hey, look what you get!

32-bit everything is activated by using -model far options while compiling and linking. The Release Notes for MPW 3.2 will explain the implementation completely; basically, the trick is that the compilers generate instructions with 32-bit addresses (instead of the normal 16-bit offsets), and that these 32-bit addresses are relocated at load time by the segment load address or by the contents of A5, as appropriate.

Finally, one can generate larger than 32K jump tables using the -wrap option. This uses unused space in the global data area for additional jump table entries when it starts to get crowded inside the 32K segment. Programmers doing large MacApp programs will love this! However, at best this utility doubles the jump table size, and if your global data area is already filled with data, you're out of luck.

If you want to use these new 32-bit everything features from MPW 3.2 with MacApp, you'll need a couple of new MacApp library files. These are available on ETO #3, as well as most of the 32-bit everything support. ETO#4 will contain the final MPW 3.2 with tools and libraries to support these new features.

THE END OF THIS ARTICLE

I hope I have helped to explain the basic issues concerning segmentation and MacApp. The brave MacApp explorer could investigate further to learn about the global functions that manipulate segments-such as GetSegFromPC, GetSegNumber, and GetResLoad. Perhaps this will be the topic of a future column.

REFERENCES

  • Mac VM Revealed, Byte November 1989
  • Inside Macintosh, II, Chapter 2 (Segment Loader)
  • MPW 3.2 Release Information (available real soon now)
  • MacApp & Object Oriented Programming (using C++) handouts, Dave Wilson
  • Many various MacApp.Tech$ links
 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 6.19.0.450 - Voice-over-internet p...
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
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
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
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Labor Day Weekend MacBook Pro sale; 15-inch m...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $125 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels Desktop... Read more
Labor Day Weekend iPad mini sale; $50 to $100...
Best Buy has the iPad mini with Retina Display (WiFi models) on sale for $50 off MSRP on their online store for Labor Day Weekend. Choose free shipping or free local store pick up. Price is for... Read more
13-inch 1.4GHz MacBook Air on sale for $899,...
Adorama has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY & NJ tax only. Their price is $100 off MSRP. Read more
It’s Official: Apple Issues Invitations To Se...
Apple has issued one of its characteristically cryptic press invitations for a special event to be held at the Flint Center for the Performing Arts in hometown Cupertino on Sept. 9, 2014 at 10:00 am... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
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
*Apple* Retail - Multiple Positions (US) - A...
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
*Apple* Retail - Multiple Positions (US) - A...
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
*Apple* Retail - Multiple Positions (US) - A...
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
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.