The Northbridge supports the use of main memory as System Management RAM (SMRAM) enabling the use of System Management Mode (SMM).
  The Northbridge supports three SMM options :
    - Compatible SMRAM (AB segment enabled)
    - High Segment (HSEG)
    - Top of Memory Segment (TSEG)
  System Management RAM (SMRAM) space provides a memory area that is available for the SMI handler’s code and data storage. This memory resource is normally hidden from the operating system so that the processor has immediate access to this memory space upon entry to SMM.
  The GMCH provides three SMRAM options :
    • Below 1 MB option that supports compatible SMI handlers.
    • Above 1 MB option that allows new SMI handlers to execute with write-back cacheable SMRAM.
    • Optional larger write-back cacheable T_SEG area of either 512 KB or 1MB in size above 1 MB that is reserved from the highest area in system DRAM memory.
  The above 1 MB solutions require changes to compatible SMRAM handler’s code to properly execute above 1 MB.
  The HSEG and TSEG SMM transaction address spaces reside in this extended memory area.
HSEG ($000A_0000 – 000B_FFFF)
  SMM-mode processor accesses to enabled HSEG are remapped to $000A_0000 – 000B_FFFF. Non-SMM-mode processor accesses to enabled HSEG are considered invalid are terminated immediately on the FSB. The exception to this is non-SMM-mode write-back cycles. They are remapped to SMM space to maintain cache coherency. AGP and hub interface originated cycles to enabled SMM space are not allowed. Physical DRAM behind the HSEG transaction address is not remapped and is not accessible.
TSEG (Top of Main Memory–TSEG)
  TSEG can be up to 1 MB and is at the top of memory. SMM-mode processor accesses to enabled TSEG access the physical DRAM at the same address. Non-SMM-mode processor accesses to enabled TSEG are considered invalid and are terminated immediately on the FSB. The exception is non-SMM-mode write-back cycles. They are directed to the physical SMM space to maintain cache coherency. AGP and hub interface originated cycle toenabled SMM space are not allowed.