[coreboot] raminit.c example
rogerx.oss at gmail.com
Mon Jan 3 10:20:00 CET 2011
On Mon, Jan 03, 2011 at 12:14:39AM -0500, Corey Osgood wrote:
>On Sun, Jan 2, 2011 at 8:03 PM, Roger <rogerx.oss at gmail.com> wrote:
>> I've read over the src/northbridge/intel/i440bx/raminit.c, but am still a
>> little mystified on setting the settings.
>> I can see where settings are set to zero (or default), but when I look at other
>> settings, the values don't seem to match up to what is well documented within
>> the comments.
>> A good easy example would be setting DRAMC:
>> DRAMC, 0x00, 0x08,
>> Is the "0x00" designated by "000" as documented? Hex?
>> What does "0x08" mean?
>See sdram_set_registers(). Those values aren't written directly to the
>register, because there may be some data stored in other bits that we
>don't want to lose. So, for DRAMC, sdram_set_registers reads the value
>of the DRAMC register (defined in i440bx.h as 0x57), then does a
>logical AND with the first value, 0x00 (which will actually clear all
>the bits), then performs a logical OR with the second value, 0x08,
>which sets bit 3 to 1, indicating the memory type is SDRAM. It then
>writes that value back to the register. In situations like this, gcc
>should just optimize the whole thing down to writing 0x08 to the
>register, so there's no problem with wasting CPU/IO cycles performing
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