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High-Level Control and Status Calls: When a Good Call Goes Bad


This Technical Note discusses situations under which high-level Status calls do not work correctly and PBStatus calls should be made instead.

Updated: [February 01 1990]

_Control and _Status traps

When Apple designed the _Control and _Status traps, it was assumed that _Control would be used to send the driver a command or information and _Status would be used to request information from the driver. The csParam parameter was meant to pass information in only one direction for each call; to a driver on control calls and from a driver on status calls. Most drivers follow this convention, and so there is no problem using the high-level Control and Status calls with them. However, some drivers depend on the bidirectional transfer of information on _Control and _Status calls through the csParam variable. For drivers of this type it behooves you to always use the PBControl and PBStatus calls. It's not hard. Honest!

Glue code is used to build the parameter block that gets passed to the _Control and _Status traps. The glue uses the refNum and csCode parameters to fill out the corresponding fields in the parameter block.

On control calls it copies the data pointed to by csParamPtr into the csParam field of the parameter block before it calls _Control, but it does not copy the information back after the call.

On status calls it only copies the csParam field to the location pointed to by csParamPtr after the _Status call. It does not copy the data pointed to by csParamPtr into the parameter block before calling _Status.

The low-level PBControl and PBStatus calls have no such problem because you are working directly with the parameter block and have direct access to the csParam field. The high-level Control and Status calls in some cases either work incorrectly, or worse, cause problems for the device driver.

An example of _Control calls that return data in csParam field are disk drivers that return a pointer to their icon on _Control calls with a csCode of 21. Here is an example of how you can use PBControl to get the icon of a drive and add it to the current resource file.

#define        kGimmeIconDataPtr 21
#define        kSizeOfIconAndMask    256
#define        kCustomAliasIconID    128

AddVolumeIconRes (short  vRefNum)
    HVolumeParam    vInfoPB;
    CntrlParam    cntlPB;
    Handle            serverIcon;
    OSErr            err;

    vInfoPB.ioNamePtr = nil;
    vInfoPB.ioVRefNum = vRefNum; // can be working // directory
    vInfoPB.ioVolIndex = 0; // use vRefNum - // don't index

    err = PBHGetVInfoSync ((HParamBlockRec *) &vInfoPB);

    if (err == noErr)
        cntlPB.ioVRefNum    = vInfoPB.ioVDrvInfo; // logical drive // number
        cntlPB.ioCRefNum    = vInfoPB.ioVDRefNum; // disk driver // reference number
        cntlPB.csCode        = kGimmeIconDataPtr;

    err = PBControl ((ParamBlockRec *) &cntlPB, false); // false = // synchronous

        if (err == noErr) // copy ICN# and add to resource fork
            serverIcon = NewHandle (kSizeOfIconAndMask);
            if (serverIcon != nil)
                BlockMove (* (Ptr *) (cntlPB.csParam), *serverIcon,
                AddResource (serverIcon, 'ICN#', kCustomAliasIconID, nil);
                if (ResError () != noErr)
                    DisposHandle (serverIcon);

The most obvious example of a device driver that expects csParam as input on a _Status call is the video device driver(s) for Macintosh II Video Cards. Almost all of the documented status calls require csParam to point to some kind of table. In this case, most of the device driver's status routines do not function properly if using the high-level Status call.

Therefore, if you are interfacing to a device driver that you either know or suspect requires csParam for its status calls, use the low-level PBStatus call instead of the high-level Status call. Likewise, if the driver returns information via the csParam field on control calls, you will need to use PBControl rather than the high-level Control call.

If you are writing a device driver, alert the users of your driver to these limitations. Alternatively, you could design your driver so that control calls only receive data and status calls only return data in the csParam field.

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Inside Macintosh, Volume II, The Device Manager

Inside Macintosh, Volume IV, The Disk Driver

Inside Macintosh, Volume V, The Disk Driver

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