
2-44
Functional Description
DMA Control (DCNTL)
–
When bit 1 in this register is set, the INTA/ (or
INTB/) pin is not asserted when an interrupt condition occurs. The
interrupt is not lost or ignored, but is merely masked at the pin. Clearing
this bit when an interrupt is pending immediately causes the INTA/ (or
INTB/) pin to assert. As with any register other than ISTAT, this register
cannot be accessed except by a SCRIPTS instruction during SCRIPTS
execution.
2.2.16.3 Fatal vs. Nonfatal Interrupts
A fatal interrupt, as the name implies, always causes the SCRIPTS to
stop running. All nonfatal interrupts become fatal when they are enabled
by setting the appropriate interrupt enable bit. Interrupt masking is
discussed in
Section 2.2.16.4, “Masking”
. All DMA interrupts (indicated by
the DIP bit in
Interrupt Status Zero (ISTAT0)
and one or more bits in
DMA
Status (DSTAT)
being set) are fatal.
Some SCSI interrupts (indicated by the SIP bit in the
Interrupt Status Zero
(ISTAT0)
and one or more bits in
SCSI Interrupt Status Zero (SIST0)
or
SCSI Interrupt Status One (SIST1)
being set) are nonfatal.
When the SYM53C896 is operating in the Initiator mode, only the
Function Complete (CMP), Selected (SEL), Reselected (RSL), General
Purpose Timer Expired (GEN), and Handshake-to-Handshake Timer
Expired (HTH) interrupts are nonfatal.
When operating in the Target mode, CMP, SEL, RSL, Target mode:
SATN/ active (M/A), GEN, and HTH are nonfatal. Refer to the description
for the Disable Halt on a Parity Error or SATN/ active (Target Mode Only)
(DHP) bit in the
SCSI Control One (SCNTL1)
register to configure the
chip’s behavior when the SATN/ interrupt is enabled during Target mode
operation. The Interrupt-on-the-Fly interrupt is also nonfatal, since
SCRIPTS can continue when it occurs.
The reason for nonfatal interrupts is to prevent the SCRIPTS from
stopping when an interrupt occurs that does not require service from the
CPU. This prevents an interrupt when arbitration is complete (CMP set),
when the SYM53C896 is selected or reselected (SEL or RSL set), when
the initiator asserts ATN (target mode: SATN/ active), or when the
General Purpose or Handshake-to-Handshake timers expire. These
interrupts are not needed for events that occur during high-level
SCRIPTS operation.