ZONE_HIGHMEM > 896MB x86 32-bit

Hi,

   I am trying to understand how linux handles memory management on
32-bit x86. I came to know about ZONE_HIGHMEM (Memory which is not
directly mapped by the kernel (> 896MB) ).

   I am not able to get any reference for from where this limit of
896 MB came from.

   I would be glad if somebody could explain why is this limit of 896
MB fixed and refernces to it.

Thanks in addvance,

Abhishek