
SCSI Functional Description
2-23
conditions applies, the contents of the prefetch unit are automatically
flushed.
On every Memory Move instruction. The Memory Move instruction is
often used to place modified code directly into memory. To make
sure that the chip executes all recent modifications, the prefetch unit
flushes its contents and loads the modified code every time an
instruction is issued. To avoid inadvertently flushing the prefetch unit
contents, use the No Flush option for all Memory Move operations
that do not modify code within the next 8 dwords. For more
information on this instruction refer to
Chapter 5, “SCSI SCRIPTS
Instruction Set”
.
On every Store instruction. The Store instruction may also be used
to place modified code directly into memory. To avoid inadvertently
flushing the prefetch unit contents use the No Flush option for all
Store operations that do not modify code within the next 8 dwords.
On every write to the
DMA SCRIPTS Pointer (DSP)
register.
On all Transfer Control instructions when the transfer conditions are
met. This is necessary because the next instruction to execute is not
the sequential next instruction in the prefetch unit.
When the Prefetch Flush bit (
DMA Control (DCNTL)
register, bit 6) is
set. The unit flushes whenever this bit is set. The bit is self-clearing.
2.2.7 Opcode Fetch Burst Capability
Setting the Burst Opcode Fetch Enable bit (bit 1) in the
DMA Mode
(DMODE)
register (0x38) causes the SYM53C896 to burst in the first two
dwords of all instruction fetches. If the instruction is a Memory-to-
Memory Move, the third dword is accessed in a separate ownership. If
the instruction is an Indirect Type, the additional dword is accessed in a
subsequent bus ownership. If the instruction is a table indirect Block
Move, the chip uses two accesses to obtain the four dwords required, in
two bursts of two dwords each.
Note:
This feature is only useful if Prefetching is disabled.
This feature is only useful if fetching SCRIPTS instructions
from main memory. Due to the short access time of
SCRIPTS RAM, burst opcode fetching is not necessary
when fetching instructions from this memory.