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December 92 - ANOTHER TAKE ON GLOBALS IN STANDALONE CODE

ANOTHER TAKE ON GLOBALS IN STANDALONE CODE

KEITH ROLLIN

[IMAGE Rollin_final_rev1.GIF]

While MPW is great for developing applications, it provides little support for creating standalone code resources such as XCMDs, drivers, and custom window, control, and menu definition procedures, especially if you have nonstandard needs. Two roadblocks developers immediately notice are the inability to create more than 32K of object code and the lack of access to global variables. This article addresses the latter issue.


The Macintosh Technical Note "Stand-Alone Code,ad nauseam " (formerly #256) does an admirable job of explaining what standalone code is and discussing the issues involved in accessing global variables from within it. I'll describe the solution proposed in that Tech Note later in this article, but you may also want to look over the Note before reading further.

It's important to realize that the Tech Note discusses just one possible solution to the problem of using global variables in standalone code. This article presents another solution, in the form of the StART package included on theDeveloper CD Series disc. Along the way, I'll talk a bit about what the issues are, describe how users of Symantec's THINK environments address the problem, recap the solution presented in the Tech Note, and show how to use MPW to implement a THINK-style solution. I'll also take a look at the advantages and disadvantages of each approach, allowing you to choose the right solution for your needs.

Note that the StART package is a solution for MPW users and that it assumes a lot about how MPW currently works. It's possible that you may not be able to use the StART package to develop standalone code that uses globals with future versions of MPW, although code already created with StART will, of course, continue to work.

WHAT IS STANDALONE CODE?

Standalone code is merely executable code that receives little to no runtime support from the Macintosh Operating System. The advantage of standalone code resources is that they can be quickly loaded into memory, executed, and dismissed without the overhead of setting up a full-fledged runtime environment for them. In addition, standalone code can execute without affecting the currently running application or relying on it for any services. This makes such resources ideal for easily extending the system's or your application's functionality. By creating the right kinds of standalone code resources, you can change how controls or windows appear, or you can dynamically extend the capabilities of your application.

Table 1 shows a list of the most common system- and application-defined standalone code resources.
Table 1Kinds of Standalone Code Resources

Resource TypeResource Function
ADBS*ADB device driver
adev*AppleTalk link access protocol
bootBoot blocks
CACHSystem RAM cache code
CDEF*Custom control definition
cdev*Control panel device
dcmd*Debugger extension
dcmpResource decompressor
DRVR*Device driver
FKEY*Function key
FMTR3.5-inch disk formatting
INIT*System extension
itl2Localized sorting routines
itl4Localized time/date routines
LDEF*Custom list display definition
MBDF*Custom menu bar definition
MDEF*Custom menu definition
mntr*Monitors control panel extension
PACKSystem package
PDEF*Printer driver
PTCHSystem patches
ptchSystem patches
rdev*Chooser device
ROvrROM resource override
RSSC*Resource editor for ResEdit
SERDSerial driver
snth*Sound Manager synthesizer
WDEF*Custom window definition
XCMD*HyperCard external command
XFCN*HyperCard external function

Note: Items marked with an asterisk are ones that you might create for your own application, extension, driver, or whatever. The rest are reserved for the system.

Standalone code differs from the executable code that makes up an application, which has a rich environment set up for it by the Segment Loader. Let's take a look at an application's runtime environment so that we can better understand the limitations we must overcome to implement standalone code.

An application runs in a section of memory referred to as its partition. Figure 1 shows the layout of an application partition. A partition consists of three major sections. At the top of the partition is the application'sA5 world , consisting of the application's global variables, the jump table used for intersegment function calls, and 32 bytes of application parameters (see "Application Parameters"). This area of memory is called the A5 world because the microprocessor's A5 register points into this data and is used for all access to it. Immediately below the A5 world is thestack , the area of memory used to contain local variables and return addresses. The stack grows downward toward theheap , which occupies the rest of the partition. The heap is used for all dynamic memory allocation, such as blocks created by NewHandle and NewPtr. Everything we see in Figure 1 -- the heap (with a valid zone header and trailer), the stack, and the filled-out global variables and initialized jump table -- is created by the Segment Loader when an application is launched.

[IMAGE Rollin_final_rev2.GIF]

Figure 1 An Application Partition



This is the application's domain, and none shall trespass against it. And therein lies the conflict between applications and standalone code: Executing code needs to use the A5 register to access its global variables, but an application's use of A5 prevents any standalone code from using it with impunity. Additionally, the A5 world is created by the Segment Loader when an application is launched. Since standalone code is not "launched" (instead, it's usually just loaded into memory and JSRed to), it doesn't get an A5 world, even if A5 were available. We must solve these two problems

-- the contention for A5 and the need to set up some sort of global variable space -- in order to use globals in standalone code.

APPLICATION PARAMETERS
Not much is known about the mysterious 32 bytes directly above A5 known as application parameters. Figures 9 and 10 on pages 19 and 21 of Inside Macintosh  Volume II indicate their existence, but the description simply says that "they're reserved for use by the system." We know that the first four bytes contain a pointer into the QuickDraw globals, but that's about it. Some MPW glue routines use some of the other bytes, but that use is undocumented. In any case, the application parameters seem pretty important. As you'll see later, we make sure our standalone code resources support them.

THE THINK SOLUTION

For years, users of THINK C and THINK Pascal have been able to use global variables in their CDEFs, LDEFs, drivers, and other types of standalone code. THINK has solved the problem of A5 contention by compiling standalone code to use the A4 register for accessing globals, leaving A5 untouched. Their solution to the need to set up global variable space is simply to attach the globals to the end of the standalone code, again leaving the application's A5 world untouched.

Figure 2 shows how standalone code created by a THINK compiler looks, both on disk and in memory. If the code was created with the C compiler, which allows preinitialized global variables, the global variable section contains the initial values. If the code was generated by the Pascal compiler, which sets all global variables to zero, the entire global section simply consists of a bunch of zeros (kind of like some guys I used to know in high school).

This is in contrast to the way globals are stored on disk for applications. MPW, for instance, uses a compressed data format to represent an application's globals on disk. When the application is launched, a small bit of initialization code is executed to read the globals from disk, expand them, and write them into the application global variable space in its A5 world.

Standalone code created by a THINK compiler accesses global variables by using A4-relative instructions. Because the use of the A4 register is ungoverned, such standalone code must manually set up A4 so that it can be used to reference its global variables. This setup is done by some macros provided by the THINK headers: RememberA0 and SetupA4. (It's called RememberA0, and not RememberA4, because the macro has to store the value in the A0 register temporarily.) When the standalone code is finished and is about to return to its caller, it must call RestoreA4 to restore the value that was in A4 before the standalone code was called.

[IMAGE Rollin_final_rev3.GIF]

Figure 2 Format of a Standalone Code Resource Created by a THINK Compiler


The solution provided by THINK offers many advantages:

  • It's simple to use. Making sure you surround the entry point of your standalone code with the appropriate macros is easy, and the macros don't require any tricky parameters. Just type them in and you're done.
  • The THINK development systems automatically insert a little bit of magic code at the beginning of standalone code resources that make the setting up of A4 as transparent as possible.
  • THINK's use of A4 means that A5 is totally undisturbed, and hence A5 continues to point to a valid A5 world with, presumably, an initialized set of QuickDraw globals. This means that standalone code can make Toolbox calls without a second thought (or even much of a first thought, for that matter).
  • Because the globals are attached to the standalone code, when the memory allocated to the standalone code resource is disposed of (for example, when the process that loaded it calls ReleaseResource on the segment), the globals are removed as well.

There are at least three disadvantages to THINK's approach, however:

  • Since A4 is now pulling duty as the global variable reference base, fewer registers are available for calculating expressions, caching pointers, and so on. This means that the code generated is less efficient than if A5 were used for referencing globals.
  • The globals are stored on disk in an uncompressed format, a fact you should be aware of before cavalierly declaring those empty 20K arrays.
  • The resources holding the standalone code must not be marked as purgeable, or the global variables will be set back to their original values when the resource is reloaded.

A fourth disadvantage could be that the combined size of the executable code and the global variables must be less than 32K. However, this is somewhat ameliorated by THINK's support of multisegmented standalone code.

THE TECH NOTE SOLUTION

Users of THINK development systems have their solution for accessing global variables in standalone code. MPW users, however, don't have an immediately obvious solution. First, MPW's compilers don't have the option of specifying that A4 should be used to access global variables. Second, the MPW linker is written to create a compressed block of data representing the global variables and to place that block of data off in its own segment. Because A4 can't be used to access globals, and because the globals aren't attached to the end of the standalone code resource, MPW users don't have the slick solution that THINK users do.

A possible alternative was presented to MPW users a couple of years ago with the publication of the Technical Note "Stand-Alone Code,ad nauseam ." Let's take a quick look at that approach, and then compare it with THINK's solution.

Let's start by examining the format of a simple application, shown in Figure 3. This is the format that MPW is designed to create, with any deviance from the standard formula being cumbersome to handle.

This application has three segments. CODE 0 contains the information used by the Segment Loader to create the jump table, the upper part of an application's A5 world. CODE 1 contains executable code, and usually contains the application's entry point. CODE 2 contains the compressed data used to initialize the global variable section of the application's A5 world, along with a little bit of executable code that does the actual decompressing. This decompression code is automatically called by some runtime setup routines linked in with the application. The purpose of the call to UnloadSeg(@_DataInit) in MPW programs is to unload the decompression code along with the compressed data that's no longer needed.

The solution proposed in the Tech Note is to use a linker option that combines segments 1 and 2. At the same time, the Note provides a couple of utility routines that create a buffer to hold the global variables and that decompress the variables into the buffer. Figure 4 shows what standalone code looks like when it's running in memory.

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Figure 3 Format of a Simple Application Created by MPW

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Figure 4 Format of Standalone Code Using the Tech Note Method

When the standalone code is called, it's responsible for creating and initializing its own A5 world. It does this by calling OpenA5World, which is directly analogous to THINK's SetupA4 macro. OpenA5World creates the buffer shown on the right in Figure 4, sets A5 to point to it, and calls the decompression routines to fill in the buffer. When the standalone code is ready to exit, it must call CloseA5World to deallocate the buffer and restore the original value of A5.

Note that this approach has an immediate disadvantage compared to the THINK approach. Because the global variables buffer is deallocated when the code exits back to the caller, all values that were calculated and stored in global variables are lost. This makes the OpenA5World/CloseA5World solution good if you simply want to use global variables in lieu of passing parameters, but lousy if you're trying to maintain any persistent data.

Fortunately, the Tech Note also presents a slight variation on the above solution that doesn't require that the global variables buffer be deallocated when the standalone code exits. However, the solution requires a little help from the host application. When the standalone code exits, it has two problems to consider. The first is that it must find some way to maintain a reference (usually a handle) to the buffer holding the global variables. After all, where can the standalone code store this reference itself? It can't store it in a global variable, because this reference will later be used to recover our global variables buffer. It can't store the reference in a local variable, because local variables are destroyed once the function that declares them exits.

The second problem that must be solved when creating a solution that doesn't require flushing the global variables is that of knowing when it actually is time to dispose of them. Globals accessed by THINK code resources are attached to the segments themselves, which means that they're disposed of at the same time as the code resource itself. What happens if the caller of a standalone code resource created using the OpenA5World technique decides that it no longer needs that resource? If it simply calls ReleaseResource on the resource, the global variables used by the standalone code will be stranded in the heap. This is known as a memory leak, and it is very bad. The block of memory holding the global variables is no longer referenced by any code, and there's no way to recover a reference to them. That block of memory will never be disposed of and will waste memory in the heap.

The approach that the Tech Note takes to solving both of these problems is to require the help of the caller (usually the host application). First, the caller must agree to maintain the reference to the standalone code's global variables buffer. After the buffer is created, the reference to it is passed back to the caller. The next time the standalone code is called, and all subsequent times, the caller passes that reference back to the standalone code, which then uses that reference to recover its globalvariables and reset A5 the way it likes it. Additionally, the caller must agree to notify the standalone code when it's about to go away. When the standalone code receives that notification, it takes the opportunity to dispose of the global variables buffer.

Our brief recap of the Tech Note outlines a workable approach that provides a few advantages over the solution provided by THINK:

  • The on-disk representation of the standalone code is usually smaller, because the combination of the compressed data and decompression routines of MPW is often smaller than the raw data generated by THINK.
  • Because the executable code and global variables are allocated in their own buffers, each of which can be 32K in length, you can create larger code resources and define more global variables. (This does not take into account the partial advantages provided by THINK's multisegmented standalone code.)
  • Because MPW doesn't use it to access the globals, the A4 register can be used to generate more efficient object code.
  • Since the globals are stored separately from the standalone code, the resource holding the standalone code can be marked as purgeable.
  • The two blocks of memory holding standalone code and global variables can be locked or unlocked separately from each other, providing greater memory management flexibility.

There are, however, some disadvantages to the OpenA5World approach. The major disadvantage concerns the persistence of the global variables buffer. Either this buffer must be deallocated every time the code resource is exited, or the help of the caller must be elicited to maintain the reference to the buffer and to tell the standalone code when the buffer must be deallocated. If you're not in a position to define the responsibilities of the caller (for instance, if you're writing a WDEF), this disadvantage could be quite serious.

The second disadvantage concerns the reuse of the A5 register. Once the standalone code changes A5 from pointing to the caller's A5 world to pointing to the standalone code's globals, A5 no longer points to a valid set of QuickDraw globals. This can easily be solved by calling InitGraf early in the standalone code, but some problems may still exist. For instance, what if the standalone code needed to draw something in the current port (as an LDEF would need to do)? The GrafPtr of the port to be used is back in the caller's A5 world. Once we switch over to the standalone code's A5 world, we no longer know what port to draw into. This problem is briefly alluded to in the Tech Note, but it's not directly addressed.

THE START SOLUTION

It's possible to combine the advantages of the two approaches we've seen so far, while at the same time eliminating some of the disadvantages. The idea behind the hybrid approach I'll now present is to con MPW into creating a standalone code resource that has the same layout as one created by THINK. Specifically, instead of being stored in a separate buffer, the globals will be tacked onto the end of the code resource. This eliminates much of the reliance the standalone code has on the caller, and, as you'll see later, still allows us to create 32K worth of object code and 32K of global data.

As we saw when discussing the Tech Note approach, we need to get MPW to take the stuff it normally puts in an application and convert it to a standalone code resource. The OpenA5World solution used a linker option to accomplish this. My solution uses a custom MPW tool instead.

Let's begin by taking a look at what we'll end up with, and then determine what it will take to get there. First, the standalone code will access its global variables by using the A5 register; there's no way around that. Even if we were to pass the object code through a postcompilation tool that converted all A5 references into A4 references, there's no way we could take care of the cases where the compiler generates code that uses A4 for other purposes. Therefore, this solution still uses A5 for accessing globals. Second, the globals will be tacked onto the end of the standalone code resource, just as they are with THINK's solution. This means that the globals will be in a known and easily determined location at all times, relieving us from having to rely on the caller to maintain our globals. When doing this, we inherit the problem THINK code has with not being purgeable, but that's a small price to pay for the ease of use we get in return.

Third, the globals will be in expanded format. The approach taken in the Tech Note requires that our standalone code carry around the baggage of the decompression routines, as well as the compressed data, long after they're no longer needed. Using pre-expanded data means a larger on- disk footprint, but again, this is a small price to pay, especially if the in-memory footprint is more of an issue (and it usually is).

Finally, we'll need routines that calculate and set our A5 value when we enter our standalone code, and that restore A5 when we leave. These routines are analogous to the macros THINK uses and to the OpenA5World and CloseA5World routines of the Tech Note solution. Figure 5 shows how our standalone code resource will end up looking, both on disk and in memory.

My system is called StART, for StandAlone RunTime. It consists of two parts: an MPW tool called MakeStandAlone that converts a simple program like the one shown in Figure 3 into a standalone code resource, and a small library file with accompanying header files for Pascal and C.

[IMAGE Rollin_final_rev6.GIF]

Figure 5 Format of Standalone Code Using StART Techniques

To show how these pieces work together, let's take a small sample that uses a global variable, and build it using the StART tools. The sample we'll use is the Persist.p program included in the Tech Note. Following is a version of the file, modified to make calls to the StART library.

 UNIT Persist;
{ This is a standalone module that maintains a running total of the }
{ squares of the parameters it receives.                            }

INTERFACE
    USES Types, StART;
    FUNCTION Main(parm: LONGINT): LONGINT;
IMPLEMENTATION
    { Define global storage to retain a running total over multiple }
    { calls to the module.  }
    VAR
        accumulation: LONGINT;
    FUNCTION Main(parm: LONGINT): LONGINT;
        VAR 
            saved:      SaveA5Rec;
    BEGIN
        UseGlobals(saved);
        accumulation := accumulation + (parm * parm);
        Main := accumulation;
        DoneWithGlobals(saved);
    END;
END.

This very simple sample performs the useless function of taking the number you pass it, squaring it, adding the result to a running total, and returning that total. UseGlobals is the StART routine that enables us to access our global variables (in this case, the lone variable named accumulation), returning the value of the caller's A5. After we've performed our mathematical wizardry, we close up shop by calling a second StART routine, DoneWithGlobals, to restore the previous A5 value.

Following is the makefile for Persist.p.

Persist     ƒƒ Persist.p.o Persist.make StARTGlue.a.o
    Link StARTGlue.a.o ð
        Persist.p.o ð
        "{Libraries}Runtime.o" ð
        "{PLibraries}PasLib.o" ð
        -sn PASLIB=Main ð
        -o Persist
    MakeStandAlone Persist -restype CUST -resnum 129 -o Persist.rsrc

Persist.p.o ƒ Persist.p Persist.make
    Pascal Persist.p

This makefile contains a couple of interesting things that are worth examining. The first point to note is that we link with a file called StARTGlue.a.o. This file contains a few useful routines, including UseGlobals and DoneWithGlobals. It also contains a special header routine that performs some crucial setup. This setup needs to be performed before any of our custom code can be executed, so StARTGlue.a.o should be the first file in the link list.

The second interesting thing about the makefile is the statement -sn PASLIB=Main. Recall that MakeStandAlone requires a file that contains the resources shown in Figure 3 in order to perform its magic. Specifically, MakeStandAlone demands that there be only three segments with a single entry point each into CODE 1 and CODE 2. However, when we link with PasLib.o, we create a fourth segment called PASLIB. We therefore get rid of this segment by merging it with the rest of our executable code in CODE 1, the Main segment.

After linking and running the resulting file through the MakeStandAlone tool, we're left with a resource containing standalone code that sets up and uses its own set of global variables. Following are highlights from the Persist sample shown above. Some routines have been removed, since we'll be examining them in depth later.

Entry
+0000   00000   BRA.S       Entry+$0014
+0002   00002   DC.B        $0000           ; flags
+0004   00004   DC.B        $43555354       ; resource type (CUST)
+0008   00008   DC.B        $0081           ; resource ID (129)
+000A   0000A   DC.B        $0000           ; version
+000C   0000C   DC.B        $00000000       ; refCon
+0010   00010   DC.B        $00000000      ; cached offset to globals
+0014   00014   BRA     MAIN

[ UseGlobals, DoneWithGlobals, GetSAA5, and CalculateOffset removed ]

MAIN                                        ; from Persist.p
+0000   000076  LINK    A6,#$FFF8
+0004   00007A  PEA     -$0008(A6)          ; UseGlobals(save);
+0008   00007E  JSR     UseGlobals
+000C   000082  MOVE.L  $0008(A6),-(A7)     ; parm * parm
+0010   000086  MOVE.L  $0008(A6),-(A7)
+0014   00008A  JSR     %I_MUL4
+0018   00008E  MOVE.L  (A7)+,D0
+001A   000090  ADD.L   D0,-$0004(A5)       ; add to accumulation
+001E   000094  MOVE.L  -$0004(A5),$000C(A6)
                                          ; return as function result
+0024   00009A  PEA     -$0008(A6)          ; DoneWithGlobals(save);
+0028   00009E  JSR     DoneWithGlobals
+002C   0000A2  UNLK    A6
+002E   0000A4  MOVE.L  (A7)+,(A7)
+0030   0000A6  RTS

[ %I_MUL4 removed ]

Globals
+0000   000E4   DC.W        $0000, $0000   ; global var accumulation
+0004   000E8   DC.W        $0000, $0000   ; 32 bytes of app parms
+0008   000EC   DC.W        $0000, $0000
+000C   000F0   DC.W        $0000, $0000
+0010   000F4   DC.W        $0000, $0000
+0014   000F8   DC.W        $0000, $0000
+0018   000FC   DC.W        $0000, $0000
+001C   00100   DC.W        $0000, $0000
+0020   00104   DC.W        $0000, $0000

Entry, UseGlobals, DoneWithGlobals, GetSAA5, and CalculateOffset are all routines linked in from the StARTGlue.a.o file; MAIN is from the Persist.p source file; and %I_MUL4 is a library routine from PasLib.o. Following these routines are 36 bytes of data. The first 4 bytes are for our global variable, accumulation. The final 32 bytes are the application parameters above A5 that the system occasionally uses.

Let's take a look at the MAIN function, which shows us accessing our global variable. First, we call UseGlobals to determine what A5 should be and to set A5 to that value. In this case, UseGlobals will set A5 to point to Globals+$0004, placing our single 4-byte global below A5, and the 32 bytes of system data above A5. Next, we push the value we want to square onto the stack twice and call %I_MUL4 to multiply the two 4-byte values.

Finally, we get to the fun part, where we add the result of %I_MUL4 to our global variable. This is done by the instruction at MAIN+$001A: ADD.L D0,-$0004(A5). This instruction says to take the value in register D0 and add it to the number stored four bytes below A5. Because A5 points to Globals+$0004, this instruction adds D0 to the value starting at Globals.

THE MAKESTANDALONE TOOL
The code above was created by the MakeStandAlone tool. Let's look now at the workhorse function of that tool, ConvertAppToStandAloneCode. It's this function that takes an application conforming to the format shown in Figure 3 and converts it to the standalone resource shown in Figure 5.

ConvertAppToStandAloneCode starts by declaring a ton of variables, all of which are actually used. It then opens the file containing the segments shown in Figure 3 by calling OpenResFile on gInputFile, a string variable set up before calling this routine. If we can't open the file, we blow out by calling ErrorExit, a routine that prints the string passed to it and then aborts back to the MPW Shell.

PROCEDURE ConvertAppToStandAloneCode;

    VAR
        refNum:             INTEGER;
        code0:              Code0Handle;
        code1:              CodeHandle;
        code2:              CodeHandle;
        sizeOfGlobals:      LONGINT;
        expandedGlobals:    Handle;
        myA5:               LONGINT;
        codeSize:           LONGINT;
        address:            CStrPtr;
        err:                OSErr;
        fndrInfo:           FInfo;
        existingResource:   Handle;

    BEGIN
        refNum := OpenResFile(gInputFile);
        IF (refNum = - 1) | (ResError = resFNotFound) THEN
            ErrorExit('Error trying to open the source file.',
                ResError);

Loading the segments. ConvertAppToStandAloneCode then scopes out the contents of the file it has just opened.

The first thing it looks at is CODE 0, which contains the application's jump table. If CODE 0 exists and we can load it, we mark it nonpurgeable and call a utility routine, ValidateCode0, to make sure that CODE 0 contains what we expect. Here's what the code looks like:

code0 := Code0Handle(Get1Resource('CODE', 0));
IF (code0 = NIL) | (ResError <> noErr) THEN
    ErrorExit('Couldn't load CODE 0 resource.', ResError);
HNoPurge(Handle(code0));
ValidateCode0(code0);

MakeStandAlone requires that the input file conform strictly to the format shown in Figure 3. Among other things, this means that there should be only two entries in the jump table, one for CODE 1 and one for CODE 2. ValidateCode0 checks for this condition and makes a few other sanity checks to make sure that CODE 0 doesn't contain any other information that we'd otherwise have to deal with. If there are any problems, ValidateCode0 calls ErrorExit with an appropriate message. Thus, if ValidateCode0 returns, everything appears to be OK with CODE 0.

At times it might be tricky or impossible to create a CODE 1 resource with only one entry point. In some cases, you can bludgeon your code into a single segment by passing-snto the Link tool, as was done earlier. Unfortunately, this won't always work. For instance, some MPW routines are compiled to require jump table entries. (Examples of such routines are sprintf and its subroutines.) If you try to use any of these routines, you'll get more than one entry point in CODE 1. The only way to avoid this problem is to keep away from library routines that require jump table entries. If you're in doubt, simply attempt to use the routine in question; the compiler, the linker, or MakeStandAlone will tell you if anything is wrong.

ConvertAppToStandAloneCode next checks the remaining resources, CODE 1 and CODE 2. CODE 1 contains the executable code that will make up the bulk of the standalone code resource, and CODE 2 contains the compressed data holding the global variables' initial values, as well as the routines that decompress that data. Each segment is loaded and passed to ValidateCode to make sure that the resource looks OK.

code1 := CodeHandle(Get1Resource('CODE', 1));
IF (code1 = NIL) | (ResError <> noErr) THEN
    ErrorExit('Couldn’t load CODE 1 resource.', ResError);
HNoPurge(Handle(code1));
ValidateCode(code1, 1, 0);

code2 := CodeHandle(Get1Resource('CODE', 2));
IF (code2 = NIL) | (ResError <> noErr) THEN
    ErrorExit('Couldn’t load CODE 2 resource.', ResError);
HNoPurge(Handle(code2));
ValidateCode(code2, 2, 8);

ValidateCode takes a handle to the segment, along with a couple of values used in the sanity check. The first number is actually the resource ID of the segment and is used when reporting any errors. The second value is the jump table offset of the entry point for this segment and is checked against the segment header (seeInside Macintosh Volume II, page 61, for a description of this header). Again, if any problems are discovered or any unexpected values encountered (such as more than one entry point per segment), ValidateCode aborts by calling ErrorExit.

Converting to a standalone resource. Once the three segments have been loaded into memory and validated, we're ready to convert these resources into a single standalone resource. We begin by decompressing the data that represents the preinitialized values for our global data. The first part of accomplishing this is getting a temporary buffer to hold the expanded values. We find the size of this buffer by looking at the belowA5 field in CODE 0. We then create a buffer this size by calling NewHandle.

sizeOfGlobals := code0^^.belowA5;
expandedGlobals := NewHandle(sizeOfGlobals);
IF expandedGlobals = NIL THEN
    ErrorExit('Couldn't allocate memory to expand A5 data.',
        MemError);

We next perform the magic that expands the global variables into the buffer. CODE 2 contains the decompression routines, so all we do is call them. The function that performs this decompression is called _DATAINIT, which our validation routines have already confirmed is the entry point to CODE 2. _DATAINIT needs to have A5 already pointing to the top of the globals area, which in our case is the end of the handle we just created. After calling SetA5 to do this, we use CallProcPtr, a little inline assembly routine, to call _DATAINIT in CODE 2. _DATAINIT fills in our handle with the initial values for our global variables and then kindly returns to us. We quickly restore the previous value of A5 so that we can access our own global variables again, and then prepare to finish with the input file. We'll need CODE 1 later, so we detach it from the input file, and then close the input file.

myA5 := SetA5(ord4(expandedGlobals^) + sizeOfGlobals);
CallProcPtr(ProcPtr(ord4(code2^) + SizeOf(CodeRecord)));
myA5 := SetA5(myA5);
DetachResource(Handle(code1));
CloseResFile(refNum);

At this point, we're done with the input file, and we have in our possession two handles. The code1 handle contains the executable code for the standalone resource, and the expandedGlobals handle contains the global data. Our task at this point is to combine these two pieces of data.

We start by getting the size of the actual object code in CODE 1. This is the size of the entire handle, less the size of the CODE resource header. The handle is then grown large enough to hold the object code, the global data, and the 32 bytes of application parameters. If we can't grow the handle, we exit. Game over.

codeSize := GetHandleSize(Handle(code1)) - SizeOf(CodeRecord);
SetHandleSize(Handle(code1),
    codeSize + sizeOfGlobals + kAppParmsSize);
IF MemError <> noErr THEN
    ErrorExit('Couldn't expand CODE 1 handle.', MemError);

Once the handle containing the code is large enough, we call BlockMove twice to put everything in place. The first call to BlockMove moves the object code down in the handle, effectively removing the segment header. This header is useful only for segments and jump table patching; we don't need it for our standalone resource. The second call to BlockMove copies the global data stored inexpandedGlobals to the end of the handle holding the object code. We finish up by calling FillChar, a built-in Pascal routine, to clear out the 32 bytes of application parameters.

BlockMove(Ptr(ord4(code1^) + SizeOf(CodeRecord)), Ptr(code1^),
    codeSize);
BlockMove(expandedGlobals^, Ptr(ord4(code1^) + codeSize),
    sizeOfGlobals);
address := CStrPtr(ord4(code1^) + codeSize + sizeOfGlobals);
FillChar(address^, 32, CHAR(0));

Filling out the header. Our standalone code resource is now almost complete. All that remains is to fill out the fields of the standard header that seems to begin most standalone code resources.

The header consists of a word for a set of flags, the type and ID of the resource, and a word for a version number. These fields were written to our original CODE 1 when we linked with StARTGlue.a.o, but they were uninitialized. We take the opportunity here to fill in these fields.

As an additional goodie, our standard header contains a 4-byte refCon that can be used for anything the standalone code wants (for example, holding some data that the calling application can access).

Once the global data has been appended to the object code handle, we no longer need the expandedGlobals handle, so we dispose of it and prepare to write out ourobjet d'art.

WITH StdHeaderHandle(code1)^^ DO BEGIN
    flags := gHdrFlags;
    itsType := gResType;
    itsID := gResID;
    version := gHdrVersion;
    refCon := 0;
END;

DisposeHandle(expandedGlobals);

Writing the standalone resource. The first step to writing out our standalone code resource is to open the file that will hold it. We do this by calling OpenResFile. If OpenResFile reports failure, it's probably because the file doesn't exist. Therefore, we try to create the file by calling CreateResFile. If that succeeds, we set the Finder information of the output file so that we can easily open it with ResEdit, and then attempt to open the file again. If that second attempt fails, we give up by calling ErrorExit.

refNum := OpenResFile(gOutputFile);
IF (refNum = - 1) | (ResError = resFNotFound) THEN BEGIN
    CreateResFile(gOutputFile);
    IF (ResError <> noErr) THEN
        ErrorExit('Error trying to create the output file.',
            ResError);

    err := GetFInfo(gOutputFile, 0, fndrInfo);
    IF err <> noErr THEN
        ErrorExit('Error getting finder information.', err);

    fndrInfo.fdType := 'rsrc';
    fndrInfo.fdCreator := 'RSED';
    err := SetFInfo(gOutputFile, 0, fndrInfo);
    IF err <> noErr THEN
        ErrorExit('Error setting finder information.', err);

    refNum := OpenResFile(gOutputFile);
    IF (refNum = - 1) | (ResError = resFNotFound) THEN
        ErrorExit('Error trying to open the output file.', ResError);
END
If our first call to OpenResFile succeeded (skipping to the ELSE clause shown below), the file already exists and may need to be cleaned up a little. If the output file already contains a resource with the same type and ID of the resource we want to write, we need to get rid of it. Calls to RmveResource and DisposeHandle accomplish that grisly task.

ELSE BEGIN
    SetResLoad(FALSE);
    existingResource := Get1Resource(gResType, gResID);
    SetResLoad(TRUE);

    IF existingResource <> NIL THEN BEGIN
        RmveResource(existingResource);
        DisposeHandle(existingResource);
    END;
END;

At this point, we have a handle that needs to be added to a file as a resource, and an open file waiting for it. Three quick calls to the AddResource, WriteResource, and SetResAttrs routines take care of the rest of our duties, and the standalone code resource is written to the designated file. We then close the file and leave ConvertAppToStandAloneCode with the knowledge of a job well done.

AddResource(Handle(code1), gResType, gResID, gResName);
IF ResError <> noErr THEN
    ErrorExit('Error adding the standalone resource.', ResError);

WriteResource(Handle(code1));
IF ResError <> noErr THEN
    ErrorExit('Error writing the standalone resource.', ResError);

SetResAttrs(Handle(code1), gResFlags);
IF ResError <> noErr THEN
    ErrorExit('Error setting the resource attributes.', ResError);

CloseResFile(refNum);
END;

UP CLOSE AND PERSONAL WITH STARTGLUE.A.O
Converting our application into a standalone code resource is only part of the process. The other part involves the routines that allow our code to execute on its own. These routines preserve the A5 world of the host application, set up the standalone code's A5 world, and restore the host application's A5 world when the standalone code is finished.

These routines are provided by StARTGlue.a.o. StARTGlue.a.o includes four client (external) routines (UseGlobals, CopyHostQD, DoneWithGlobals, and GetSAA5), an internal routine (CalculateOffset), and a block of public and private data. Because of this embedded block of data, the library is written in assembly language. Let's take a look at the source file, StARTGlue.a.

                 CASE        OFF

                 INCLUDE     'Traps.a'
                 INCLUDE     'QuickEqu.a'
                 INCLUDE     'SysEqu.a'
FirstByte        MAIN
                 IMPORT      Main, _DATAINIT
                 ENTRY       gGlobalsOffset
                 bra.s       Island

                 dc.w        0                   ; flags
                 dc.l        0                   ; resType
                 dc.w        0                   ; ID
                 dc.w        0                   ; version
                 dc.l        0                   ; refCon 

gGlobalsOffset   dc.l        0                   ; offset to globals

By convention, standalone code resources start with a standard header having the format shown in Table 2.


Table 2Standard Header for Standalone Code Resources

FieldSizeContents
entry2 bytesBranch instruction to first byte of executable code. flags 2 bytes User-defined flags. You can set and define this field any way you want.
resType4 bytesResource type.
resID2 bytesResource ID.
version 2bytesVersion number. The values for this field are unregulated, but usually follow the same format as the version numbers in 'vers' resources.
refCon4 bytesUser-defined reference constant. Use this field for anything you want, including communicating with the host.

Nothing requires standalone code to include this header. However, it's nice to follow convention, and including the resource type and ID makes identifying blocks in the heap easier.

When you compile and link with StARTGlue.a.o, these fields are empty (set to zero). However, the MakeStandAlone tool automatically fills in these fields based on command-line options when it converts your code.

StARTGlue.a.o's entry point branches to the following code, which then branches to a function called Main. The reason for this double jump is to maintain the standard header for a standalone code resource. The first two bytes are used to jump to the code's entry point. However, we can jump only 128 bytes with the 68000's 2-byte relative branch instruction. If Main happens to be further than 128 bytes from the start of the code resource, we would need to use the 4-byte branch instruction. To provide for this contingency, we have our 2-byte branch instruction jump to the 4-byte branch instruction, which can then jump to anywhere that it wants with impunity.

Island
        bra     Main
        lea     _DATAINIT,A0    ; dummy line to reference   _DATAINIT

The LEA instruction that follows the branch is a dummy statement. Its sole purpose is to trick the linker into including _DATAINIT, the routine that the MakeStandAlone tool calls to decompress the global data. Because the LEA instruction immediately follows an unconditional branch, and because it doesn't have a label that can be jumped to, it's never actually executed.

UseGlobals. The UseGlobals function is used to set up the standalone code's A5 world. An example of this is shown earlier in the Persist program.

UseGlobals performs three functions:

  • It sets the A5 register and the low-memory location CurrentA5 to the correct value for the standalone code. It determines the standalone code's A5 value by calling the GetSAA5 function, described later.
  • It copies the host application's QuickDraw globals pointer to the standalone code's QuickDraw globals pointer (this pointer is the 4-byte value to which A5 normally points). By copying this pointer, the standalone code can call Toolbox routines knowing that A5 references a valid set of QuickDraw globals.
  • It returns the host application's A5 and CurrentA5 values so that they can later be restored.

;
; PROCEDURE UseGlobals(VAR save: SavedA5Rec);
; { Balance with DoneWithGlobals. }
;
UseGlobals   PROC        EXPORT
             IMPORT      GetSAA5

             move.l      4(sp),A0         ; get ptr to save record
             move.l      A5,(A0)          ; save A5
             move.l      CurrentA5,4(A0)  ; save low-memory value
             clr.l       -(sp)            ; make room for function 
                                          ;   result
             bsr.s       GetSAA5          ; get our own A5
             move.l      (sp)+,A5         ; make it real
             move.l      A5,CurrentA5     ; make it really real
             move.l      4(sp),A0         ; get ptr to save record
             move.l      (A0),A0          ; get host’s A5
             move.l      (A0),(A5)        ; copy his QD globals ptr
             move.l      (sp)+,(sp)       ; remove parameters
             rts                          ; return to caller

CopyHostQD. The CopyHostQD routine is an optional utility routine. You don't need to call it unless you have to ensure that the host's QuickDraw globals remain undisturbed. By default, your standalone code shares the same set of QuickDraw globals as the host application. However, if you have unusual requirements, you may need to establish your own set of QuickDraw globals.

A simple way to set up your own QuickDraw globals would be to call InitGraf(@thePort) after you called UseGlobals. This would create a valid set of QuickDraw globals. However, some standalone code resources initially need to work with information provided by the host application. For instance, a custom MDEF normally draws in the currently set port. To inherit such information, you can call CopyHostQD just after you call UseGlobals.

; PROCEDURE CopyHostQD(thePort: Ptr; oldA5: Ptr);
;       { Balance with DoneWithGlobals. }
;   assumes that A5 has already been set up to our globals
;
CopyHostQD      PROC            EXPORT

returnAddress   EQU         0
oldA5                   EQU         returnAddress+4
thePortPtr      EQU     oldA5+4
parameterSize   EQU     thePortPtr-oldA5+4

                move.l  oldA5(sp),A0        ; get oldA5
                move.l  (A0),(A5)           ; make (A5) point to 
                                                ;   thePort

                move.l  (A0),A0             ; get host’s thePort 
                                                ;   pointer
                move.l  thePortPtr(sp),A1   ; get our thePort pointer
                move.l  #grafSize,D0        ; copy whole grafPort
                move.l  D0,D1               ; since the pointers
                subq.l  #4,D1               ;   point near the end of
                sub.l   D1,A0               ;   the QD globals, move 
                sub.l   D1,A2               ;   them down to point 
                                                ;   to the beginning
                _BlockMove

                move.l  (sp)+,A0            ; pop return address
                add     #parameterSize,sp   ; pop parameters
                jmp     (A0)                ; return to caller

DoneWithGlobals. The DoneWithGlobals routine reverses the effects of UseGlobals. It simply restores the values of the A5 register and low-memory global CurrentA5 to the values saved by UseGlobals.

;
; PROCEDURE DoneWithGlobals(restore: SaveA5Rec);
;
DoneWithGlobals PROC            EXPORT

                move.l  (sp)+,A0        ; pull off return address
                move.l  (sp)+,A1        ; address of record 
                                        ;   holding info
                move.l  (A1),A5         ; first restore A5
                move.l  4(A1),CurrentA5 ; then restore low-memory
                                        ;   value
                jmp     (A0)            ; return to caller

GetSAA5. You probably won't need to call GetSAA5. This function is called by UseGlobals to return the value that's used to refer to the standalone code's A5 world. The first time this function is called, this value needs to be calculated. After that, the offset from the beginning of the code to the global data is cached and is used in subsequent calls to GetSAA5. Once the offset has been determined, it's added to the address of the start of the standalone code and returned to the caller.

;; FUNCTION GetSAA5: LONGINT;
;
GetSAA5 PROC        EXPORT
        IMPORT      CalculateOffset

        move.l      gGlobalsOffset,D0   ; have we done this 
                                        ;   before?
        bne.s       @1                  ; yes, so use cached 
                                        ;   value
        bsr.s       CalculateOffset     ; nope, so calculate it
@1
        lea         FirstByte,A0        ; get base address
        add.l       A0,D0               ; add offset to top of 
                                        ;   globals
        move.l      D0,4(sp)            ; set function result

       rts                              ; return to caller

CalculateOffset. CalculateOffset determines the offset from the beginning of the code resource to the location that A5 should point to. We see from Figure 5 that A5 should point to the location 32 bytes before the end of the resource. Therefore, we get a handle to the code resource, get the code resource's size, subtract 32 from it, and return the result as the needed offset.

CalculateOffset PROC

                lea         FirstByte,A0    ; get pointer to us
                _RecoverHandle              ; get handle to us
                _GetHandleSize              ; find our size (= offset
                                            ;   to end of globals)
                sub.l       #32,D0          ; account for 32 bytes of
                                            ;   appParms
                lea         gGlobalsOffset,a0   ; get address to save
                                            ;   result
                move.l      D0,(A0)         ; save this offset for 
                                            ;   later
                rts

SUMMARY OF THE THREE SOLUTIONS

This article has explored three ways to access global variables in standalone code: the THINK method, the OpenA5World method, and the StART method.

The THINK method uses the A4 register to access the global variables. The A4 register is managed by the RememberA0, SetUpA4, and RestoreA4 functions. The advantages of the THINK method are as follows:

  • The host's A5 register is untouched.
  • The storage for globals is coupled with the storage for the code itself, meaning that no additional storage needs to be allocated or disposed of.

The disadvantages of the THINK method are:

  • The A4 register cannot be used for code optimization.
  • Standalone code resources cannot be marked purgeable without the risk of losing any values stored in global variables.
  • Unless you use the multisegmented standalone code features of the THINK environments, you're limited to a combined total of 32K of code and data.
  • The global data is stored in an uncompressed format on disk.

Because MPW doesn't provide the compiler support that THINK does, the approach described in the Tech Note reuses register A5 to access global variables. Support is provided by the functions MakeA5World, SetA5World, RestoreA5World, DisposeA5World, OpenA5World, and CloseA5World. The advantages of this method are as follows:

  • It has a compact on-disk format (global data is compressed).
  • A4 is free for code optimization.
  • The code resource can be marked purgeable.
  • You can access 32K of code and 32K of data.

The disadvantages of the Tech Note method are:

  • It requires support from the host application for persistence of globals.
  • Care must be taken to restore the host's A5 when control is returned to the host (which can include callbacks, a la HyperCard).

The StART solution tries to incorporate the best of both worlds. StART's use of the A5 register is managed by calls to UseGlobals, DoneWithGlobals, and (optionally) CopyHostQD. Its advantages are as follows:

  • A4 is free for code optimization.
  • You can access 32K of code and 32K of data.
  • The storage for globals is coupled with the storage for the code itself, meaning that no additional storage needs to be allocated or disposed of.

The disadvantages it doesn't address are:

  • Care must be taken to restore the host's A5 when control is returned to the host (which can include callbacks).
  • Standalone code resources cannot be marked purgeable without the risk of losing any values stored in global variables.
  • The global data is stored in an uncompressed format on disk.

There's one major limitation that none of these techniques address. Neither MPW nor THINK can handle certain kinds of global variables -- ones that get preinitialized to some absolute address -- in standalone code. For instance, consider the following C source:

char *myStrings[] = {
    "Macintosh",
    "Programming",
    "Secrets",
    "2nd Edition"
};

This declares an array of pointers to the four given strings. When this definition appears in source code in a THINK C project, the compiler will tell you that this sort of initialization is illegal in standalone code. However, MPW's compilers aren't as integrated into the build process as THINK's are, and they don't know to give you a similar warning. Thus, we can compile an array like the one just shown without an error. When the MakeStandAlone tool is later executed, it will dutifully initialize the array with pointers to the given strings. However, these pointers are in the form of absolute memory locations, which are valid only at the time the globals are expanded. When it's time to execute the standalone code, it's almost certain that the strings won't be loaded into the same place they were in when the globals were expanded, making the pointers in our array invalid.

All you can do to avoid this problem is make sure that you don't have any global variables that are preinitialized to the addresses of other objects (such as strings, functions, and other variables). Without knowing the format of the compressed global data that _DATAINIT expands, it isn't possible to program the MakeStandAlone tool to look for the problem globals.

WHERE TO GO FROM HERE

This article just scratches the surface of what can be done with MPW. It gives a little behind-the- scenes information and describes how to take advantage of that information with a custom tool. The intrepid explorer may want to apply what's learned here to some other topics.

32-BIT EVERYTHING
With MPW 3.2, Apple has eliminated most of the traditional 32K barriers imposed by 16-bit fields. By expanding fields in the jump table to 32 bits, replacing the Segment Loader, patching object code with absolute addresses, and providing user-callable runtime routines, MPW allows you to create code and data blocks of practically any size. It may be interesting to explore the new formats and data structures used with 32-bit everything to see how you can use them in the same way we used the old 16-bit information.

MERGING START TECHNIQUES WITH THOSE OF THE TECH NOTE
The StART method uses a bit of assembly language to provide some runtime support for standalone code. Specifically, it maintains a reference to the code's global variables in a local data field. This same technique could be used to partially remove the dependency of code created with the Tech Note method on the host application.

JUMP TABLE
We've fully explored the area below A5, but only a small part of the area above A5. We've looked at the globals area below A5 and the application parameters area above A5, but the majority of the "above A5 world" is normally occupied by a jump table that supports multisegmented applications. With a little more work and runtime support, it may be possible to write multisegmented standalone code in MPW.

Multisegmented standalone code offers more benefits than simply allowing you to write huge chunks of standalone code. Programmers using Object Pascal and readers of the Macintosh Technical Note "Inside Object Pascal" (formerly #239) know that polymorphism requires the use of a jump table. By implementing support for a jump table in standalone code, it should be possible to write standalone code with Object Pascal or C++'s PascalObjects. C++ programmers writing native C++ classes or classes based on HandleObject should refer to Patrick Beard's article, "Polymorphic Code Resources," indevelop Issue 4.

THANKS DEPARTMENT
This article would not have existed if not for the help and inspiration of the following individuals and nonindividuals:

  • The creators of the A4 method used in the THINK products for showing that globals could be used in standalone code
  • The authors of the BuildDCMD tool for MacsBug, a tool that proved that applications conforming to a certain guideline could be converted to standalone code
  • Larry Rosenstein, who, thanks to file sharing, unknowingly provided the source code shell for the MakeStandAlone tool (all the stuff that deals with error handling and command-line parsing)

KEITH ROLLIN is one of Taligent's charter members, sporting the obligatory snide business title of Phantom Programmer (he got this title after buying that lakefront property in the fifth basement of the Grand Opera House in Paris). When not fending off people asking him what he does at Taligent, Keith skis, rides his bike, reads voraciously, watches 1940s movies at the local oldies theater, and comes up with reasons not to shave. Look for his latest book, Macintosh Programming Secrets,  2nd edition, co-authored with Scott Knaster, at your local bookstore (he needs the money). *For the sake of brevity, I occasionally refer to both the THINK C and THINK Pascal compilers simply as "THINK." *

THANKS TO OUR TECHNICAL REVIEWERSC. K. Haun, Pete Helme, Craig Prouse *

 
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**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
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