Technical Q&A DV18

Detecting a CD-ROM

Q: My application runs from the hard disk, but it requires a specific CD-ROM to be present on the system. I am concerned about the possibility that a user may have a hard-disk volume or a diskette with the same name as my CD-ROM. To check for this, my application searches for a specifically named locked volume that contains a certain file, and I'd like to simplify this process. What is the best way to determine if a mounted volume or SCSI address is a CD-ROM drive?

A: In the past, there was no foolproof way to determine whether a drive on the Macintosh is a CD-ROM, so developers tried several methods:

1. Inspect the drive-queue element for a mounted drive to determine which driver the dQRefNum points to. Then, inspect the driver's name. If the name is ".AppleCD", it's probably a CD-ROM drive.

2. This approach fails for non-Apple CD-ROM drives, since they have different driver names.

3. Inspect the drive's attributes in the four bytes preceding the drive-queue entry. If the drive is "locked in hardware" and is removable, it's probably a CD-ROM drive.

4. Issue a SCSI request directly to the drive.

5. This fails for some early CD-ROM drives (non-SCSI-2 compliant). Before SCSI-2, there was no CD-ROM device type defined, and some CD-ROM drives reported that they were hard disks. There is an additional complication introduced by SCSI Manager 4.3, and IDE CD-ROM drives require a completely different calling architecture.

6. Furthermore, some newer Macintosh models such as the PowerBook 1400 and many of the clones use an ATAPI protocol-based CD-ROM drive and therefore won't show up in any SCSI probing you do.

However, since the introduction of version 5.2.X. of the .AppleCD (and any .AppleCD compliant) driver, CD-ROM drivers have supported the DriverGestalt csCode.

To find the inserted CD-ROM drives using these drivers, you can scan the Drive Table and issue DriverGestalt commands to the drivers to determine the device type. The following snippet will illustrate this:

#include <DriverGestalt.h>
void FindTheCD-ROMS(void)                   //  Scan Drive table for CD-ROMS
{
    DriverGestaltParam          pb;
    DrvQEl  *dqp;
    OSErr   status;
    pb.csCode       = kDriverGestaltCode;       // Setup Driver Gestalt PB
    pb.driverGestaltSelector = kdgDeviceType;   // ask for Device Type
    dqp = (DrvQEl *) GetDrvQHdr()->qHead;   // Start with head of drive queue

    while (dqp != NULL) {                   // for each device in drive queue
        pb.ioCRefNum    = dqp->dQRefNum;            // Get the driver refNum
        pb.ioVRefNum    = dqp->dQDrive;             // get the drive  refNum
        status = PBStatusSync((ParmBlkPtr) &pb);    // Do a Driver Gestalt call
        if (status == noErr)
            if(pb.driverGestaltResponse == kdgCDType ) // Device type is 'cdrm'
            {
                    printf("Drive: %d Driver:(%d) ",
                             (int) dqp->dQDrive, dqp->dQRefNum);
            }
        dqp = (DrvQEl *) dqp->qLink;                // Next drive
    }
}

A complete sample entitled Macintosh Disk Driver Gestalt Sample is available from DTS which illustrates how to query devices.

To discover any unmounted drives, you need to use either the SCSI or ATA manager, whichever is appropriate. The 'ATA demo' DTS code sample illustrates how to scan the ATA bus and identify the available devices.

There is also a driver Status call for available on the .AppleCD driver starting in version 5.2.X. that will return information on the type of CD-ROM device.

Issue a Status Call to the .AppleCD Driver with the following parameters:

    csCode = 120
    csParam[ 0 -1]   will return a DeviceIdent of the form
typedef struct DeviceIdent{
  uchar busType;                // SCSI -  0                ATAPI = 7
  uchar bus;                    // SCSI - Bus#              ATAPI = 0
  uchar targetID;               // SCSI - Target SCSI ID    ATAPI = Bus #
  uchar partition ;             // SCSI - LUN               ATAPI  = 0
}

One thing to note: if you are using an older version of the .AppleCD driver, the above calls might not work, in which case you may consider either falling back to previous methods or requesting the user update their CD-ROM driver.