Tips & Tidbits

By Neil Ticktin, Editor-in-Chief

Tip of the Month

In the September, 1993 issue of MacTech Magazine, one reader wrote in asking about finding out if QuickTime is installed via the Gestalt call. We asked you, the readers, to respond and you a number of you did. Thanks to all of you who responded including: Gary Woodcock, Gary McGath and Mats Hummel who were among the first to respond.

The first answer came from Robert Howe with a nice “clean” answer. The most interesting answer came from Alain Danteny who not only answered this question, but more about Gestalt. Robert and Alain are splitting this month’s prize.

First, the direct answer from Robert Howe: There is a new gestalt selector. You can modify your GestaltEqu.h file (with Think C) to include the following:

/* 1 */

#define gestaltQuickTime 'qtim'

(Realize that this may be included in the new header files that come with the THINK C 6.0.1 update). You may then call it as you would any other gestalt call. If QuickTime is not installed, gestalt returns an error.

A quick boolean check could be written as

/* 2 */

#include <GestaltEqu.h>
 
if (QuickTimeInstalled());
     // play a movie
 

Boolean QuickTimeInstalled ( void ) {
     return (noErr == gestalt('qtim', &result)); 
}

- Robert Howe,

More Gestalt information

Next, comes the additional information from Alain Danteny. If you want to have a full view of all currently installed and registered gestaltSelectors, follow these steps: First, call Gestalt with selector "tabl". The response (a pointer) is the starting address of an array containing all the gestaltSelector and procAddress. The format of each entry is:

{ 3 }

gestaltTableEntry = RECORD
      gestaltSelector : OSType;
      gestaltAddr : LongInt;
    END; {Pascal-like}

/* 4 */

struct{
    OSType gestaltSelector;
    long gestaltAddr;
}gestaltTableEntry;//C-like

The very first 2 entries are:

0000 000C xxxx xxxx ; (selector = $0000 000C in hexa)
0000 0007 yyyy yyyy ;

Those are undoubtly the selectors of Gestalt's developers at Apple: Carl C. Hewitt and <unknown?>. Moreover, under macsbug, if you type:

ip xxxx xxxx (see above)

you will decompile the gestaltProc and see that those selectors are probably used internaly by the traps Gestalt, NewGestalt, ReplaceGestalt... the gestaltProc uses two signatures: "carl" and "bbmc" (who's bbmc?)

Following entries are non-Apple gestaltSelectors: mainly INITS and cdev signatures (ADex, ADrk, MClk, NowT aso). Last are well-known Apple's system Selectors. This array ends up with longint $7FFF FFFF.

- Alain Danteny,

MOVEM.L Warning!

The MOVEM is a 680x0 assembler instruction. It is used a lot for saving and restoring the values of multiple data and address registers. I tried to save off two address register values with the lower register first and then restoring it later using two MOVE.L instructions. I ran into an unexpected surprise. The instruction:

MOVEM.L A2/A4,-(A7)

is not equivalent as you would expect to the 2 instructions:

MOVE.L  A2,-(A7)
MOVE.L  A4,-(A7)

but instead to:

MOVE.L  A4,-(A7)
MOVE.L  A2,-(A7)

The instruction MOVEM.L stores the registers from high to low, ignoring the order you specify. In other words,

MOVEM.L A2/A4,-(A7)

does the same thing as:

MOVEM.L A4/A2,-(A7)

Using MOVEM pre-decriment, -(A7), in conjunction with MOVEM postincrement, (A7)+, addressing, saving off and restoring the same registers is what the instruction was apparently designed for and works just fine for that purpose.

- Marek Hajek

Toggle invisibles in MPW

I often need to see “invisible” characters in text files, because many of mine come from BBS captures and may have glitches in them. MPW’s sequence for turning the display of invisible characters on and off is cumbersome, so I added a menu and keyboard shortcut; a single command to toggle the display. The following command adds a “Toggle Invisibles” to the Edit menu, so Command-Option-Y (displayed as ¥, option-Y) does the trick.

/* 5 */

AddMenu Edit "Toggle Invisibles/¥" 
 'If `Format -x a "{Active}"` == "Ail" ; 
   Format -a AIl "{Active}" ; 
  Else ; 
   Format -a Ail "{Active}" ; 
  End'

What this does: The first Format command, `Format -x a "{Active}"`, emits a string describing the active window’s format attributes. The If statement compares this to a default string of “Ail”: “A” for Auto-indent on, “i” for invisibles off, and “l” for lock-scroll off. Based on the result of that comparison, another Format command is issued to turn the display of invisible characters on or off (Format -a AIl or Format -ia Ail, respectively).

The format attribute strings “Ail” and “AIl” assume the other defaults for the file format attributes are Auto-indent ON and lock-scroll OFF. These are the common defaults, but you can change them by switching the case of the A and L in the format command. For example, if you prefer to not auto-indent, the format attribute strings would be “ail” and “aIl” instead.

- Lee David Rimar

Need information on your Macintosh?

Need some extensive information about your Mac? In AppleLink, press Option - Command - D.

- William Modesitt