2-40 Functional Description
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
forthe 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/inte rrupt is enabled during Targetm ode
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 fr om the
CPU. This prevents an interrupt when arbitration is complete (CMP set),
when the LSI53C875A 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.
2.2.16.4 Masking
Masking an interrupt means disabling or ignoring that interrupt. Inte rrupts
canbemaskedbyclearingbitsintheSCSIInterrupt En ableZe ro
(SIEN0) and SCSI Interrupt Enable One (SIEN1) (for SCSI interrupts)
registers or DMA Interrupt Enable (DIEN) (for DMA interrupts) register.
How the chip responds to masked interrupts depends on: whether polling
or hardware interrupts are being used; whether the interrupt is fatal or
nonfatal; and whether the chip is operating in the Initiator or Target mode.
If a nonfatal interrupt is masked and that condition occurs, the SCRIPTS
do not stop, the appropriate bit in the SCSI Interrupt Status Zero (SIST0)
or SCSI Interrupt Status One (SIST1) is still set, the SIP bit in the
Interrupt Status Zero (ISTAT0)is not set, a nd the IRQ/ pin is not
asserted.
If a fatal interrupt is masked and that condition occurs, then the SCRIPTS
still stop, the appropriate bit in the DMA Status (DSTAT),SCSIInterrupt
Status Zero (SIST0),orSCSI Interrupt Status One (SIST1) register is
set, and the SIP or DIP bit in the Interrupt Status Zero (ISTAT0) register
is set, but the IRQ/ pin is not asserted.