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SCSI Registers
4-69
conditions are met), on writes to the
DMA SCRIPTS
Pointer (DSP)
, on regular MMOV instructions, and when
an interrupt is generated. Based on the burst length as
determined by the values in the
DMA Mode (DMODE)
register, the unit automatically determines the maximum
burst size that it is capable of performing. If the burst
threshold is set to 8 Dwords, the prefetch unit fetches
instructions in two bursts of 4 Dwords. If the burst
threshold is set to 16 Dwords or greater, the prefetch unit
fetches instructions in one burst of 8 Dwords. Burst
thresholds of less than 8 Dwords cause the prefetch unit
to be disabled. PCI Cache commands (Read Line and
Read Multiple) are issued if PCI caching is enabled.
Prefetching from SCRIPTS RAM is not supported and is
unnecessary due to the speed of the fetches. When
fetching from SCRIPTS RAM, the setting of this bit has
no effect on the fetch mechanism from SCRIPTS RAM.
The prefetch unit does not support 64-bit data instruction
fetches across the PCI bus. Prefetches of SCRIPTS
instructions are 32-bits in width.
SSM
Single-Step Mode
Setting this bit causes the SYM53C1010 SCSI function to
stop after executing each SCRIPTS instruction and to
generate a single step interrupt. When this bit is cleared
the SYM53C1010 SCSI function does not stop after each
instruction. It continues fetching and executing
instructions until an interrupt condition occurs. For normal
SCSI SCRIPTS operation, keep this bit cleared. To
restart the SYM53C1010 SCSI function after it generates
a SCRIPTS Step interrupt, read the
Interrupt Status Zero
(ISTAT0)
and
DMA Status (DSTAT)
registers to recognize
and clear the interrupt. Then set the START DMA bit in
this register.
4
IRQM
IRQ Mode
When set, this bit enables a totem pole driver for the
INTA/, or INTB/ pin. When cleared, this bit enables an
open drain driver for the INTA/, or INTB/, pin with an
internal weak pull-up. The bit should remain cleared to
retain full PCI compliance.
3
STD
Start DMA Operation
The SYM53C1010 SCSI function fetches a SCSI
SCRIPTS instruction from the address contained in the
2