Avago Technologies LSI53C896 User Manual

Page 185

SCSI Registers

4-73

Version 3.3

PFEN

Prefetch Enable

Setting this bit enables an 8-Dword SCRIPTS instruction
prefetch unit. The prefetch unit, when enabled, fetches
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 flushes 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 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
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 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
always 32 bits in width.

SSM

Single Step Mode

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 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.