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Boot Disk Icon
Volume Number:5
Issue Number:5
Column Tag:Programmer's Forum

Related Info: Finder Interface Device Manager

Change the Boot Disk Icon

By David Dunham, Goleta, CA

I’ve been asked several times how my bootDiskIcon INIT works. I told the questioners that I’d answer in MacTutor.

Also, this is a short article. MacTutor needs more of them -- I know how to initialize Managers and handle events, and I certainly don’t need to see pictures in hexadecimal. (This isn’t just a snide remark directed at the editor, it’s a plea to those with nifty ideas that don’t need an entire application to show them off. Write a one or two page article!)

bootDiskIcon is a hack that came about because I don’t own an HD20. Apple insists on portraying their internal hard disk as an HD 20 on my desktop. I got so disgusted with this that I wrote an INIT to show something else, anything but an HD 20. ICN# 256, in fact. bootDiskIcon is inferior to programs like Façade-- it’s less flexible, and may not work with all hard disks -- but it was better than nothing, and it’s easy to understand.

The key to bootDiskIcon is the way the Finder handles disk icons. A pedantic digression: how does one find out how Finder handles icons? It’s not in Inside Macintosh. Therefore, it must be in a Technical Note. But Finder isn’t a category in the table of contents. So it’s off to the index but it’s not listed under “icon.” Luckily, it was one of the “Finder” Tech Notes, number 28. How did I know to keep looking? Because I’d seen it years ago when it came out, and knew it was there somewhere. Always skim every Tech Note -- not to memorize them, but to remember that they’re there.

When the Finder wants a disk icon, it gives the disk driver a Control call with csCode = 21. The driver returns a pointer to an ICN#. My first inclination was, why write code when you can change data? The ICN# must be part of the disk driver. The disk driver is part of Apple HD SC Setup. I found it there (it’s not a resource), changed it, and reinstalled the driver. This failed utterly -- there’s apparently a checksum.

If I couldn’t change the ICN# on disk, how about changing it in memory? I can get its address in memory by giving the driver a csCode = 21 call, then simply change the data. This is what bootDiskIcon does.

Before I get to what you’re all waiting for, the source code, I’ll mention the difference between patches and INITs. A patch is a change to a piece of code, almost always one of the traps. An INIT resource is any piece of code that runs at startup time (typically in a file of type INIT, cdev, or rdev). An INIT frequently installs a patch, but it could do anything at all, like playing a tune or subverting a disk driver’s icon.

More ideological baggage: INITs should announce their presence. Paul Mercer has a good way to do this, called ShowINIT. I’m told the source code’s available on AppleLink. I’ve modified it to handle color icons, and compiled the whole thing as a separate resource, SHOW 0. bootDiskIcon calls this code. You’re welcome to use it in any non-commercial (free or shareware) programs -- just use ResEdit to paste SHOW 0 into your own INIT file. You may not use it in public domain programs. These are those without a copyright holder, and I don’t want my code in the public domain. Don’t make the mistake of calling your free program “public domain.” Once you do, you can’t get it back (despite what Adobe thinks).

We can almost get to the code, but you might be wondering how to test it. It’s real inconvenient to reboot, just to see if a change worked. Luckily, you don’t have to. Remember that an INIT is just a piece of code that happens to get executed at boot time. You can often execute the same code at any time, and instruct your development system to make an INIT resource when you’re finished debugging. In bootDiskIcon’s case, I simply executed it, juggled into the Finder layer, and verified that I’d chant the code. It starts out by calling my SHOW resource -- the parameters are reversed from the way they’re defined because my ShowICON is defined as a Pascal procedure. My sloppiness in not defining it as such in this C program probably works only because this is an INIT -- if I trash the stack, it doesn’t much matter, since I’ll be returning to INIT 31 very soon.

The real work begins with finding out the driver refNum and drive of the first mounted disk (now you know why it’s called bootDiskIcon -- I told you it was a quick hack). I then find the address of the driver’s ICN#, get an ICN# of my own choice, and overwrite the driver’s using BlockMove.

#include <deviceMgr.h>
#include <hfs.h>

#define NIL 0L

void  main(void);/* Always use prototypes! */

void main() {
 Ptr    icn;
 int    error;   /* Shouldn’t really be write-only  */
 HParamBlockRec  pb;
 cntrlParam cpb;
 Handle handle;

 /* Show icon on the bottom of the screen */
 handle = GetResource(‘SHOW’,0); /* Get handle to the PROC */
 if (handle == 0L) { /* Something’s wrong (couldn’t load) */
 SysBeep(32);    /* Let somebody know */
 return;/* Don’t try to call it! */
 HLock(handle);  /* Hold down the PROC */
 (**(ProcPtr *)handle)(-1,128);  
 /* ShowICON() -- note reversed parameters */
 HUnlock(handle);/* Let it float in the heap again */

 /* Change the icon */
 pb.volumeParam.ioNamePtr = NIL;
 pb.volumeParam.ioVolIndex = 1;  /* First mounted volume */
 error = PBHGetVInfo(&pb,FALSE);
 cpb.ioRefNum = pb.volumeParam.ioVDRefNum;
 cpb.ioVRefNum = pb.volumeParam.ioVDrvInfo;  
 /* drive (ioDrvNum) */
 cpb.csCode = 21;
 error = PBControl(&cpb,FALSE);
 icn = *(Ptr *)(&cpb.csParam[0]);
 handle = GetResource(‘ICN#’,256);

Known Bugs

Actually, these are rumored bugs, because it works for me! I’m told it doesn’t work with Jasmine disks where the driver puts the SCSI ID inside the flower. It probably doesn’t work with a boot floppy, because the Finder normally special cases the disk icon. (Furthermore, Finder probably doesn’t consider floppies to be “foreign drives.”)


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