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How To Create A Mixed-Partition CD-ROM

How To Create A Mixed-Partition CD-ROM


Since the original Phil & Dave's Excellent CD was released, containing both Macintosh HFS and Apple II ProDOS volumes, DTS has gotten many questions about how it was done. Some ask just out of curiosity, while others want to create their own mixed-partition CD-ROMs. This article gives a detailed account of how any developer can prepare a mixed-partition hard disk whose image can be pressed onto CD-ROM.

The process of producing a CD-ROM disc containing both HFS and ProDOS volumes is relatively simple and straightforward. It's facilitated by the fact that Apple's operating systems recognize the data track of a CD-ROM as if it were a SCSI hard disk. You prepare a hard disk exactly as you wish it to appear on CD-ROM, ship it off to a CD production company, and they send you back a CD.

Mixing partitions is easiest if you have a 600+ MB hard disk, but you can also mix partitions if you have two or more smaller hard disks. We'll get down to the brass tacks of this procedure after a preliminary discussion of why you might want to mix HFS and ProDOS partitions, and some background information about partitions that you need to know if you're to fully understand the procedure.


Why would a developer want to create a CD-ROM that mixes HFS and ProDOS volumes? For one thing, combining HFS and ProDOS volumes on one CD is a way for developers of Apple II applications to make their applications and files available both locally and through AppleShare ® fileservers (which only read CD-ROMs in HFS format) with minimal additional effort. For another, mixing partitions is a way to distribute applications and files so they can be read by both the Macintosh and the Apple II.

Perhaps a more obvious solution to the problem of creating a CD-ROM readable by both the Macintosh and the Apple II would be to convert all volumes to the ISO 9660 format, described in Brian Bechtel's article in this issue. A CD-ROM in ISO 9660 format is readable not only by both the Macintosh and the Apple IIGS but by other operating systems as well.

For developers who rely on CD-ROM to store large amounts of information accessible by only one application, this is adequate and in some cases preferred. On the other hand, for developers who wish to use CD-ROM to distribute multiple applications, graphics and sound files, or other files that the user can browse through and launch using the Finder (such as Apple's Developer CD Series and the public domain CDs being released by user groups), using the ISO 9660 format presents certain problems. Storing files in ISO 9660 format strips the Finder of its ability to read desktop information about those files. On the Macintosh desktop, applications on ISO-format CD-ROMs are shown as generic application icons, documents as generic document icons, and folder and file placement information is lost. In addition, although there are supported extensions to ISO to handle Apple II GS filetype and auxtype information, software is not available at this time to apply these extensions before pressing.

Another reason for choosing to use native file formats rather than ISO 9660 is that conversion into the latter format involves an additional step in the process of pressing a CD-ROM: a premastering system must be used to create a tape that a production company then uses to create a CD-ROM. If your CD-ROM will be used only on an Apple computer, there's really no need for you to go through this additional step.


Partitions are logical volumes on a hard disk.

ProDOS is limited to 32 MB volumes, so under the ProDOS file system, a 20 MB hard disk would usually have only one ProDOS volume on it, while a 650 MB hard disk would probably be partitioned into several ProDOS volumes. HFS can handle very large volumes, so there is rarely a need for more than one HFS volume on one disk. (Note that the Macintosh driver currently supplied with Apple HD SC and CD SC drives will support only one HFS partition. Most large third-party drives will support multiple HFS partitions. Apple does not recommend shipping CD- ROM with multiple HFS partitions.)

A disk is partitioned and the partitions are initialized with software that is included on the system disks or with the hard disk drive. Advanced Disk Utility (ADU) for the Apple II GS, included with System Disk 5.0 or later, will satisfy most Apple II partitioning needs. It supports all drives that follow the Apple extensions to the ANSI SCSI standard, and most that follow the ANSI SCSI standard faithfully, even without the Apple extensions. For the Macintosh, partitioning software is usually included with a hard disk drive.

SCSI hard disks store block allocation information (that is, number and size of the partitions and drivers on the disk) in the first few physical blocks of the disk. The hard disk driver creates logical volumes from this information at boot time and mounts these partitions as volumes on the desktop. Figure 1 illustrates the layout of a typical hard disk with mixed partitions.

[IMAGE Mixed-Partition1.GIF]

Figure 1 The Layout of a Typical Hard Disk With Mixed Partitions

Physical block 0 of the disk contains the driver descriptor map (DDM), which describes the drivers on the disk. When the disk is mounted, this information is used to load the necessary drivers, as detailed in Inside Macintosh , volume V, page 576. The Macintosh requires that a driver be resident on the disk; the Apple II supports drivers if they are resident on the disk, while not requiring them to be.

Starting at physical block 1 of the disk is the partition map. Each partition on the disk is described in its own partition map entry (PME) in this partition map. A PME, which occupies one block and is built when the partition is initialized, consists of a series of data fields describing the size and state of a specific partition. With the exception of physical block 0, every block on the disk must be accounted for in a PME, as belonging to a partition. The partition map is itself a partition and contains a PME describing itself. The PME format is shown in Figure 2.

[IMAGE Mixed-Partition2.GIF]

Figure 2 The Format of a Partition Map Entry

In condensed form, the partition map for a hard disk with both HFS and ProDOS partitions looks like this:

BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType

pmMapBlkCnt is a count of valid PMEs on the hard disk. This longword is contained in each valid PME. If it is modified in one, it must be modified in all PMEs. If a partition has been added manually (that is, with a SCSI block editor) and is not recognized by the operating system, the cause is usually an incorrect value in pmMapBlkCnt.

pmPyPartStart is the address of the first physical block of the partition. If the first physical block of a partition (logical block 0) is at physical block $200 of the hard disk, then reading block $20 of the partition actually reads physical block $220 of the disk. pmPartBlkCnt is the size, in blocks, of the partition. The size of the last partition on the disk is arrived at by subtracting the address in pmPyPartStart for this partition from the total number of blocks on the disk.

pmPartName is the name of the partition. It serves to identify the partition and should not be confused with the volume name.

pmPartType is the partition type and can contain (but is not limited to) the following:

APPLE_DRIVERPartition contains a device driver
APPLE_PARTITION_MAPPartition contains a partition map
APPLE_SCRATCHPartition is unused and free for use
APPLE_HFSPartition contains Macintosh HFS volume
APPLE_PRODOSPartition contains Apple II ProDOS volume
APPLE_FREEPartition is unused and unusable

APPLE_SCRATCH partitions are areas of the disk that are currently unused, but that can be recognized and initialized by the operating system. In the process of creating a mixed-partition disk, this is the type to assign to partitions that will later be initialized in ProDOS format (assuming that HFS partitions are formatted first). APPLE_FREE is the type to assign to partitions consisting of blocks that will not be used but must be accounted for in order to fulfill the requirement that all blocks on the disk belong to a partition.

When you go about mixing partitions, as described in the following section, you may need to change some of the fields in a PME, and to copy blocks from one disk to another. PMEs can be browsed and edited with SEDIT, a utility written at Apple by David Shayer. Figure 3 shows a PME viewed in SEDIT. This utility also makes it easy to perform block editing at a device level on SCSI hard disks. SEDIT can copy blocks on the same or between separate devices, and provides nifty templates for editing blocks of data. You'll find SEDIT included, along with documentation, on theDeveloper Essentials disc. (A word to the wise: SEDIT also has the wonderful ability to scramble any SCSI device that is connected, so be sure to look at the warning message under the File menu and to read the documentation before trying anything you're not sure of.)

[IMAGE Mixed-Partition3.GIF]

Figure 3 SEDIT View of the PME for the HFS Partition of A Disc Called Wanda


Now that you understand the layout of the disk and the importance of the partition map, you're ready to mix your own partitions. You can choose to either include the same information on both partitions (for example, large databases) or arrange the files so that Apple II-specific information is on a ProDOS volume and Macintosh-specific information is on an HFS volume. And you have a choice of whether to start out with one large hard disk, or two or more smaller hard disks. The first way is easiest.

[IMAGE Mixed-Partition4.GIF]

Figure 4 Two Ways to Create a CD-ROM

In the processes described here, every attempt has been made to let the existing system software and utilities do the work, with a minimum of "twiddling" necessary by the developer. This ensures that the CD-ROM will work properly and will be compatible with future system software.


The simplest method to prepare a mixed-partition disk from which to press a CD is as follows:
  1. Beg, borrow, steal, or even (gasp!) buy a 600+ MB hard disk drive that will work with both the Apple II GS and the Macintosh. Software to partition the hard disk for the Macintosh is usually included with the drive.
  2. Use the Macintosh partitioning software on the hard disk. This will create the DDM in block 0. Create an APPLE_SCRATCHpartition for each ProDOS volume you wish to include on the disk. Remember that ProDOS volumes are limited to 32 MB and that only the first two volumes will be accessible under ProDOS 8 (but all will be accessible under GS/OS). Make the HFS partition the last one on the disk, to allow for changing the size of this partition without disturbing the ProDOS partitions.

    If you create two APPLE_SCRATCH partitions, the partition map will look something like this:

    BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType
  3. Disconnect the drive from the Macintosh (with the power off, of course), and connect it to the Apple II GS. Boot the system with System Disk 5.0.2 or later (to take advantage of the new SCSI Manager and drivers). When the Finder's desktop appears, a dialog will be presented declaring that the disk is unreadable. Click Initialize for each of the APPLE_SCRATCH partitions (that's twice for the above example).

    Warning: Do not initialize the HFS partition! The Finder will also want to initalize the HFS partition, since it doesn't recognize it, and you may politely decline by clicking Eject in the dialog box.

    [IMAGE Mixed-Partition5.GIF]

    Figure 5 The Dialog Box to Initialize Partitions

    Every time the GS Finder is launched (booting, quitting from an application, and so forth) it will ask if you wish to initialize the HFS partition. This annoying behavior will disappear when the CD- ROM with the image of the hard disk is mounted, since it is write-protected.

    The hard disk is now fully prepared. In our example, it contains two ProDOS volumes and one HFS volume, which are fully initialized and ready for files to be copied onto them.

  4. Copy the desired files to their respective volumes. Transfer the hard disk drive between the Apple IIGS and the Macintosh as needed, until the files and folders for all volumes are arranged as you wish them to appear on the CD.
  5. Mail your hard disk drive to the CD production company of your choice, asking them to place an image of the hard disk on the data track of the CD.

In cases where you wish to combine partitions from separate hard disks on one large hard disk, more work is required, but it is certainly not impossible.

The same process I'm about to describe can also be done using a CD-ROM premastering system that allows block manipulation by a Macintosh, but you will definitely need technical assistance from the premastering system's engineer. With such a system, it is possible to manually create partition maps and block copy the desired volumes over. The end result is an image of a large hard disk identical to the image achieved by the process described in the preceding section and below. As an example to illustrate the process of combining smaller hard disks on a larger one, let's say we're starting with two hard disks--one 80 MB hard disk formatted as a large HFS volume and one 80 MB hard disk with two 32 MB ProDOS partitions. The partitioning utilities and system software have already done most of the work for us: the partitions are initialized and the partition maps built.

The partition map for our first hard disk, SCSI ID 1, looks like this:

BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType

Note that because not all hard disks, even of the same capacity, have the same block count, the value in PmPartBlkCnt for the last partition could differ if a different hard disk were being used.

The partition map for our second hard disk, SCSI ID 2, looks like this:

BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType

Manually combining the partition maps on paper, we come up with the desired partition map for the large hard disk, SCSI ID 3:

BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType
BlockpmMapBlkCntpmPyPartStart pmPartBlkCntpmPartNamepmPartType

What remains is to combine all of the partitions on the third hard disk. To do so, we first copy block 0 (the DDM) from hard disk 1 to hard disk 3. Then we copy the partition map from hard disk 1 to hard disk 3. We copy physical blocks 3 and 4 from hard disk 1 to physical blocks 5 and 6 on hard disk 3.

From hard disk 2, we copy physical blocks 2 and 3 to blocks 3 and 4 on hard disk 3. The final partition map is now in place, although the values in some of the fields are incorrect. We use SEDIT to update the fields according to the manually created table, remembering to update pmPyPartStart in each entry and PmPartBlkCnt for the last partition on the disk (to adjust for the changed number of unused blocks in the APPLE_FREE partition). Now hard disk 3 is ready to have the volumes copied to it.

Using the SEDIT Copy Blocks command, we copy the volumes from the smaller hard disks to the proper locations on the large hard disk:

   From SCSI ID  To SCSI ID  From Block  To Block  # of Blocks
1. 1             3           0            0        1   DDM
2. 1             3           40           40       20  Mac Driver
3. 1             3           60           20060    2626E   HFS Partition
4. 2             3           40           60       10000   ProDOS partition
5. 2             3           10040        10060    10000   ProDOS partition

After we copy the DDM from hard disk 1 to hard disk 3 it is no longer valid, so we zero out the DDM's first 24 bytes. (If we planned to use hard disk 3 from the Macintosh and not as a master for a CD-ROM, we would update these bytes to make the DDM valid for hard disk 3.) We also zero out the first 8 bytes (the boot block) of the HFS partition to ensure that the CD doesn't attempt to boot.

The large hard disk is now fully prepared and ready for shipment to the CD production company.


If after reading this article you're eager to try creating your own mixed-partition CD-ROM, you'll want to refer to the sidebar on CD-ROM production companies in Brian Bechtel's article in this issue. There you'll find names and addresses of places to send your hard disk. Brian's paper CD-ROM and the Macintosh Computer , found on the accompanying Developer Essentials disc, covers basic details of cost and time required to get a CD-ROM pressed.

And here's a final note to round out your understanding of mixed- partition CD-ROMs. When the CD-ROM resulting from the process described in this article is mounted, the partitioned volumes on its data track are recognized and mounted on the desktop. The Macintosh will currently mount only the first HFS partitioned volume that it finds. The Apple II will try to mount the HFS volume but will not find a file system translator to read it with, and so will effectively ignore it.

LLEW ROBERTS became an Apple person by accident, although we're not sure exactly which accident that was. (Lately there have been several.) He says he works on just too many different things to actually specify what he does for a living. We think he may be a DTS engineer, because he was recently overheard answering questions regarding mixing HFS and ProDOS partitions on a single drive. His only REAL hobby is collecting originals and English translations of Japanese manga and anime (comic books and animated video). His favorite is AppleSeed. Llew also dabbles in subliminal suggestion. Can you find the hidden message in his article? You'll know for sure when you awaken in the middle of the night craving some manga or a CD-ROM drive. *

Thanks to Our Technical Reviewers: Bryan Atsatt, Matt Gulick, Jim Luther, Dave Lyons, Jim Reekes, Dave Shayer


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