50 www.xilinx.com PowerPC™ 405 Processor Block Reference Guide
1-800-255-7778 UG018 (v2.0) August 20, 2004
Chapter 2: Input/Output Interfaces
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xThe prefetch address does not fall outside the current 1 KB physical page.
Address pipelining of cacheable prefetch requests can occur if all of the following
conditions are met:
xAddress pipelining is supported by the PLB slave.
xThe ICU is not already involved in an address-pipelined PLB transfer.
xA branch or interrupt does not modify the sequential execution of the current (first)
instruction-fetch request.
xCacheable prefetching is enabled (CCR0[PFC] 1).
xA cacheable instruction-prefetch is requested, and the instruction is not in the
instruction cache, the fill buffer, or being returned over the ISOCM interface.
xThe prefetch address does not fall outside the current 1 KB physical page.

Guarded Storage

Accesses to guarded storage are not indicated by the ISPLB interface. This is because the
PowerPC Architecture allows instruction prefetching when:
xThe processor is in real mode (instruction address translation is disabled).
xThe fetched instruction is located in the same physical page (1 KB) as an instruction
that is required by the sequential execution model.
xThe fetched instruction is located in the next physical page (1 KB) as an instruction
that is required by the sequential execution model.
Memory should be organized such that real-mode instruction prefetching from the same
or next 1 KB page does not affect sensitive addresses, such as memory-mapped I/O
devices.
If the processor is in virtual mode, an attempt to prefetch from guarded storage causes an
instruction-storage interrupt. In this case, the prefetch never appears on the ISPLB.
Instruction-Side PLB I/O Signal Table
Figure 2-4 shows the block symbol for the instruction-side PLB interface. The signals are
summarized in Tab le 2-7 .
Figure 2-4: Instruction-Side PLB Interface Block Symbol
UG018_04_051204
PPC405
PLBC405ICUADDRACK
PLBC405ICUSSIZE1
PLBC405ICURDDACK
PLBC405ICURDDBUS[0:63]
PLBC405ICURDWDADDR[1:3]
PLBC405ICUBUSY
PLBC405ICUERR
C405PLBICUREQUEST
C405PLBICUABUS[0:29]
C405PLBICUSIZE[2:3]
C405PLBICUCACHEABLE
C405PLBICUU0ATTR
C405PLBICUPRIORITY[0:1]
C405PLBICUABORT