DDR SDRAM Initialization.

[Eric W. Biederman]

sivakumar <sivakumar.subramani at wipro.com> writes:

> Thanks for the information.
> 
> I have gone through the code and have following query.
> Following is the one of the step in Initialization of DDR SDRAM.
> 
> -Next a MODE REGISTER SET command should be issued for the Extended
>  Mode Register to enable the DLL,
> 
>         /* 5. Issue EMRS to enable DLL */
>         SET_RAM_COMMAND(RAM_COMMAND_EMRS)
>         movl (0x0000<<MD_SHIFT), %eax
>         DO_DELAY
> 
> First we are setting the Mode select bit of DRAM controller Mode
> Register to Extended Mode Register Set Enable. 
> 
> >From DDR spec, it is given that zero bit of EMRS will enable the DLL.
> 
> Here I want to know how by moving the (0x0000 << MD_SHIFT) to eax
> register, we are writting to Extended mode register.
> 
> Can any one explain this?

It is performing a memory read.   A memory read in intel chipsets
when the command register is set properly causes commands to be sent
to memory.  Reads seem to work a little bit more reliably than writes
in my experience.

Eric