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Inside Macintosh: Devices /
Chapter 3 - SCSI Manager / SCSI Manager Reference
Data Structures

Driver Descriptor Record

The driver descriptor record contains information about the device drivers resident on a SCSI peripheral device. The driver descriptor record is defined by the Block0 data type.

TYPE Block0 = 
   sbSig:         Integer;    {device signature}
   sbBlkSize:     Integer;    {block size of the device}
   sbBlkCount:    LongInt;    {number of blocks on the device}
   sbDevType:     Integer;    {reserved}
   sbDevId:       Integer;    {reserved}
   sbData:        LongInt;    {reserved}
   sbDrvrCount:   Integer;    {number of driver descriptor entries}
   ddBlock:       LongInt;    {first driver's starting block}
   ddSize:        Integer;    {size of the driver, in 512-byte blocks}
   ddType:        Integer;    {operating system type (MacOS = 1)}
   ddPad:         ARRAY [0..242] OF Integer; {additional drivers, if any}
Field Description
The device signature. This field should contain the value of the sbSIGWord constant ($4552) to indicate that the driver descriptor record is valid (meaning that the disk has been formatted).
The size of the blocks on the device, in bytes.
The number of blocks on the device.
The number of drivers installed on the disk. More than one driver may be included when multiple operating systems or processors are supported. The drivers can be located anywhere on the device and can be as large as necessary.
The physical block number of the first block of the first device driver on the disk.
The size of the device driver, in 512-byte blocks.
The operating system or processor supported by the driver. A value of 1 specifies the Macintosh Operating System. The values 0 through 15 are reserved for use by Apple Computer, Inc.
Additional ddBlock, ddSize, and ddType entries for other device drivers on the disk.
If multiple device drivers exist on the device, you can use the Start Manager SetOSDefault function to control which operating system is loaded at startup by specifying a value that corresponds to the ddType field of the appropriate device driver. For more information on the startup process, see the chapter "Start Manager" in Inside Macintosh: Operating System Utilities.

See "The Structure of Block Devices," beginning on page 3-12, for more information about this data structure.

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Apple Computer, Inc.
3 JUL 1996

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