
SCSI Registers
5-65
it continues fetching and executing instructions until an
interrupt condition occurs. For normal SCSI SCRIPTS
operation, this bit should be clear. To restart the
SYM53C895 after it generates a SCRIPTS Step interrupt,
the ISTAT and DSTAT registers should be read to
recognize and clear the interrupt and then the START
DMA bit in this register should be set.
IRQM
IRQ Mode
3
When set, this bit enables a totem pole driver for the IRQ
pin. When reset, this bit enables an open drain driver for
the IRQ pin with a internal weak pull-up. This bit is reset
at power up. This bit should remain clear to retain full PCI
compliance.
STD
Start DMA Operation
2
The SYM53C895 fetches a SCSI SCRIPTS instruction
from the address contained in the DSP register when this
bit is set. This bit is required if the SYM53C895 is in one
of the following modes:
Manual start mode – Bit 0 in the DMODE register is
set.
Single-step mode – Bit 4 in the DCNTL register is set.
When the SYM53C895 is executing SCRIPTS in manual
start mode, the Start DMA bit needs to be set to start
instruction fetches, but does not need to be set again
until an interrupt occurs. When the SYM53C895 is in
single-step mode, the Start DMA bit needs to be set to
restart execution of SCRIPTS after a single-step
interrupt.
IRQD
IRQ Disable
1
Setting this bit disables the IRQ pin and clearing this bit
enables normal operation. As with any other register
other than ISTAT, this register cannot be accessed except
by a SCRIPTS instruction during SCRIPTS execution.
For more information on the use of this bit in interrupt
handling, refer to Chapter 2, "Functional Desription."
COM
SYM53C700 Compatibility
0
When this bit is clear, the SYM53C895 behaves in a
manner compatible with the SYM53C700 in that
selection/reselection IDs are stored in both the SSID and
SFBR registers.