3
31
puts them at the top of the 32-bit address space. For many years it wasn't possible or practical
to put that much RAM into a PC. But now it is, so it's up to the memory controller and host
bridge to figure out what to do. Many systems cause that high RAM to simply be ignored,
resulting in the loss of effective RAM. More complex systems will take the RAM that would
occupy that 3.5-4GB address space and re-map it into the 4.0-4.5 address space. The RAM
doesn't care because it's just an array of storage cells, it's up to the memory controller to as-
sociate addresses with those storage cells.
Of course, that only works if you're using a 64-bit (or 32bit physical address extension (PAE)
enabled) OS that can deal with physical addresses larger than 32 bits.
Once this option is enabled, the BIOS can see 4096MB of memory.
► DCT Unganged Mode
DCT stands for DRAM Controller.
Ganged refers to the use of both DRAM controllers within a memory controller acting in con-
cert to access memory. For a description of ganged (128-bit DRAM data width) and unganged
(64-bit DRAM data width) DRAM modes :
Ganged channels (DDR3) :
■
DCT channels A and B can be ganged as a single logical 128-bit DIMM.
■
Offers highest DDR3 bandwidth.
■
Requires both DIMMs in a logical pair to have identical size and timing parameters, both
DCTs programmed identically.
Unganged channels
■
DCT channels A and B operate as two completely independent 64-bit channels (both chan-
nels operate at the same frequency).
■ Reduce DRAM page conflicts – more concurrent open dram pages.
■ Better bus efficiency.
Burst lengths supported
When both DCTs are enabled in unganged mode, BIOS must initialize the frequency of each
DCT in order.
► Power Down Enable
When power down mode is enabled, if all pages of the DRAMs associated with a CKE pin are
closed, then these parts are placed in power down mode.
► Power Down Mode
For non-mobile systems, power down mode should be set to [Channel] CKE control.
A DIMM or a group of DIMMs enters power down mode by deasserting the corresponding
clock enable signal when the DRAM controller detects that there are no transactions
scheduled to any of the DIMMs connected to the clock enable signal. A DIMM or a group of
DIMMs exits power down mode by asserting the corresponding clock enable signal when a
transaction is scheduled to any DIMM connected to the clock enable signal. There are two
CKE pins per DRAM channel. For each channel :
[Channel] CKE control. The DRAM channel is placed in power down when all chip selects
associated with the channel are idle.
[Chip Select] CKE control. A chip select or pair of chip selects is placed in power down
when no transactions are pending for the chip select(s).
