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2-38 Functional Description
polled when polled interrupts are used. It is also the first register that
should be read after the IRQ/ pin is asserted in association with a
hardware interrupt. The INTF (Interrupt-on-the-Fly) bit should be the first
interrupt serviced. It must be written to one to be cleared. This interrupt
must be cleared before servicing any other interrupts.
See Register 0x14, Interrupt Status Zero (ISTAT0) register, bit 5 Signal
process in Chapter 4, “Registers,” for additional information.
The host (C Code) or the SCRIPTS code could potentially try to access
the mailbox bits at the same time.
If the SIP bit in the Interrupt Status Zero (ISTAT0) register is set, then a
SCSI-type interrupt has occurred and the SCSI Interrupt Status Zero
(SIST0) and SCSI Interrupt Status One (SIST1) registers should be read.
If the DIP bit in the Interrupt Status Zero (ISTAT0) register is set, then a
DMA-type interrupt has occurred and the DMA Status (DSTAT) register
should be read.
SCSI-type and DMA-type interrupts may occur simultaneously, so in
some cases both SIP and DIP may be set.
SIST0 and SIST1 – The SCSI Interrupt Status Zero (SIST0) and SCSI
Interrupt Status One (SIST1) registers contain SCSI-type interrupt bits.
Reading these registers determines which condition or conditions caused
the SCSI-type interrupt, and clears that SCSI interrupt condition.
If the LSI53C875A is receiving data from the SCSI bus and a fatal
interrupt condition occurs, the chip attempts to send the contents of the
DMA FIFO to memory before generating the interrupt.
If the LSI53C875A is sending data to the SCSI bus and a fatal SCSI
interrupt condition occurs, data could be left in the DMA FIFO. Because
of this the DMA FIFO Empty (DFE) bit in DMA Status (DSTAT) should be
checked.
If this bit is cleared, set the CLF (Clear DMA FIFO) and CSF (Clear SCSI
FIFO) bits before continuing. The CLF bit is bit 2 in Chip Test Three
(CTEST3). The CSF bit is bit 1 in SCSI Test Three (STEST3).
DSTAT – The DMA Status (DSTAT) register contains the DMA-type
interrupt bits. Reading this register determines which condition or