PowerPC™ 405 Processor Block Reference Guide www.xilinx.com 159
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locations. These bits are decoded against PPC405 address bits 0:7. These eight most
significant address bits permit the OCM controllers to reside independently in any 16 MB,
non-cacheable, memory range within the PPC405 32bit address (4 GB) memory space
The ISOCM and DSOCM hardware outputs a maximum of 22 address bits (data-side
address bits [8:29] and instruction-side address bits [8:28]) to address memory contained in
the FPGA fabric.
DSCNTL Registers
Tabl e 3-9 and Table 3 -10 describe the DSCNTL registers in Virtex-II Pro and Virtex-4
devices. For additional information, refer to Figure 3-11, page 162 (Virtex-II Pro) and
Figure 3-12, page 163 (Virtex-4).

Table 3-9: DSCNTL Register for Virtex-II Pro

Bit 0 DSOCM Enable If set to 1, address decoding based on the value of DSARC will be
enabled. If set to 0, the content in DSARC will be ignored.
Bit 1 DISABLEOPERANDFWD If set to 1, load data from the DSOCM goes directly into a latch in
the processor block. This causes an additional cycle (a total of two
cycles) of latency between a load instructions which is followed by
an instruction that requires the load data as an operand.
If set to 0, load data from the DSOCM/ must pass through steering
logic before arriving at a latch. This causes a single cycle of latency
between a load instruction which is followed by an instruction that
requires the load data as an operand.
Bit 2 DSOCMBUSY This status bit can be used as a flag indicator to the FPGA fabric.
This is an optional signal.
Bit 3 Reserved. This bit must be configured to 0.
Bit 4 Reserved. This bit must be configured to 0.
Bit 5:7 DSOCMMCM CPU Clock and DSOCM Clock ratio. For Virtex-II Pro user s , us er s
must setup the ratio in this field with valid clock ratios used in the
application system. Then the processor gasket will issue
appropriate transaction based on this ratio.