learning ddr mode reigsters

For application flexibility, various functions, features, and modes are programmable in four Mode Registers,
provided by the DDR3 SDRAM, as user defined variables and they must be programmed via a Mode Register Set (MRS) command.

As the default values of the Mode Registers (MR#) are not defined, contents of
Mode Registers must be fully initialized and/or re-initialized, i.e., written, after power up and/or reset for
proper operation. Also the contents of the Mode Registers can be altered by re-executing the MRS command
during normal operation. When programming the mode registers, even if the user chooses to modify only a
sub-set of the MRS fields, all address fields within the accessed mode register must be redefined when the
MRS command is issued. MRS command and DLL Reset do not affect array contents, which means these
commands can be executed any time after power-up without affecting the array contents