TweetFollow Us on Twitter

A4, Code Resources
Volume Number:9
Issue Number:12
Column Tag:Think Top 10

Code Resources
and the A4 World

By Scott Shurr, Symantec Technical Support, Symantec Corp.

This article will describe the various kinds of code resources, and talk about the differences between writing applications and code resources. In applications, the machine code produced by the compiler uses the A5 register as a base address from which to access the applications global variables. Code resources are intended to be run simultaneously with applications, so they use a scheme designed to prevent interference with the application which called them up. In this case the A4 register is used as the base address for the resource’s global variables, as illustrated in the following diagram:

It is important when writing a code resource to be aware of how and when to set up A4. The different types of code resources require different declarations of the main() routine. The example consists of a single segment code resource which takes 2 arguments, which are a number of beeps, and the address of a string to receive a value. The code resource beeps the appropriate number of times, and then puts “Hello world” into the string. Here is the first version of the code resource:

/* 1 */

/* CodeResource.c */
void CRdoBeep(int beepSize);

void main(int beepSize, char *receiveString)
{
static char sLiteral[] = "Hello world"; 

strcpy(receiveString,sLiteral);
CRdoBeep(beepSize);
}

void CRdoBeep(int beepSize)
{
int i;

for (i=0; i<beepSize; i++)
  SysBeep(beepSize);
}

Create a new project, Resource.pi, and add the CodeResource.c file and the ANSI-A4 library to the project. Go to Set Project Type... and select the Code Resource button. Set Name to myCodeResource, Type to CODE, and ID to 31. One of the effects of changing the project type to anything other than Application is to cause the compiler to use the A4 register instead of the A5 as the address of the beginning of the application globals. Now select Build Code Resource..., and save the resource into Main.pi.rsrc. Another project called Main.pi will be used to test the code resource, and it will expect to find the code resource in that file in the folder. Create another new project in the same folder as Resource.pi, and add this program to it:

/* 2 */

/* UsesCodeResource.c */
#include <stdio.h>

typedef void (*CRPtr) (int,char*);

void main()
{
char myString[20];
Handle myCRhandle;

myCRhandle = GetNamedResource('CODE',"\pmyCodeResource");
HLock(myCRhandle);
(* (CRPtr) (*myCRhandle))(5,myString);
HUnlock(myCRhandle);
printf("myString=\"%s\"\n",myString);
}

Add UsesCodeResource.c and the ANSI library, and give it a try. The code resource will be loaded, it will beep 5 times, and then display some garbage instead of “Hello world”. The Disassemble command is useful in pointing out the problem. For the code resource:

main:
00000000: 4E56 0000          LINK      A6,#$0000
00000004: 486C 0000          PEA       sLiteral(A4)
00000008: 2F2E 000A          MOVE.L    receiveString(A6),-(A7)
0000000C: 4EAC 0000          JSR       strcpy(A4)
00000010: 3EAE 0008          MOVE.W    beepSize(A6),(A7)
00000014: 4EAC 0000          JSR       CRdoBeep(A4)
00000018: 4E5E               UNLK      A6
0000001A: 4E75               RTS
0000001C

CRdoBeep:
00000000: 4E56 0000          LINK      A6,#$0000
00000004: 2F07               MOVE.L    D7,-(A7)
00000006: 7E00               MOVEQ     #$00,D7
00000008: 6008               BRA.S     *+$000A      ; 00000012
0000000A: 3F2E 0008          MOVE.W    beepSize(A6),-(A7)
0000000E: A9C8               _SysBeep
00000010: 5247               ADDQ.W    #$1,D7
00000012: BE6E 0008          CMP.W     beepSize(A6),D7
00000016: 6DF2               BLT.S     *-$000C      ; 0000000A
00000018: 2E1F               MOVE.L    (A7)+,D7
0000001A: 4E5E               UNLK      A6
0000001C: 4E75               RTS
0000001E

Notice the instruction PEA sLiteral(A4). This depends on having A4 set to the address of the beginning of the code resource, so that sLiteral can be found. If MacsBug is used by putting a Debugger() call in CallsCodeResource.c, it can be seen that upon entry to the code resource’s main, A0 is set to the correct place, but A4 isn’t. Global variables always use offsets from A4 in code resources. In multi-segment code resources, all string literals also use an A4 offset. This works the same in C++, except that C++ has an option “Put string literals in code”, that will keep them from using A4. To fix the bug, change the code resource to look like this:

/* 3 */

/* CodeResource.c */
#include <SetUpA4.h>
void CRdoBeep(int beepSize);

void main(int beepSize, char *receiveString)
{
static char sLiteral[] = "Hello world"; 

RememberA0();
SetUpA4();
strcpy(receiveString,sLiteral);
CRdoBeep(beepSize);
RestoreA4();
}

void CRdoBeep(int beepSize)
{
int i;

for (i=0; i<beepSize; i++)
  SysBeep(beepSize);
}

To understand why this change makes the code resource work, see the disassembly of CallsCodeResource.c:

main:
00000000: 4E56 FFEC          LINK      A6,#$FFEC
00000004: 2F0C               MOVE.L    A4,-(A7)
00000006: 42A7               CLR.L     -(A7)
00000008: 2F3C 434F 4445     MOVE.L    #$434F4445,-(A7)
  ; 'CODE'
0000000E: 486D 0000          PEA       "\pmyCodeResource"
00000012: A9A1               _GetNamedResource
00000014: 285F               MOVEA.L   (A7)+,A4
00000016: 204C               MOVEA.L   A4,A0
00000018: A029               _HLock
0000001A: 486E FFEC          PEA       myString(A6)
0000001E: 3F3C 0005          MOVE.W    #$0005,-(A7)
00000022: 2054               MOVEA.L   (A4),A0
00000024: 4E90               JSR       (A0)
00000026: 204C               MOVEA.L   A4,A0
00000028: A02A               _HUnlock
0000002A: 486E FFEC          PEA       myString(A6)
0000002E: 486D 0000          PEA       "myString="%s""
00000032: 4EAD 0000          JSR       printf
00000036: 286E FFE8          MOVEA.L   $FFE8(A6),A4
0000003A: 4E5E               UNLK      A6
0000003C: 4E75               RTS

The _GetNamedResource trap returns a handle to the code resource on the stack. It is saved in A4 (A4 is not special in this case, it’s just what the compiler chose to use). Its dereferenced value is placed in A0, and the program does a JSR to the address in A0, the start of the code resource. The code resource itself begins with a header. It is possible to write your own header by including it in the source code for the code resource, and checking the Custom Header option in the Set Project Type dialog. In most cases, the option will not be selected, and the compiler will provide a default header for the code resource, like this:

00000000: BRA.S  *+$0010
     ...
00000010: LEA    start-of-resource,A0
00000014: NOP
00000016: NOP
00000018: BRA    main-of-code-resource

This sets A0 to the current address of the beginning of the code resource, and then jumps to the LINK instruction in the main function of the resource. A commented version of SetUpA4.h is useful in seeing the change made in the second version of the code resource:

/* 4 */

/*
 *  SetUpA4.h
 *
 *  Copyright (c) 1991 Symantec Corporation.
 *   All rights reserved.
 *
 *  This defines "SetUpA4()" and "RestoreA4()" routines that 
 *  will work in all A4-based projects.
 *
 *  "RememberA4()" or "RememberA0()" must be called in advance 
 *  to store away the value of A4 where it can be found by 
 *  "SetUpA4()".  The matching calls to "RememberA4()" (or 
 *  "RememberA0()") and "SetUpA4()" *MUST* occur in the same 
 *  file.
 *
 *  Note that "RememberA4()", "RememberA0()" "SetUpA4()", and
 *  "RestoreA4()" are not external.  Each file that uses them 
 *  must include its own copy.
 *
 *  If this file is used in the main file of a code resource 
 *  with "Custom Headers", be sure to #include it *AFTER* the 
 *  custom header!  Otherwise, the code resource will begin 
 *  with the code for the function "__GetA4()", defined below.
 *
 */

static void
__GetA4(void)
{
 asm {
 bsr.s  @1;  PC, (points to next line) goes on the stack
 ;  Branch to @1
 
 dc.l 0 ;  store A4 here
@1 move.l (sp)+,a1 ;  pop item from stack (which points
 ;  to previous line) into a1
 }
}

#define RememberA4() do { __GetA4(); asm { move.l a4,(a1) } } while (0)
 // __GetA4() puts pointer to spot for storing a4 in a1
 // move a4 to spot a1 points to
 
#define RememberA0() do { __GetA4(); asm { move.l a0,(a1) } } while (0)
 // __GetA4() puts pointer to spot for storing a4 in a1
 // move a0 to spot a1 points to
 
#define SetUpA4()do { asm { move.l a4,-(sp) } __GetA4(); /
  asm { move.l (a1),a4 } } while (0);
 // push a4 onto the stack
 // Call __GetA4 which puts a pointer to the space into a1
 // move whatever a1 is pointing to into a4
 
#define RestoreA4()do { asm { move.l (sp)+,a4 } } while (0)
 // pop item from stack into a4
 
/* In other words:

 RememberA4 puts the value in A4 in special spot.
 RememberA0 puts the value in A0 in special spot.
 
 SetUpA4 puts current value in A4 on the stack and gets 
 value in special spot and puts it in A4.
 RestoreA4 pops saved value of A4 from stack into A4.
*/

Here is the disassembly of the changed portion of the code resource:

__GetA4:
00000000: 6104               BSR.S     *+$0006      ; 00000006
00000002: 0000 0000          ORI.B     #$00,D0
00000006: 225F               MOVEA.L   (A7)+,A1
00000008: 4E75               RTS
0000000A

main:
00000000: 4E56 0000          LINK      A6,#$0000
00000004: 4EAC 0000          JSR       __GetA4(A4)
00000008: 2288               MOVE.L    A0,(A1)
0000000A: 2F0C               MOVE.L    A4,-(A7)
0000000C: 4EAC 0000          JSR       __GetA4(A4)
00000010: 2851               MOVEA.L   (A1),A4
00000012: 486C 0000          PEA       sLiteral(A4)
00000016: 2F2E 000A          MOVE.L    receiveString(A6),-(A7)
0000001A: 4EAC 0000          JSR       strcpy(A4)
0000001E: 508F               ADDQ.L    #$8,A7
00000020: 3F2E 0008          MOVE.W    beepSize(A6),-(A7)
00000024: 4EAC 0000          JSR       CRdoBeep(A4)
00000028: 548F               ADDQ.L    #$2,A7
0000002A: 285F               MOVEA.L   (A7)+,A4
0000002C: 4E5E               UNLK      A6
0000002E: 4E75               RTS
00000030

The RememberA0() in the source creates the 2 instructions after the LINK. It first does a JSR to __GetA4, whose job it is to get an address into A1. Notice that the second line of __GetA4 is a DC.L, which allocates 4 bytes, which are used for storing the addresses. The fact that the SetUpA4.h actually reserves some memory, unlike most other header files, is why it must always be included in each segment that uses any of RememberA4(), RememberA0(), SetUpA4(), or RestoreA4(). The BSR.S pushes the address of the 4 bytes onto the stack and then jumps over the 4 bytes. Then the address is popped from the stack into A1, and the function returns. The value of A0 is then saved at that address. The next 3 instructions in the code resource’s main are generated from SetUpA4(). It first pushes the current value of A4 onto the stack, so that it can be restored before returning to the application, which was probably using A4 for its own purposes. Then, the JSR to __GetA4 again gets the address of the storage area into A1. The address stored there is then placed into A4, which now holds the correct base address for the code resource. After the code resource does its work, it finishes up with RestoreA4(). That generates the MOVEA.L instruction which restores the old value of A4 from the stack before the code resource returns to the application which called it. This example may have raised some more questions, which I will try to answer here.

Q. Does this work the same in C++ as it works in C?

A. The code produced by C++ for a code resource that contains #include <SetUpA4.h> is almost identical to that produced by C, but the mechanism for producing it is different. If you look at SetUpA4.h (in Mac #includes: THINK #includes) you will see #if THINK_C which will select the C or the C++ version of the source. The __GetA4 is essentially the same for both versions, but the C++ version reflects its less powerful asm capabilities. In C, the calls like RememberA4() are implemented as #defines, while in C++ they are inline functions.

Q. Does any of this also apply to the other non-application projects, like Desk Accessories and Device Drivers?

A. The non-application projects are similar in that they all use offsets from A4 instead of A5 to access globals. You don't need to use SetUpA4 for desk accessories and device drivers, because the compiler provides a header which sets up the A4 for you.

Q. Why would one want to use a custom header in a code resource?

A. You can use the 18 bytes of the header for any purpose, as long as the first word is reserved for the code which will branch to where the resource is to begin execution. You could put flags for the version of the resource, or similar information, to be read by the code resource or by the program that is calling it. Just remember when you do that to put the #include <SetUpA4.h> after the header, or the code resource will start in the code SetUpA4.h generates, which isn’t desireable.

Q. What are the prototypes for the main() functions of various kinds of code resources?

A.

CDEF control definition
pascal long main(short varCode, ControlHandle theControl, 
 short message, long param);
FKEY function key
main();
INIT
main();
LDEF list definition
pascal main(short lMessage, Boolean lSelect, Rect lRect, 
 Cell lCell, short lDataOffset, short lDataLen, 
 ListHandle lHandle);
MDEF menu definition
pascal main(short message, MenuHandle theMenu, 
 Rect *menuRect, Point hitPt, short *whichItem);
WDEF window definition
pascal long main(short varCode, WindowPtr theWindow, 
 short message, long param);

 
AAPL
$108.00
Apple Inc.
+1.02
MSFT
$46.95
Microsoft Corpora
+0.90
GOOG
$559.08
Google Inc.
+8.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Vitamin-R 2.20b1 - Personal productivity...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dropbox 2.10.44 - Cloud synchronization...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Sandvox 2.9.2 - Easily build eye-catchin...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
Cocktail 8.0.1 - General maintenance and...
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
LibreOffice 4.3.3.2 - 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
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more

Latest Forum Discussions

See All

Monster Flash Review
Monster Flash Review By Jordan Minor on October 31st, 2014 Our Rating: :: ALONE IN THE DARKUniversal App - Designed for iPhone and iPad Solid shooting and a surprising amount of spooky tension make Monster Flash a great portable... | Read more »
Retry Review
Retry Review By Rob Thomas on October 31st, 2014 Our Rating: :: SOARING HIGHUniversal App - Designed for iPhone and iPad Flappy who? Let Retry wash all those bad bird-related memories away on a cool retro-flavored flight… right... | Read more »
Dementia: Book of the Dead Review
Dementia: Book of the Dead Review By Lee Hamlet on October 31st, 2014 Our Rating: :: A TOUGH READUniversal App - Designed for iPhone and iPad A witch hunter is sent after a demonic book in the spooky but short-lived Dementia: Book... | Read more »
Card Dungeon, the Semi-Board Game Roguel...
Card Dungeon, the Semi-Board Game Roguelike, Has Been Renovated Posted by Jessica Fisher on October 31st, 2014 [ permalink ] | Read more »
Logitech Protection + Power iPhone5/5S C...
Made by: Logitech Price: $99.99 Hardware/iOS Integration Rating: 3 out of 5 stars Usability Rating: 0.5 out of 5 stars Reuse Value Rating: 0.75 out of 5 stars Build Quality Rating: 0.75 out of 5 stars Overall Rating: 1.25 out of 5 stars | Read more »
This Is Not a Test Goes Free, Permanentl...
This Is Not a Test Goes Free, Permanently Posted by Jessica Fisher on October 31st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swap Heroes Review
Swap Heroes Review By Campbell Bird on October 31st, 2014 Our Rating: :: STRATEGIC SWAPPINGUniversal App - Designed for iPhone and iPad Rotate a cast of heroes to fend of waves of monsters in this difficult, puzzle rpg.   | Read more »
Night Sky Pro™ (Reference)
Night Sky Pro™ 3.0.1 Device: iOS Universal Category: Reference Price: $2.99, Version: 3.0.1 (iTunes) Description: Night Sky Pro™Wonder No More™ Night Sky Pro™ is the ultimate stargazing experience. From the creators of the original... | Read more »
Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 2014 13-inch R...
The Apple Store is now offering Apple Certified Refurbished 2014 13″ Retina MacBook Pros for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more

Jobs Board

Solutions Specialist with *Apple* Knowledge...
Company Description: We are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** Every day, business customers come to the Apple Store to discover what powerful, easy-to-use Apple products can do for them. As a Business Leader, Read more
Sr. Manager, *Apple* Deployment Programs fo...
**Job Summary** Apple is seeking candidates for a new position on the Education Content and Technology team. iPad and Mac is in the hands of millions of teachers and Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.