> Two thresholds of high memory exist on 32-bit x86 systems, one
> > at 4GiB and a second at 64GiB. The 4GiB limit is related to the amount
> > of memory that may be addressed by a 32-bit physical address. To
> > access memory between the range of 1GiB and 4GiB, the kernel
>
> 896 MB to 4GB actually...
896MB to available phy. memory.
Actually at 4GB there is one threshold as said above. It is impossible for a 32-bit processor to address an address more than 4GB, so the x86 processors use PAE (Physical Address Extension)
http://en.wikipedia.org/wiki/Physical_Address_Extension
