SMM mode RAM usage -- open discussion

maman

I've been thinking about the SMM(System Management Mode) RAM (address range A_0000h - D_FFFFh) usage for a while.
As per every chipset datasheet I've read, it seems that this area of RAM is "missing" or "not seen" or "inaccessible" during normal runtime (operating system already take control) to the processor. I mean this area is "shadowed" by other hardware devices (not the RAM itself). It should be that way to maintain legacy compatibility (with DOS), hence the RAM in this area is totally unseen by the system unless it is in SMM. The devices that occupy this area during runtime includes part of the VGA memory mapped video buffer, etc.

This is my opinion and understanding of SMM. For those who are more knowledgeable, would you please corrrect if there are any misunderstanding of this point of view Wink

_________________
-- Human knowledge belongs to the world --

maman · Posted: Tue Oct 25, 2005 7:38 am Post subject:

well, after reading Intel's System Programming Guide and some chipsets datasheet, it seems that my statement above is partly right. In addition, the SMM "handler" area can be relocated as needed, so it doesn't have to be in the "ISA" video memory area. it can relocated high in the address space, such as above the top of DRAM memory, etc. also correction that the video memory area (VGA) is only A_0000h - B_FFFFh. C_0000h - D_FFFFh is used for "runtime" expansion ROM codes.
_________________
-- Human knowledge belongs to the world --