TweetFollow Us on Twitter

XCMD Chat
Volume Number:4
Issue Number:11
Column Tag:HyperChat™

XCMD Cookbook

By Donald Koscheka, Apple Computer, Inc.

on HyperChat

A Nice Chat

While doing the Sunday crossword puzzle recently, I was hit with a very interesting thought. One of the clues in this particular puzzle called for a synonym for “small talk”. The answer was “chat”. This struck me as odd because in many ways HyperTalk is very much the preferred object oriented programming language for the Macintosh. One of the earliest object oriented languages, as you know, is called “SmallTalk”. By calling this column “HyperChat”, Fred makes a rather obscure reference to HyperTalk’s philosophical roots. Chat implies a looseness of speech, a vernacular that is easy to master. That is exactly what HyperTalk is - an easy to grasp interface to the Macintosh Toolbox. What HyperTalk lacks in power, it certainly makes up for in ease of understanding!

At the opposite end of the spectrum is Assembly language programming. This much maligned language conjures all sorts of anxieties in the minds of otherwise fearless programmers. Yet most professional programmers will admit that an understanding of assembly language can improve one’s ability to write efficient code in a higher-level language as well as better understand those mysterious looking dumps that one gets when suddenly presented with a memory dump from TMON or MACSBUG.

Worse yet, those of us who choose to work in higher level languages provide a great disservice to the assembly language programmer by not showing them how to interface with our languages. HyperTalk documentation is certainly sparse in describing how an assembly language programmer can write an XCMD in assembly.

Assembly Interface

I started writing this month’s column hoping to be of service to the assembly language programming community. I wasn’t very far along when I realized that this column offers a second service, at no extra charge to the reader.

The process of showing you how to interface to HyperCard in assembly language also introduces the Pascal and C programmer to debugging in Macsbug or TMON (Forth programmers take heart: Jörg Langkowski’s article in the December, 1987 issue of Mactutor provides you with the information you’ll need to write XCMDs; even if you don’t program in Forth-like languages, you should read Jörg’s column; his insights into the Macintosh are often astonishing).

The most important reason to code in Assembly language is that it provides a rich opportunity to improve on the more generic code generated by Pascal and C compilers. Consider the following string comparison in Pascal:

len: INTEGER;
Match : Boolean;
Str1  : Str255;
Str2  : Str255;

Str1 := ‘Hello World’;
Str2 := ‘GoodBye Cruel World’;


IF Length(Str1) <> Length( Str2 ) THEN 
 match := FALSE
ELSE
BEGIN
 Match := True;  { assume they will match}
 k := 1;

 While (k <= Length( Str1)) AND (match) DO
 IF Str1[k] <> Str2[k] THEN 
 match := False
 ELSE
 k := k + 1;
END;

Listing 1. String Comparisons in Pascal

String compares can be made more efficient than this in Pascal. I chose this example for its simplicity. In comparing strings, we must consider the end points, if the strings are not the same length, they are not equal. If you haven’t been programming in Assembly language, you may not see how this routine could be made more efficient. One measure of a program’s efficiency is a count of the number of bytes of instructions needed to execute the program. If you compile listing 1 and dump the object code, you would discover that the while loop requires 82 bytes of instructions to execute. Assembly language programmers know by instinct that 82 bytes is too much code to perform a single string compare.

An assembly language equivalent of the while loop in listing 1 can be shrunk by an order of magnitude as in listing 2. If you’re trying to speed up a particularly slow loop, a few bytes of well-written assembler might be just what the doctor ordered. But optimizing code is just one reason to familiarize yourself with assembly language. A more compelling reason for the high-level language is that an understanding of assembler will help you make sense of your TMON or MacsBug dumps.

 ; A0, A1 point to the 2 strings
 Move.b (A0)+, D1; get the length of string 1
 Move.b (A1)+, D2; get the length of string 2
 Cmp.b  D1,D2  ; are they the same length?
 Beq  CompareChars ; yes, go ahead and compare them
 Move #0, D0; set the result to false
 Bra  Done; and exit

CompareChars     ; compare Str1<->Str2
 Cmp.b  (A0)+,(A1)+; do the characters match?      
 Beq  Done; yes, see if we’re at end of string
 Dbra D1, CompareChars
 Move #1, D0; they match, set the result true

DONE  sne D0       ; Result is true if strings match,          
 false otherwise

Listing 2. String Compare in Assembler

Entering XCMDs

The real trick to writing an XCMD in assembly language is understanding that HyperCard expects to see an XCMD that was generated by the Pascal compiler. This implies that parameters are pushed on the stack from left to right and that subroutines are responsible for removing the pushed parameters from the stack. XCMDs receive only one parameter so the push order isn’t important. What is important is remembering to remove that parameter from the stack before returning to HyperCard. Listing 3 is the assembly language interface for XCMDs. The fields in the paramBlock record are identical to the Paramblock record in Pascal or C.

On entry to a subroutine in assembly language, the last item on the stack is the return address of the calling routine. Normally, when we are done with the subroutine, an rts (return from subroutine) instruction will pop this return address off the stack and into the Program Counter resuming execution at the instruction pointed to by that location. This won’t do for Pascal routines since convention dictates that we also remove the parameters from the stack. One way to do this would be to pop the return address into a temporary register, say D0, remove the parameters from the stack by adding the size of the parameters to the stack pointer and then pushing the contents of D0 onto the stack and executing an RTS. A more efficient method exists: Pop the return address into an address register, say A0. Unbias the stack parameters by adding 4 to the stack (the size in bytes of the parameter block pointer). Finally, since register A0 contains the return address, execute the Jmp Indirect instruction on A0: Jmp (A0).

Globals in XCMDs

If your XCMD requires local variables, you’re going to need a place to store them. Assembly language XCMDs bear the same restriction placed on high-level languages: you don’t have access to the globals. A simple solution would be to allocate a handle large enough to store all your globals and keep that handle available in an address register. This works fine if the data is very static but not very well otherwise since you could easily run out of registers while manipulating even a small number of handles. Pascal and C use a more efficient approach, one that you’ve already seen if you’ve done any debugging in TMON or Macsbug.

On entry to the subroutine, we know that the stack pointer, register A7, already points to the next available space on the stack (the bottom of the stack). Why don’t we allocate our data on the stack by pointing an address register, say A6, to the bottom of the stack, subtracting the number of bytes that we need for our locals from A7 effectively growing the stack by the amount we need (the stack grows downward). Now A6 points to our local variables and A7 continues its role as stack pointer below our local stack frame (figure 1 ).

The process of creating a stack frame can be performed with one assembly language instruction: link. Used judiciously, the link instruction buys us a whole lot more: it saves the old value of the address register and gives a reference point to the parameters passed by the caller.

By executing link A6,#LocalSize, before doing anything else, we set up up a stack frame at the stack bottom. If stacksize were set to zero, we wouldn’t actually allocate any stack space for globals, but the instruction would still provide a payback. Because nothing else was put on the stack between the call to this routine and our link instruction, A6 also doubles as a pointer to our parameters! First, 0(A6) contains the previous value of A6 (can you think of anything this might be useful for?), next 4(A6) is the return address, and 8(A6) is the paramblock pointer passed to us by HyperCard. By putting 8(A6) into A3, we save the pointer to our parameters in an address register.

The offsets defined in the parameter block equates now become offsets off A3 for each field. The first field in the record is paramCount(A3) the count of the number of parameters in the params array. Accessing the handles in the params array poses another question. The first handle in the array is at params(A3). Getting to the other handles in the params array requires a straightforward application of arithmetic. By definition, handles occupy 4 bytes in the Mac. Any array element, i, is at offset (i-1)*4 byte in the array. We subtract 1 from the element number because array indices count from 0 not from 1. If we put the array index into D0, subtract 1 and multiply by 4 we have the offset from the beginning of the array. All we need to do is add this number to params[A3] and we have the handle. The 68000 has just an instruction: indexed addressing with offset. Here is how this instruction can be used to get the third parameter in the list:

Moveq     #3, D0   ; Access the third parameter in the list 
Sub.w     #1, D0   ; arrays count from 0, not 1
Asl.w     #2, D0   ; shift left by 2 is same as multiply by 4! 
Move.l    params(A3,D0.w), A0 ; A0 now holds the third handle 

Since stack frames are oriented from the highest memory location they occupy to the lowest, local variables are always referred to by negative offsets. In Pascal, the following declaration

VAR
myInt   : INTEGER;
myLong  : LongINt;
myRect  : Rect;

would have the following counterpart in assembly language:

myInt        EQU -4        ; locals start at offset -4 in the frame
myLong  EQU myInt-2; Integers are 2 bytes
myRect      EQU myLong-4  ; longs are 4 bytes
LocalSize EQU myRect-8  ; rectangles are 8 bytes.
LocalSize is equal to 14 bytes.  The stack frame is set up to read:
LINK    A6,#LocalSize; create the local variable pool.
Move.w  (A0),D0

If you’re having a bit of difficulty with this material, try writing a simple XCMD in Pascal or C and disassembling it in TMON or Macsbug. You can do this by invoking the Debugger call as the first statement in you XCMD.

Exiting the XCMD

Leaving the XCMD requires a little house cleaning. First, we restore the registers that were saved onto the stack. Next, we execute an unlink (UNLK) instruction to undo the last Link instruction. At this point, the stack looks just like it did when we entered the XCMD. More importantly (A7) is the return address of the calling routine. Popping this value into A0 allows us to save the return address in a safe place so that we can remove the parameters from the stack. We know how much space the parameters take. The stack contains only one parameter, XCmdBlkPtr, whose length is four bytes. Adding 4 to A7 shrinks the stack to the right size. Now all that’s left is to Jmp to the location pointed to in A0 and we’re done!

Conclusion

If you’re already programming in assembly language, this article is enough to get you started with XCMDs, especially if you’re not familiar with Pascal calling conventions. If you’re a Pascal or C programmer, the above discussion should help you to debug your code by explaining some of the assembly code you may have been looking at in TMON or Macsbug. In any case, understanding how compilers take your statements and convert them into machine executable code is a great way to learn more about the inner-workings of your programs and those seemingly mysterious bugs that just are not obvious in your Pascal or C source code. Hopefully, I’ve been able to fill some gaps in your debugging skills with this information. If the information in this article was useful, please let me know, I’ll be happy to offer more information on assembly language and debugging techniques.


;******************************
;* File: SimpleXCMD.a*
;* *
;* A simple XCMD written in *
;* Assembly language to show*
;* how XCMDs are written in *
;* assembler...  *
;* -------------------------- *
;* By:  Donald Koscheka   *
;* Date:16 July, 1988*
;******************************

;******************************
;* Build Sequence
;*
;* asm -w SimpleXCMD.a
;* link  -rt XCMD=1200 -sn Main=SimpleXCMD
;* SimpleXCMD.a.o
;* -o “YourStackName”
;******************************

;***  ParamBlock structure***
paramCountEQU    0   
params  EQU paramCount+2
returnValue EQU  params+( 16 * 4 )     
passFlagEQU returnValue + 4 
entryPointEQU    passFlag + 2   
request EQU entryPoint + 4  
result  EQU request + 2
inArgs  EQU result + 2
outArgs EQU inArgs + ( 8 * 4 )
pBlkSizeEQU outArgs + (4 * 4 )

;*** ------------------ ***
;*** THE LOCAL VARIABLES  ***
;*** (WILL GO ON STACK)   ***
;*** Note that the stack frame***
;*** counts backwards from 0***
;*** so that the value of ***
;*** LocalSiz will always be  ***
;*** negative    ***

LOCALS  EQU 0
LASTLOCAL EQU    LOCALS
LOCALSIZEQU LASTLOCAL
;*** ------------------ ***


SimpleXCMDMAIN   EXPORT
;******************************
;* In:
;*
;* 0(A7) == Return Address
;* 4(A7) == ParamBlockPtr
;* 
;* Link A6 to create a stackframe
;* that points to these vars.
;******************************

 ;*** Set Up Stack Frame
 LINK   A6,#LOCALSIZ ; Size of the local frame
 MOVEM.LD5-D7/A3/A4,-(SP) ; save some registers
 
 ;*** Get Pointer to paramblock
 MOVE.L 8(A6), A3; Point to parameters
 CLR.L  returnValue(A3) ; set to “empty”
 TST.W  paramCount(A3)  ; Any Parameters?
 BEQ    DONE; no, just return
 
   ;*** Insert your code here.  If your XCMD doesn’t take any 
 ;*** parameters eliminate the atest on paramcount ...
 
DONE    ;*** Prepare for Return to HyperCard
 MOVEM.L(SP)+,D5-D7/A3/A4 ; restore registers
 UNLK   A6; wipe out stack frame
 MOVE.L (A7)+, A0; get the return address
 ADD.L  #4, A7   ; unbias the stack
 JMP    (A0); return to HyperCard
 END

Listing 3. SimpleXCMD in Assembly Language

end HyperChat
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Minecraft 1.20.2 - Popular sandbox build...
Minecraft allows players to build constructions out of textured cubes in a 3D procedurally generated world. Other activities in the game include exploration, gathering resources, crafting, and combat... Read more
HoudahSpot 6.4.1 - Advanced file-search...
HoudahSpot is a versatile desktop search tool. Use HoudahSpot to locate hard-to-find files and keep frequently used files within reach. HoudahSpot is a productivity tool. It is the hub where all the... Read more
coconutBattery 3.9.14 - Displays info ab...
With coconutBattery you're always aware of your current battery health. It shows you live information about your battery such as how often it was charged and how is the current maximum capacity in... Read more
Keynote 13.2 - Apple's presentation...
Easily create gorgeous presentations with the all-new Keynote, featuring powerful yet easy-to-use tools and dazzling effects that will make you a very hard act to follow. The Theme Chooser lets you... Read more
Apple Pages 13.2 - Apple's word pro...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices, and... Read more
Numbers 13.2 - Apple's spreadsheet...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
Ableton Live 11.3.11 - Record music usin...
Ableton Live lets you create and record music on your Mac. Use digital instruments, pre-recorded sounds, and sampled loops to arrange, produce, and perform your music like never before. Ableton Live... Read more
Affinity Photo 2.2.0 - Digital editing f...
Affinity Photo - redefines the boundaries for professional photo editing software for the Mac. With a meticulous focus on workflow it offers sophisticated tools for enhancing, editing and retouching... Read more
SpamSieve 3.0 - Robust spam filter for m...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
WhatsApp 2.2338.12 - Desktop client for...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more

Latest Forum Discussions

See All

‘Resident Evil 4’ Remake Pre-Orders Are...
Over the weekend, Capcom revealed the Japanese price points for both upcoming iOS and iPadOS ports of Resident Evil Village and Resident Evil 4 Remake , in addition to confirming the release date for Resident Evil Village. Since then, pre-orders... | Read more »
Square Enix commemorates one of its grea...
One of the most criminally underused properties in the Square Enix roster is undoubtedly Parasite Eve, a fantastic fusion of Resident Evil and Final Fantasy that deserved far more than two PlayStation One Games and a PSP follow-up. Now, however,... | Read more »
Resident Evil Village for iPhone 15 Pro...
During its TGS 2023 stream, Capcom showcased the Following upcoming ports revealed during the Apple iPhone 15 event. Capcom also announced pricing for the mobile (and macOS in the case of the former) ports of Resident Evil 4 Remake and Resident Evil... | Read more »
The iPhone 15 Episode – The TouchArcade...
After a 3 week hiatus The TouchArcade Show returns with another action-packed episode! Well, maybe not so much “action-packed" as it is “packed with talk about the iPhone 15 Pro". Eli, being in a time zone 3 hours ahead of me, as well as being smart... | Read more »
TouchArcade Game of the Week: ‘DERE Veng...
Developer Appsir Games have been putting out genre-defying titles on mobile (and other platforms) for a number of years now, and this week marks the release of their magnum opus DERE Vengeance which has been many years in the making. In fact, if the... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for September 22nd, 2023. I’ve had a good night’s sleep, and though my body aches down to the last bit of sinew and meat, I’m at least thinking straight again. We’ve got a lot to look at... | Read more »
TGS 2023: Level-5 Celebrates 25 Years Wi...
Back when I first started covering the Tokyo Game Show for TouchArcade, prolific RPG producer Level-5 could always be counted on for a fairly big booth with a blend of mobile and console games on offer. At recent shows, the company’s presence has... | Read more »
TGS 2023: ‘Final Fantasy’ & ‘Dragon...
Square Enix usually has one of the bigger, more attention-grabbing booths at the Tokyo Game Show, and this year was no different in that sense. The line-ups to play pretty much anything there were among the lengthiest of the show, and there were... | Read more »
Valve Says To Not Expect a Faster Steam...
With the big 20% off discount for the Steam Deck available to celebrate Steam’s 20th anniversary, Valve had a good presence at TGS 2023 with interviews and more. | Read more »
‘Honkai Impact 3rd Part 2’ Revealed at T...
At TGS 2023, HoYoverse had a big presence with new trailers for the usual suspects, but I didn’t expect a big announcement for Honkai Impact 3rd (Free). | Read more »

Price Scanner via MacPrices.net

New low price: 13″ M2 MacBook Pro for $1049,...
Amazon has the Space Gray 13″ MacBook Pro with an Apple M2 CPU and 256GB of storage in stock and on sale today for $250 off MSRP. Their price is the lowest we’ve seen for this configuration from any... Read more
Apple AirPods 2 with USB-C now in stock and o...
Amazon has Apple’s 2023 AirPods Pro with USB-C now in stock and on sale for $199.99 including free shipping. Their price is $50 off MSRP, and it’s currently the lowest price available for new AirPods... Read more
New low prices: Apple’s 15″ M2 MacBook Airs w...
Amazon has 15″ MacBook Airs with M2 CPUs and 512GB of storage in stock and on sale for $1249 shipped. That’s $250 off Apple’s MSRP, and it’s the lowest price available for these M2-powered MacBook... Read more
New low price: Clearance 16″ Apple MacBook Pr...
B&H Photo has clearance 16″ M1 Max MacBook Pros, 10-core CPU/32-core GPU/1TB SSD/Space Gray or Silver, in stock today for $2399 including free 1-2 day delivery to most US addresses. Their price... Read more
Switch to Red Pocket Mobile and get a new iPh...
Red Pocket Mobile has new Apple iPhone 15 and 15 Pro models on sale for $300 off MSRP when you switch and open up a new line of service. Red Pocket Mobile is a nationwide service using all the major... Read more
Apple continues to offer a $350 discount on 2...
Apple has Studio Display models available in their Certified Refurbished store for up to $350 off MSRP. Each display comes with Apple’s one-year warranty, with new glass and a case, and ships free.... Read more
Apple’s 16-inch MacBook Pros with M2 Pro CPUs...
Amazon is offering a $250 discount on new Apple 16-inch M2 Pro MacBook Pros for a limited time. Their prices are currently the lowest available for these models from any Apple retailer: – 16″ MacBook... Read more
Closeout Sale: Apple Watch Ultra with Green A...
Adorama haș the Apple Watch Ultra with a Green Alpine Loop on clearance sale for $699 including free shipping. Their price is $100 off original MSRP, and it’s the lowest price we’ve seen for an Apple... Read more
Use this promo code at Verizon to take $150 o...
Verizon is offering a $150 discount on cellular-capable Apple Watch Series 9 and Ultra 2 models for a limited time. Use code WATCH150 at checkout to take advantage of this offer. The fine print: “Up... Read more
New low price: Apple’s 10th generation iPads...
B&H Photo has the 10th generation 64GB WiFi iPad (Blue and Silver colors) in stock and on sale for $379 for a limited time. B&H’s price is $70 off Apple’s MSRP, and it’s the lowest price... Read more

Jobs Board

Housekeeper, *Apple* Valley Villa - Cassia...
Apple Valley Villa, part of a 4-star senior living community, is hiring entry-level Full-Time Housekeepers to join our team! We will train you for this position and Read more
Housekeeper, *Apple* Valley Village - Cassi...
Apple Valley Village Health Care Center, a 4-star rated senior care campus, is hiring a Part-Time Housekeeper to join our team! We will train you for this position! Read more
Optometrist- *Apple* Valley, CA- Target Opt...
Optometrist- Apple Valley, CA- Target Optical Date: Sep 23, 2023 Brand: Target Optical Location: Apple Valley, CA, US, 92308 **Requisition ID:** 796045 At Target Read more
Senior *Apple* iOS CNO Developer (Onsite) -...
…Offense and Defense Experts (CODEX) is in need of smart, motivated and self-driven Apple iOS CNO Developers to join our team to solve real-time cyber challenges. Read more
*Apple* Systems Administrator - JAMF - Activ...
…**Public Trust/Other Required:** None **Job Family:** Systems Administration **Skills:** Apple Platforms,Computer Servers,Jamf Pro **Experience:** 3 + years of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.