BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
122
Section 6: DRAM
Document
1250_1125-UM100CB-R
This will give a good starting point and should give acceptable results in most systems. Modifications can then
be made as a result of performance monitoring.
P
AGE
P
OLICY
The memory controller supports page mode access to the DDR SDRAMs, and provides several different
policies for controlling page accesses. The FCRAMs do not support page mode (they always auto-precharge
following CAS).
The Closed Page policy is the simplest. Pages are always closed by using CAS cycles with AutoPrecharge. If
the memory uses FCRAMs the controller must be configured to use this policy.
The CAS time check policy will check the request queue just before the CAS cycle is issued to a channel. If
there are requests in the queue that hit on the page then it is left open (and the same test will be performed
when the new request is serviced) otherwise it is closed by issuing the AutoPrecharge command along with
the CAS. If a sequence of accesses are done to the same page there is a good chance the page will be open
when required.
The Hint Based policy is the same as the CAS time check, except that it accepts a hint with each memory
request. The hint is part of the command on the ZBbus, and will be asserted by the DMA engines, PCI and
HyperTransport interfaces when doing block moves. If the hint is set on a request then even if there are no
more requests the page is left open following its CAS time check. If the hint is clear for the request then the
behavior matches the CAS time check policy. Pages that are left open by hints will be closed by subsequent
accesses that have the hint clear and fail the CAS time check, or will be closed using an explicit Precharge
instruction if a page conflict occurs to the same bank.
The Open Page policy leaves the page open until a page conflict occurs to the same bank or an AutoRefresh
is done. In the case of a page conflict an explicit Precharge command is performed to close the open page,
delaying the RAS cycle to open the new page.
The page policy is set per chip select, allowing different areas of memory to have different policies. The optimal
policy can therefore be selected based on the data access patterns. A good starting point is to use the CAS
time check policy with hints enabled.
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