LSI LSI53C896, Technical Manual

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SCSI Registers 4-73
PFF Prefetch Flush 6
Setting this bit causes the prefetch unit to flush its
contents. This bit clears after the flush is complete.
PFEN Prefetch Enable 5
Setting this bit enables an 8-Dword SCRIPTS instruction
prefetch unit. The prefetch unit, when enabled, will fetch
8 Dwords of instructions and instruction operands in
bursts of 4 or 8 Dwords. Prefetching instructions allows
the LSI53C896 SCSI function to make more efficient use
of the system PCI bus, thus improving overall system
performance. The unit will flush whenever the PFF bit is
set, as well as on all transfer control instructions when
the transfer conditions are met, on every write to the
DMA SCRIPTS Pointer (DSP) , on every regular MMOV
instruction, and when any interrupt is generated. The unit
automatically determines the maximum burst size that it
is capable of performing based on the burst length as
determined by the values in the
DMA Mode (DMODE)
register. If the burst threshold is set to 8 Dwords the
prefetch unit will fetch instructions in two bursts of 4
Dwords. If the burst threshold is set to 16 Dwords or
greater the prefetch unit will fetch instructions in one burst
of 8 Dwords. Burst thresholds of less than 8 Dwords will
cause the prefetch unit to be disabled. PCI Cache
commands (Read Line and Read Multiple) will be issued
appropriately 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 will have 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 will always be 32 bits in width.
SSM Single Step Mode 4
Setting this bit causes the LSI53C896 SCSI function to
stop after executing each SCRIPTS instruction, and
generate a single step interrupt. When this bit is cleared
the LSI53C896 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 LSI53C896 SCSI function after it generates a
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