conditions caused the DMA-type interrupt, and clears that DMA interrupt condition. Bit 7 in DSTAT, DFE, is purely a status bit; it will not generate an interrupt under any circumstances and will not be cleared when read. DMA interrupts flush neither the DMA nor SCSI FIFOs before generating the interrupt, so the DFE bit in the DMA Status (DSTAT) register should be checked after any DMA interrupt.

If the DFE bit is cleared, then the FIFOs must be cleared by setting the CLF (Clear DMA FIFO) and CSF (Clear SCSI FIFO) bits, or flushed by setting the FLF (Flush DMA FIFO) bit.

SIEN0 and SIEN1 – The SCSI Interrupt Enable Zero (SIEN0) and SCSI Interrupt Enable One (SIEN1) registers are the interrupt enable registers for the SCSI interrupts in SCSI Interrupt Status Zero (SIST0) and SCSI Interrupt Status One (SIST1).

DIEN – The DMA Interrupt Enable (DIEN) register is the interrupt enable register for DMA interrupts in DMA Status (DSTAT).

DCNTL – When bit 1 in the DMA Control (DCNTL) register is set, the IRQ/ 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 IRQ/ 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 ISTAT 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 LSI53C875A is operating in the Initiator mode, only the Function Complete (CMP), Selected (SEL), Reselected (RSL), General

SCSI Functional Description

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LSI 53C875A technical manual Fatal vs. Nonfatal Interrupts