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SCSI FIRMWARE INTERRUPT STRUCTURE
the REACTIVATION entry point of the SCSI firmware. The WAIT FOR INTERRUPT (OPEN) status is usually given when a particular TARGET is "threaded" to the MVME147 on the SCSI bus, and is slow in transitioning between information transfer phases. A bus phase interrupt brings the processor back to the SCSI firmware to finish the command execution that was temporarily slowed down by the TARGET.
For the WAIT FOR INTERRUPT (OPEN) status, the user may send a new command because the SCSI bus is free.
The second method of returning control involves the direct branch or jump to the REACTIVATION entry point of the firmware. For all the statuses involved (WAIT TIME, MESSAGE RECEIVED, COMMAND RECEIVED), the MVME147 is the current SCSI bus initiator and the user may only service the current "thread".
The SCSI firmware was designed to operate in both interrupt and non- interrupt modes. When the user chooses the interrupt mode of operation, the WD33C93 interrupt is enabled at the level specified in command packet in the MVME147 interrupt handler. Vector number $45 is used by the WD33C93 for the SCSI bus interrupts and the SCSI firmware initializes vector offset $114 to point to the SCSI firmware interrupt handler.
Whenever processor control is passed to the SCSI firmware interrupt handler, the MC68030 interrupt mask must be at a level no lower than that specified in the command packet. As processor control is switched out of the interrupt handler, the MC68030 interrupt mask is still at the same level.
