Need of different memory zones

[Niroj Pokhrel]

Hi All,

I have been studying Memory Management in linux. But I am confused with the
division of different ZONE. The use of ZONE_DMA and ZONE_NORMAL is fine.
But I am confused with ZONE_HIGHMEM, if the system is 32 bit then why can't
it map 4GB memory (2^32). I had thought that ZONE_NORMAL was for the kernel
usage so directly mapped and ZONE_HIGHMEM for the user process and is not
directly mapped to facilitate virtual addressing. But I know I am not
getting something right. While going through Ch - 15 of Rubini's Linux
Device Driver (Memory Management and DMA) I came across following line

"a 64-bit architecture such as Intel’s x86-64 can fully map and handle
64-bits of memory.Thus, x86-64 has no ZONE_HIGHMEM and all physical memory
is contained within ZONE_DMA and ZONE_NORMAL."  So, if it can be directly
mapped why not in 32 bit architecture.

I am really confused. Please Help. Thanking you in Advance


Yours,
Niroj Pokhrel
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