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SCSI FIRMWARE INTERRUPT STRUCTURE
4-2
4
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.