Storing Tape Miniclass Context in Optional Extensions
A tape miniclass driver can store context in two optional extensions:
A driver-specific minitape extension
When used, this extension typically stores information about device capabilities (SCSI inquiry data or equivalent). A driver that supports several devices can store details about each device, rather than repeatedly querying the devices.
A miniclass driver specifies the size of the minitape extension in the MinitapeExtensionSize member of the TAPE_INIT_DATA_EX structure it passes to TapeClassInitialize from its DriverEntry routine. The tape class driver allocates the requested storage on behalf of the miniclass driver. The miniclass driver initializes the optional extension with a TapeMiniExtensionInit routine. The minitape extension remains valid until the driver is unloaded.
A command-specific command extension
When used, this extension stores command-specific context between calls to a tape miniclass routine that might be called more than once to process a single request. For example, a tape miniclass driver's TapeMiniGetStatus routine might store the status from a TEST_UNIT_READY command in the command extension while it determines whether the tape drive also requires cleaning.
A miniclass driver specifies the size of the command extension in the CommandExtensionSize member of the TAPE_INIT_DATA_EX structure it passes to TapeClassInitialize from its DriverEntry routine.
All tape miniclass routines that handle device-specific aspects of device-control requests are given a pointer to the command extension when they are called. The tape class driver allocates storage for the command extension before calling such a miniclass routine. The miniclass routine initializes the command extension on the first call to process a given request; that is, when the CallNumber parameter to the routine equals zero. The command extension remains valid until the tape miniclass routine returns either TAPE_STATUS_SUCCESS or an error status.